Electron microscopy apparatus and method for a time resolved low energy electron microscopy investigation of a sample

Abstract

An electron microscopy apparatus (100) for time resolved low energy electron microscopy investigation of a sample (1) comprises a tip-shaped photo-emitter source (11), which is arranged for radiation-induced emission of source electrons (2) towards the sample (1), a radiation source device (20) for creating emitter excitation radiation (3) with an emitter excitation waveform and for irradiating the photo-emitter source (11) with the emitter excitation radiation (3), a sample excitation device (30) for applying a sample excitation (4) to the sample (1), and a detector device (40) for collecting sample electrons emerging at the sample (1) in response to an interaction of the source electrons (2) with the sample (1), wherein the electron microscopy apparatus (100) is configured for the time-resolved investigation of the sample (1) based on a synchronization of the sample excitation (4) applied to the sample (1) with the source electrons (2) received by the sample (1), and wherein the photo-emitter source (11) is configured for creating the source electrons (2) in response to the irradiation with the emitter excitation radiation (3) by an emission process in which the number of emitted electrons is linear with respect to the irradiated power of the emitter excitation radiation (3) on the photo- emitter source (11). Furthermore, an electron microscopy method of investigating a sample (1) by time resolved low energy electron microscopy is described.

Description

ELECTRON MICROSCOPY APPARATUS AND METHOD FOR A TIME RESOLVED LOW ENERGY ELECTRON MICROSCOPY INVESTIGATION OF A SAMPLEField of the inventionThe invention relates to an electron microscopy apparatus for investigating a sample by time resolved low energy electron microscopy and to an electron microscopy method of investigating a sample by time resolved low energy electron microscopy. Applications of the invention are available e. g. in the fields of material analyses, material investigations, investigations of electron-photon interactions, or monitoring chemical surface reactions.Technical backgroundIn the present specification, reference is made to the following prior art illustrating technical background of the invention and related techniques:[1] E. Bauer, Basic Interactions, in Surface Microscopy with Low Energy Electrons (Springer New York, New York, NY, 2014), pp. 21-88;[2] W. Telieps et al., An Analytical Reflection and Emission UHV Surface Electron Microscope, Ultramicroscopy 17, 57 (1975);[3] R. M. Tromp et al., Design of a New Photo-Emission / Low-Energy Electron Microscope for Surface Studies, Ultramicroscopy 36, 99 (1991);[4] R. M. Tromp et al., A New Low Energy Electron Microscope, Surf. Rev. Lett. 05, 1189 (1998);[5] EP 1994544 Bl;[6] US 7453 062 B2;[7] W. Wan, et al., Design and Commissioning of an Aberration-Corrected Ultrafast Spin-Polarized Low Energy Electron Microscope with Multiple Electron Sources, Ultramicroscopy 174, 89 (2017);[8] A. Feist et al., Ultrafast Transmission Electron Microscopy Using a Laser-Driven Field Emitter: Femtosecond Resolution with a High Coherence Electron Beam, Ultramicroscopy 176, 63 (2017);[9] A. Feist, et al., Quantum Coherent Optical Phase Modulation in an Ultrafast Transmission Electron Microscope, Nature 521, 200 (2015);

[0010] M. Guide et al., Ultrafast Low-Energy Electron Diffraction in Transmission Resolves Poly- mer / Graphene Superstructure Dynamics, Science 345, 200 (2014);

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[0012] S. Vogelgesang et al., Phase Ordering of Charge Density Waves Traced by Ultrafast Low- Energy Electron Diffraction, Nat. Phys. 14, 174 (2018);

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[0024] Jan Gerrit Horstmann "Ultrafast Probing and Coherent Vibrational Control of a Surface Structural Phase Transition" Dissertation, Georg-August-Universitat Goettingen, Goettingen, 2021.Low energy electron microscopy (LEEM) is an analytical electron microscopy technique for investigating a sample surface (see e. g. [1] to [4]), which employs low-energy electron scattering off the surface to obtain real-space and / or diffraction information on surface structures and / or surface substances with resolutions down to 2 nm (see e. g. [5], [6],

[0020] and

[0021] ). Various contrast mechanisms are available which include different methods tailored to specific structural and electronic observables, like e. g. bright field imaging providing contrast for variations in backscattering diffraction efficiency, or defocus phase contrast visualizing step edges, or dark-field imaging providing for high contrast in mapping different structural domains. Other variations include LEEM potentiometry, micro- and nanobeam diffraction, or electron energy-loss spectroscopy. Further strengths ofLEEM are its intrinsic suitability to in-situ conditions, compatibility with standard surface preparation techniques, capability of implementation at high pressures and temperatures, and capability of characterizing sample changes, e.g., during growth, surface chemistry and catalysis

[0023] , Generally, a great variability of the incident electron energy and beam properties, as well as the possible choice of contrast apertures, render LEEM an important tool to understand surface structures and domain textures.LEEM has been extended to time-resolved investigations (Ultrafast Low-Energy Electron Microscopy, ULEEM). Conventional ULEEM is based e. g. on an optical-pump / electron-probe scheme, in which the momentary state of a temporarily evolving system is captured by short pulses of electrons. Time-dependent changes of a structure are induced by a short pump pulse exciting the sample, and images are recorded as a function of a controlled time delay between the laser-pump and electron-probe beams. Operating in a stroboscopic fashion, ULEEM in particular allows to track those aspects of dynamical processes which progress in a repeatable manner. Advantageously, ULEEM opens up unprecedented insights into spatiotemporal dynamics at surfaces. For example, ULEEM contributes direct structural information on the nanometer scale with the intrinsic advantages of a full-field imaging method. Moreover, its flexibility to easily switch between microdiffraction, dark-field imaging, phase contrast, and several other variants, equips it with numerous alternatives to gather precise structural information from resolving atomic-scale orientations and symmetries in diffraction to the mapping of mesoscale textures. Conventional ULEEM schemes are discussed e. g. in [7] and

[0024] , According to [7], a planar photocathode is employed as an electron source, which has a substantial disadvantage in terms of limited spatial coherence, possibly longer electron pulse durations, and lower position stability of the electron source.For improving the spatial coherence, employing a tip-shaped electrode is proposed in

[0024] , as it is known e. g. in time resolved transmission electron microscopy (Ultrafast TEM, see [8], [9],

[0014] ,

[0015] ) or time resolved low energy electron diffraction investigations (Ultrafast LEED, see

[0010] to

[0012] ). With more details, the electron gun employed in

[0024] is a tungsten needle emitter driven by means of two-photon photoemission.As a disadvantage, the ULEEM scheme of

[0024] only allows for limited control over the temporal structure of the electron beam. In particular, it necessarily requires a fs laser as an optical source for driving the two-photon photoemission. This prevents arbitrary time-structuring of the electron beam and due to the nonlinear nature of the emission process the generation of longer electron pulses (i.e. with picosecond, nanosecond or microsecond duration) strongly increases the thermalload on the tip emitter and may lead to its destruction or quick degradation over time. Furthermore, it has been found in practice that the imaging quality, the energy width of the beam and the reliability of the source can be impaired when using non-linear photoemission from needle emitters.Objective of the inventionIt is an objective of the invention to provide an improved electron microscopy apparatus for time resolved low energy electron microscopy investigations of a sample and an improved electron microscopy method of investigating a sample by time resolved low energy electron microscopy, being capable of avoiding limitations and disadvantages of conventional techniques. In particular, the time resolved low energy electron microscopy is to be provided with variable configurations of the microscopy setup, reduced complexity of the microscopy setup, improved spectral resolution, improved imaging quality and / or reduced instabilities and noise effects. Furthermore, the time resolved low energy electron microscopy is to be provided with new configuration variants and / or with an extended range of applications.Summary of the inventionThese objectives are solved by an electron microscopy apparatus and / or an electron microscopy method comprising the features of the independent claims. Advantageous embodiments and applications of the invention are defined in the dependent claims.According to a first general aspect of the invention, the above objective is solved by an electron microscopy apparatus, being configured for a time resolved low energy electron microscopy investigation of a sample (ULEEM apparatus). The ULEEM apparatus comprises an electron source device having a tip-shaped photo-emitter source, which is arranged for radiation-induced emission of source electrons (photo electrons) towards the sample, a radiation source device being arranged for creating emitter excitation radiation with an emitter excitation waveform and for irradiating the tip-shaped photo-emitter source (cathode) with the emitter excitation radiation, a sample excitation device being arranged for applying a sample excitation to the sample, and a detector device being arranged for collecting sample electrons emerging at the sample in response to an interaction of the source electrons with the sample, wherein the electron microscopy apparatus is configured for the time-resolved investigation of the sample based on a synchronization of the sample excitation applied to the sample with the source electrons received by the sample. According to the invention, the tip-shaped photo-emitter source is configured for creating thesource electrons by an emission process in which the number of emitted electrons is linear with respect to the irradiated power of the emitter excitation radiation on the tip-shaped photo-emitter source (linear photoemission) in response to the irradiation with the emitter excitation radiation. Accordingly, a temporal modulation of the intensity of the emitter excitation radiation, e.g., laser intensity, received by the tip-shaped photo-emitter source, thus also leads to a temporal modulation of the electric current of emitted source electrons. The tip-shaped photo-emitter source may be called linear photo-emitter source.According to a second general aspect of the invention, the above objective is solved by an electron microscopy method of investigating a sample by time resolved low energy electron microscopy (ULEEM method). The ULEEM method comprises the steps of irradiating a tip-shaped photoemitter source with emitter excitation radiation being created with a radiation source device and having an emitter excitation waveform, radiation-induced emission of source electrons from the photo-emitter source towards the sample in response to irradiating the photo-emitter source, and collecting sample electrons emerging at the sample in response to an interaction of the source electrons with the sample with a detector device, wherein the time-resolved investigation of the sample is based on a synchronization of the sample excitation applied to the sample with the source electrons received by the sample. According to the invention, the source electrons are created by an emission process in which the number of emitted electrons is linear with respect to the irradiated power of the emitter excitation radiation on the tip-shaped photo-emitter source in response to irradiating the photo-emitter source. Preferably, the inventive ULEEM method or an embodiment thereof is executed with an ULEEM apparatus according to the first general aspect of the invention or an embodiment thereof.Advantageously, with the inventive provision of a photo-emitter source with linear photoemission (linear photo-emitter source), the application of the ULEEM technique is substantially improved. Emitter excitation radiation can be created with any selectable time structure (i.e., temporal shape, temporal pattern of field amplitude maxima and / or minima). The radiation source device is no longer restricted to a fs laser. Other pulsed or even cw laser types can be employed as outlined below. Thus, new configuration variants of the ULEEM apparatus are available. The linear photoemission improves the capability of the electron source device to provide a predetermined modification of longitudinal electron density within the electron beam with increased precision and variability and with the capability of setting freely selectable temporal patterns of the electron beam.Preferably, an electron beam path from the tip-shaped photo-emitter source to the sample is configured such that the time structure, i.e. the repetition rate and temporal pulse structure, of the pulsed electron beam waveform imprinted to the source electrons by the emitter excitation waveform is kept unchanged from the emission at the tip-shaped photo-emitter source until the irradiation on the sample. A specific temporal pattern (provided by field amplitude maxima and / or minima) of the source electrons received by the sample closely resembles to the temporal pattern of the source electrons when emitted from the photo-emitter source. Accordingly, the temporal pattern of the source electrons received by the sample is equal to or includes subtle in- tra-pulse changes of the electron pules compared with the temporal pattern of the emitted source electrons, while a makro structure of the temporal pattern as used for the time resolved electron microscopy investigation of the sample is not changed.The electron beam path from the tip-shaped photo-emitter source to the sample is free of a temporal modulation of the time structure of the source electrons, so that changes of the waveform are excluded or minimized to a degree neglectable for the time resolved electron microscopy investigation of the sample. However, the electron beam path may include static electro-optical elements, like elements for static recompression compressing and / or dispersion compensating elements, which create exclusively static electric and / or magnetic fields during the ULEEM operation and which in particular may change a shape of field amplitude maxima and / or minima of the source electrons without changing a mutual temporal relation between pulses thereof. Changes of the shape of field amplitude maxima and / or minima of the source electrons due to static electro-optical elements may be applied within individual electron pulses, preferably on a sub-picosecond timescale, but may occur on up to 10 picoseconds. Furthermore, the electron beam path from the tip-shaped photo-emitter source to the sample preferably is made as short and as undisturbing as possible, so that changes of the waveform are excluded or minimized.Furthermore, the inventors have found, that the noise of the fs laser is amplified by the non-linear photoemission employed in prior art, e. g. according to

[0024] , This can be avoided with the invention as noise of the radiation source device is no longer amplified with higher orders, when exciting the electron emission. With an improved imaging quality, new ULEEM applications are obtained by implementing new imaging modes with time-resolution. As a further advantage of the invention, the inventors have found that the linear photo-emitter source allows the creation of the electrons with a reduced energy width.Generally, the low energy electron microscopy is adapted for investigating the sample with source electrons having a low kinetic energy when interacting with the sample, i. e. the source electrons arriving at the sample have a kinetic energy (relative to the potential of the sample) in an energy range such that the source electrons are capable of directly interacting with the surface of the sample, in particular with sample states, that can be directly excited by the received source electrons. An imaging contrast is determined by at least one of different mechanisms at the surface, such as inelastic and / or elastic scattering which depends e.g., on the surface structural composition. Depending on the electron energy, the interaction with the surface potential field or the occupation of the unoccupied states in the material band structure can be a dominant contrast mechanism as well.The energy of the source electrons with respect to the sample is defined by the difference in electric potential between the tip-shaped photo-emitter source and the sample. Preferably, this difference is in a range of -500 V to 500 V, e. g. in a range from - 200 V to + 200 V. The electron energy preferably can be changed continuously, assuming even 0 V relative energy (so that the source electrons are reflected exactly at the sample surface). Optionally, an energy range around zero, e. g. - 1 V to + 1 V, can be excluded.The source electrons are created by inducing photoemission at the tip-shaped photo-emitter source irradiated with the emitter excitation radiation. The material of the tip-shaped photo-emitter source is selected such that linear photoemission is obtained, i. e. the number of created source electrons is proportional to the intensity of the emitter excitation radiation.The photo-emitter source has a tip shape, i. e. source electrons are emitted from a section of the photo-emitter source restricted to a spot, e. g. from a tip of a needle or cone of source material. Preferably, the tip or otherwise restricted emitter section has a cross-sectional dimension below 1 pm, particularly preferred below 100 nm. Advantageously, the tip-shaped photo-emitter source facilitates the generation of a high-coherence electron beam (see e. g.

[0013] ). Advantageously, employing the tip-shaped photo-emitter source provides for electron pulses focused down to e. g. 20 nm diameter, with e. g. 0.6 eV energy width and a pulse duration at the sample down to 200 fs (see

[0018] ).The high coherence of the generated electron beam enables applications in nanoscale diffraction using a small electron focus (e.g. analyzing small crystals), or applications using phase-contrast imaging or holography. Advantageously, the invention combines diffraction and / or imaging modeswith time resolution, e. g. femtosecond time resolution. A particular advantage is that high-coher- ence electron beams with a time structure can be generated, wherein the time structure can be selected in dependency on the application requirements of the ULEEM apparatus. The time structure may comprise e. g. femtosecond peaks at a high repetition rate, or waveforms, such as sine waves. The emitter excitation may be created even using a continuous-wave laser combined with an emitter excitation modulation as outlined below.Based on the invention, more complex schemes of electron beam manipulation can be introduced, such as compression, streaking and / or manipulation of electron pulses by an electromagnetic optic (i.e. electrostatic mirror, see g. g.

[0016] ) or light-driven electron optics. These enable unprecedented control of the electron pulses and maintain the highest possible temporal resolution at the sample. This is especially crucial since electrons in the LEEM may be decelerated before the sample, leading to a significant temporal spread (of at least several picoseconds), which can be prevented by pre-compression with an electron mirror (e. g. pulsed waveforms having a duration < 100 fs might be feasible).The sample excitation device generally is arranged for creating a sample excitation and applying it to the sample, wherein the sample excitation may comprise e.g., an electromagnetic field with a predetermined time structure, i. e. it has a predetermined sample excitation waveform varying as a function of time. The excitation field is capable of interacting with the sample so that sample states, like electronic states and / or vibrational states of the sample, are excited. The sample excitation is applied e. g. as at least one of a radiation field, an electrical current and a magnetic field.The sample excitation applied to the sample is synchronized with the source electrons received by the sample, i. e. the time structure of the sample excitation and the time structure of the source electrons in front of the sample have a predetermined temporal relationship relative to each other. Preferably, the source electrons and the sample excitation have periodic time structures, e. g., with equal or multiples of the periodicity and variable phase difference. Alternatively, the source electrons and the sample excitation may have a non-periodic time structure with a predetermined pattern of field amplitude maxima and minima.The ULEEM apparatus is configured for relaying sample electrons emerging at the sample in response to an interaction of the source electrons with the sample to the detector device. Depending on the imaging mode applied, the sample electrons can comprise electrons created by the sample in response to the interaction of the source electrons with the sample (e. g. secondaryelectrons) or source electrons reflected and / or scattered at the sample. Accordingly, the term "emerging at the sample" can include that the sample electrons after the sample are identical to the electrons before the sample, except that they have been scattered and / or reflected at, above and / or in the sample.Preferably, the source electrons reaching the sample have a predetermined time structure, i. e. they have a predetermined source electron waveform varying as a function of time. The time structure of the source electrons can be created by various modulation mechanisms being provided by or with the radiation source device and acting on the emitter excitation radiation, as described with reference to preferred embodiments of the invention below.Generally, the electron microscopy apparatus is configured for a time-resolved investigation of the sample, wherein the time-resolved investigation comprises e. g., an imaging of a surface section of the sample or a probing of sample features at at least one certain location at the sample surface in dependency on time, e. g. in dependency on a time delay after sample excitation. Time resolution of the sample investigation is obtained from the temporal synchronization of the sample excitation with the source electrons, in particular the source electrons received by the sample. Accordingly, detector signals of the detector device, like image data or probing data, can be collected as a function of time, in particular as a function of the time delay between the sample excitation waveform and the source electron waveform. Advantageously, a temporal resolution on the picosecond and femtosecond timescale can be obtained.As a further advantage, the inventive ULEEM apparatus is compatible with existing conventional LEEMs, resulting in a design that can be retrofitted to those instruments. In particular, a modular electron-optical setting of LEEM can be provided, thus allowing for a simple switching between a conventional continuous-beam electron gun, e.g., a cold field emission gun, and a laser-triggered gun. This scheme will greatly ease alignment of the microscope and will facilitate the introduction of the electron microscopy apparatus in routine investigations.Advantageously, the invention can be implemented with various geometries of the photo-emitter source. Preferably, the photo-emitter source may comprise at least one of a Schottky-type emitter (in particular a tip-shaped cathode combined with a suppressor electrode and an extractor electrode), a cold-field-type emitter (in particular a tip-shaped cathode combined with an extractor electrode, without a suppressor electrode) and a thermionic emitter (in particular a tip-shapedcathode combined with a Wehnelt cylinder electrode). Advantageously, the cold-field-type emitter features the smallest electron source size with high degree-of-coherence and high brightness of the photo-emitted emitted electron beam, and the Schottky-type emitter achieves a slightly larger source size and lower coherence while operating more stable under relaxed vacuum conditions (IO-9mbar). The thermionic-type emitter features significantly larger source size (>1 pm) and higher beam currents, but can be used in lower quality vacuum conditions (10-6-10-7mbar). A further advantage of the cold-field-type emitter and Schottky-type emitter is their overall robustness and capability for extended operation over several months and years, while the thermionic emitter if subjected to a degradation can be more easily exchanged due to the relaxed vacuum conditions. In principle, if operated in photoemission mode, the cold-field-type emitter is expected to produce the narrowest energy distribution of electrons if a thermal broadening is dominating the emission spectra.Optionally, the photo-emitter source may be combined with at least one additional magnetic and / or electric focussing electrode.Furthermore, a plurality of materials is available which can provide the photo-emitter source, in particular according to each of the above emitter types as outlined with the following preferred embodiments. According to a first variant, the photo-emitter source may comprise a tungsten tip having a (100) crystal facet covered by a zirconium-oxide thin film. This material combination preferably is used for providing a Schottky-type emitter. Advantageously, tungsten has high stability under laser-irradiation.Alternatively, according to a second variant, the photo-emitter source may comprise a composite tip made from a material having an electron work function equal to or below the electron work function of Lanthanum Hexaboride, LaBs, or Cerium Hexaboride, CeBg. In particular, the photoemitter may consist of LaBs or CeBg.As a further alternative, the photo-emitter source may comprise a metal tip with a crystal tip facet having a reduced electron work function compared with the remaining tip material. In particular, the photo-emitter source may comprise a pure metal tip, e. g. being made of W, e. g. W(310), Mo, Re, Ir, Ta, He, Pt, or Ni, or the photo-emitter source may comprise a transition metal carbide tip, in particular being made of HfC, ZrC, NbC, TaC, TiC, or VC. One of the pure metals preferably is used for providing a cold field -type emitter. With a further alternative variant, the photo-emittersource may comprise a carbon-cone emitter or a single carbon nanotube tip. The various materials feature individual advantages: Single-crystal metals are in particular suited as cold field emitters because they can be prepared with small radius of curvature and exhibit good electrical conductivity. Transition metal carbide emitters promise higher beam currents with slightly larger emitter size. Carbon-cone and carbon-nanotube have been shown to emit electron beams from down to few-atom source sizes and thus high brightness and high degree-of-coherence.According to a further preferred embodiment of the ULEEM apparatus, the radiation source device may be configured for creating the emitter excitation waveform as a time-modulated waveform. In terms of the ULEEM method, the emitter excitation waveform, i. e. a radiation light field, is created as a time-modulated waveform with a varying amplitude. Particularly preferred, the emitter excitation waveform is a periodic sequence of radiation pulses or a digital on / off sequence of radiation sections. Advantageously, with this embodiment, the linear photoemission of the source electrons with a predetermined source electron waveform is directly determined by the emitter excitation waveform as it is created by the radiation source device (or radiation source device combined with modulation). Thus, the complexity of the ULEEM apparatus configuration can be reduced. The radiation source device, like a laser source, creates a light field with the emitter excitation waveform, e. g. by a pulsed operation of the radiation source device, wherein the emitter excitation waveform directly determines the photoemission waveform in a linear relation and a temporal shape and intensity of the emitter excitation is directly imprinted onto the electron beam.With a further advantageous embodiment of the ULEEM apparatus, an emitter excitation modulator device may be arranged between the radiation source device and the photo-emitter source for modulating the emitter excitation radiation created by the radiation source device, in particular as a periodic sequence of radiation pulses or as a digital on / off sequence of radiation sections. In terms of the method, the ULEEM method includes a step of applying a modulation to the emitter excitation radiation, in particular as a periodic sequence of radiation pulses or as a digital on / off sequence of radiation sections, with an emitter excitation modulator device which is arranged between the radiation source device and the photo-emitter source.The emitter excitation modulator device may be an additional component, like e. g. an acousto- optical modulator (AOM), a beam chopper and / or a radiofrequency amplitude modulator (RF optic), in the beam path from the radiation source device to the photo-emitter source. The emitterexcitation modulator device may be used in combination with a modulated radiation source device or with a cw radiation source device. Advantageously, the emitter excitation modulator device offers additional flexibility of setting the emitter excitation waveform independently on the operation requirements of the radiation source device.In particular, with the provision of the emitter excitation modulator device, the radiation source device may be configured for creating a continuous wave (cw) waveform output to which a modulation is applied with the emitter excitation modulator device for creating the emitter excitation waveform exciting the photo-emitter source. The waveform output by the radiation source device may be a continuous wave waveform, i. e. a radiation light field with a time-constant amplitude. Advantageously, with this embodiment, the emitter excitation waveform allows a reduction of the energy width of the source electrons, e. g. compared with a pulsed emitter excitation waveform. As a further advantage, the configuration of the radiation source device may be facilitated.According to a further preferred variant of the invention, the radiation source device comprises a laser source, like e. g. a pulsed laser or a cw laser. Employing the laser source has particular advantages in terms of noise reduction and precision of emitter excitation modulation. Advantageously, a broad range of emitter excitation photon energies (wavelengths) is available. In particular, the radiation source device may be configured for creating the emitter excitation radiation with a wavelength in UV-, VIS, NIR, and / or MIR-wavelengths ranges or correspondingly e.g., in a range from 1 nm to 200 nm or from 200 nm to 1500 nm or from 1500 nm to 16 pm. Practically relevant is in particular especially the range from 400 nm to 800 nm, i.e. in the visible spectrum. Additionally or alternatively, the radiation source device may be configured for creating the emitter excitation radiation with a wavelength of THz, RF, or microwave radiation.According to a particularly preferred embodiment of the invention, the emitter excitation radiation is created such that the waveform of the source electrons has an electron density of less than 100 electrons per pulse in temporal average. Preferably, in terms of the apparatus, the radiation source device may be arranged for creating the emitter excitation radiation such that the waveform of the source electrons has the electron density of less than 100 electrons per pulse in temporal average. In particular, an electron density in temporal average of, e.g., 10 to 100 electrons per pulse, or 1 to 10 electrons per pulse or even less than 1 electron per pulse may be obtained. Advantageously, in particular by reducing the electron density, the pulse duration and energy width of the electron pulses can be reduced, e. g. down to a 200 fs pulse duration and an energy width below 1 eV.Advantageously, multiple variants of creating and applying the sample excitation to the sample are available, which can be selected in dependency on the particular application conditions of the invention, e. g. in dependency on the sample to be investigated. According to a preferred embodiment of the invention, the radiation source device is further arranged for applying the sample excitation to the sample by creating sample excitation radiation with a sample excitation waveform and by irradiating the sample with the sample excitation radiation, wherein the sample excitation waveform has a predetermined temporal relationship relative to the source electrons received by the sample. In terms of the method, the sample excitation is applied as a radiation field to the sample with the same radiation source device, which primarily is arranged for creating the emitter excitation radiation and for irradiating the tip-shaped photo-emitter source with the emitter excitation radiation. Advantageously, the emitter excitation radiation and the sample excitation radiation have an inherent synchronisation, and the complexity of the electron microscopy apparatus is reduced by the double function of the radiation source device.For allowing the time-resolved sample investigation, the sample excitation has a time structure. With employing the emitter excitation radiation, the time structure is obtained with the modulated radiation source device inherently creating the time structure or an emitter excitation modulator device combined with the radiation source device. Advantageously, the emitter excitation radiation may be used directly as provided by the radiation source device or the emitter excitation modulator device, i. e. without further waveform or wavelength variation, for exciting the sample. Alternatively, the emitter excitation radiation may be used indirectly, e. g. by driving an optical process, like an amplification and / or wavelength shifting process, for creating the sample excitation radiation for exciting the sample.According to further or alternative embodiments of the invention, an electrical sample excitation device and / or a sample excitation radiation source may be arranged, in particular in addition to the radiation source device, for applying the sample excitation to the sample. Accordingly, the sample excitation may comprise an electrical sample excitation applied to the sample with an electrical sample excitation device and / or a radiation sample excitation applied to the sample with a sample excitation radiation source. Advantageously, an adaptation of the sample excitation to features of the sample to be investigated is facilitated with these embodiments. Furthermore, another degree of freedom is provided for adjusting the time structure of the sample excitation, independently on the time structure of the emitter excitation radiation.The electrical sample excitation device is configured for applying an electrical current and / or a magnetic field to the sample, for exciting sample states to be investigated. Accordingly, the electrical sample excitation device may comprise a current source and / or a magnetic field coil, optionally integrated in the sample holder. The electrical sample excitation may have particular advantages for executing ultrafast potentiometry imaging, wherein a time-dependent current evolution upon the application of a current pulse e. g. on a semiconductor structure could be measured.The sample excitation radiation source is a radiation source, like e. g. a pulsed laser, which may be provided additionally to the emitter excitation radiation source.According to a further preferred variant of the inventive electron microscopy apparatus, a delay device may be provided, which is configured for adjusting a temporal relationship between the sample excitation and the source electrons received by the sample. For investigating the sample with time resolution, the temporal relationship between the sample excitation and the source electrons received by the sample, preferably the time interval between sample excitation and subsequently arriving electrons, is adjusted with the delay device. The delay device generally is an optical and / or electrical component acting on the sample excitation and / or the source electrons. Preferably, a temporal setting of the sample excitation relative to the source electrons is tuned with the delay device, e. g. by an adjustable optical delay line transmitting the sample excitation and / or a time control of an electrical sample excitation device. The delay device provides particular advantages for the investigation of the sample with time resolution, in particular for investigating repeatable processes at the sample. By periodically changing the temporal relationship between the sample excitation and the source electrons received by the sample, the repeatable processes can be probed at varying times after sample excitation.With another embodiment of the invention, an auxiliary electron source device, in particular comprising a cold field emission gun, may be arranged for an emission of adjustment electrons towards the sample. Advantageously, by creating the adjustment electrons with the auxiliary electron source device and emitting the adjustment electrons towards the sample, advantages for adjusting and / or calibrating the electron microscopy apparatus can be obtained.Brief description of the drawingsFurther advantages and details of the invention are described in the following with reference to the attached drawings, which schematically show in:Figure 1: an electron microscopy apparatus according to a first embodiment of the invention;Figures 2 and 3: further examples of photo-emitter sources;Figure 4: an illustration of the photoemission of source electrons;Figure 5: another variant of the electron microscopy apparatus according to a second embodiment of the invention, including a compression mirror;Figure 6: an illustration of the compression mirror included in the embodiment of Figure 5; andFigure 7: an electron microscopy apparatus according to a fourth embodiment of the invention, including an optically controlled electrical sample excitation.Preferred embodiments of the inventionFeatures of preferred embodiments of the inventive electron microscopy apparatus are described in the following with particular reference to the provision of a tip shaped photo-emitter source of an electron gun, emitter excitation modulation procedures applied for creating an electron beam with an arbitrary modification of longitudinal electron density, sample excitation procedures, and features of the electron optical imaging. The invention preferably is implemented on the basis of a LEEM apparatus as it is known per se, like e. g. an electron microscope of the type "FE-LEEM / PEEM P90 Series" (manufacturer: SPECS Surface Nano Analysis GmbH, Germany). Accordingly, particular details of a conventional LEEM apparatus, the UHV chamber, the frame and major components thereof, the available control schemes thereof, the available schemes of detector signal acquisition and analysis, and the available processes of sample preparation are not described as they are known from prior art.Figures 1, 5 and 7 schematically illustrate basic embodiments of the inventive electron microscopy apparatus 100. It is emphasised that the implementation of the invention is not restricted tothese embodiments, but rather possible with further modifications, in particular in terms of the sample excitation 4. Features disclosed with reference to one of the embodiments can be employed also with the other embodiments. Components with the same reference numeral are provided as described e. g. with reference to Figure 1.The electron microscopy apparatus 100 of Figures 1, 5 and 7 includes an electron source device 10 with a tip shaped photo-emitter source 11, a radiation source device 20 for creating emitter excitation radiation 3 and inducing photoemission of source electrons 2 at the photo-emitter source 11, a sample excitation device 30 for applying a sample excitation 4 to a sample 1, a detector device 40 for collecting sample electrons 5 emerging at the sample 1 in response to an interaction of the source electrons 2 with the sample 1, a sample holder 50 accommodating the sample 1, an electro-optical system 60, in particular including an electro-optical sample prism 62, for electro- optical relaying the source electrons 2 from the electron source device 10 to the sample 1 and directing the sample electrons 5 from the sample 1 to the detector device 40, an optional auxiliary electron source device 80 and a control device 70, being arranged for controlling the components 10 to 80 and / or parts thereof. The control device 70 comprises at least one computer circuit being coupled with the components to be controlled. As in a conventional LEEM microscope, the sample holder 50 can be provided with a tempering device (not shown), for instance with the cryogenic cooling device, for cooling the sample to the temperature of liquid nitrogen.According to the embodiment shown in Figure 1, the electron source device 10 comprises a laser driven gun with a tip shaped photo-emitter source 11 (cathode). The photo-emitter source 11 is arranged e. g. as a cold-field emitter-type assembly, wherein the cathode is combined with an extractor electrode 12. Optionally, a focussing electrode 13 can be provided. In a cold-field emitter mode, the cathode is operated at a cathode voltage UtiPof e. g. -15 kV, and stepwise increasing voltages are applied to the extractor electrode 12 and the optional focussing electrode 13. With the focussing electrode 13, the source electrons can be focussed into an imaging plane of an electron condenser optic 61 of the electro-optical system 60. The photo-emitter source 11 may comprise various pure materials, e.g. W(310), Mo, or carbon. The tip has a cone shape, e. g. with a radius < lOOnm. The photo-emitter source 11 is provided as a part of an electron gun, e. g. as manufactured by York Probe Sources Ltd, United Kingdom or DELONG INSTRUMENTS a. s., Czech Republic.Alternative configurations of the photo-emitter source 11 are shown in Figures 2 and 3. According to Figure 2, a thermionic emitter type assembly is provided, including the tip shaped photo-emitter source 11, a Wehnelt cylinder electrode 14 and an anode 15. The Schottky-type emitter assembly of Figure 3 is similar to the configuration of Figure 1, wherein a suppressor electrode 16 with a suppressor voltage below the voltage cathode voltage UtiPis additionally provided. The photo-emitter source 11 of the Schottky-type emitter comprises e. g. a tungsten tip with a (100) crystal facet covered by a zirconium oxide monolayer. The photo-emitter source configurations of Figures 1 to 3 can be provided with an additional magnetic and / or electric focus electrode unit, like the focussing electrode 13 shown in Figures 1 and 3.The radiation source device 20 comprises for instance a femtosecond laser source, like the laser system PHAROS (manufacturer: Light Conversion, UAB, Lithuania), emitting the emitter excitation radiation 3 as a pulsed laser beam, i. e. a laser beam with a pulsed emitter excitation waveform. For example, the pulsed laser beam has a centre wavelength of 515 nm, a repetition frequency of 300 kHz and a pulse duration of 200 fs. Alternatively, radiation source device 20 may be adapted for emitting the emitter excitation radiation 3 in another wavelength range, e. g. as UV-radiation, light at other optical frequencies, IR radiation, or THz radiation.Optionally, the radiation source device 20 may be combined with an emitter excitation modulator device 21 for modulating the photoemission of the source electrons 2. The emitter excitation modulator device 21 is arranged in the emitter excitation beam path between the radiation source device 20 and the photo-emitter source 11, and it comprises for instance an acousto-opti- cal modulator. By controlling the emitter excitation modulator device 21 with a periodic or nonperiodic waveform, for instance with a radiofrequency (RF) waveform, the amplitude of the laser beam created by the radiation source device 20 may be RF-modulated. With the emitter excitation modulator device 21, the radiation source device 20 may create the emitter excitation waveform as a continuous wave waveform. Alternatively, the emitter excitation modulator device 21 may be arranged for a modulation with a frequency of a microwave field.As shown in Figure 4, the laser-triggered photo-emitter source 11 provides a short electron pulse of source electrons 2 in response to the irradiation with a pulse of the emitter excitation radiation 3 created by the radiation source device 20, optionally in combination with the emitter excitation modulator device 21. The source electrons 2 are passed through the electro-optical system 60 of the ULEEM. Electron pulse dispersion occurs along the subsequent electron beam path, as illustrated in Figure 4. The electro-optical system 60 and / or any compression element includedtherein is adapted for compensating the electron pulse dispersion by electric and / or magnetic fields that are exclusively static during the ULEEM operation.Due to the tip configuration, e. g. with the W(310) tip of the cold-field emitter or with the (100) crystal facet and the covering film at the exposed tip end of the Schottky-type emitter, the photoemitter source 11 has a linear photoemission characteristic. Accordingly, the number of source electrons 2 emitted from the photo-emitter source 11 is proportional to the intensity of the emitter excitation radiation 3. Furthermore, resulting from the tip shape, the source electrons 2 have a high spatial coherence. With the pulsed emitter excitation radiation 3, the beam of source electrons 2 is created with a pulsed electron beam waveform equal to the emitter excitation waveform of the pulsed laser beam. Accordingly, a pulsed beam of source electrons 2 is created with the repetition frequency of e. g. 300 kHz.According to Figure 1, the radiation source device 20 simultaneously is used as the sample excitation device 30. The pulsed laser beam is directed as the sample excitation 4 to the sample 1. Due to a delay stage 23 included in the beam path towards the sample 1, a delay between the sample excitation pulses of the sample excitation 4 and emitter excitation pulses of the emitter excitation radiation 3 (and correspondingly electron pulses created with the photo-emitter source 11) can be adjusted and optionally varied.The electro-optical system 60 comprises the electron condenser optic 61, the electro-optical sample prism 62 and sample optics 63 along the electron beam path from the photo-emitter source 11 to the sample 1. Subsequently, the electro-optical system 60 comprises the sample optics 63, the electro-optical sample prism 62 and projector optics 64 (details see e. g., Figure 5) along the electron beam path from the sample 1 to the detector device 40. The electro-optical system 60 is configured and controlled as it is known from conventional electron microscopes, in particular conventional LEEM microscopes.In operation, an electron pulse of source electrons 2 is created with each photoemission pulse of the emitter excitation radiation 3, resulting from the linear photoemission at the photo-emitter source 11. The electro-optical system 60 directs the source electrons 2 towards the sample 1, where sample electrons 5 are provided by an interaction of the source electrons 2 with the sample 1, e. g. by scattering the source electrons 2 or by creating secondary electrons. The sample electrons 5 emerging at the sample 1 are relayed via the electro-optical sample prism 62 towardsthe detector device 40. The detector device 40 comprises an imaging detector, for instance a TemCam-XF416ES (TVIPS GmbH, Germany).By creating the emitter excitation radiation 3 and the sample excitation 4 with the common radiation source device 20, the pulsed beam of source electrons 2 is inherently synchronised with the sample excitation 4. With the detector device 40, the sample electrons 5 can be detected in dependency on a delay between the pulsed beam of source electrons 2 and the sample excitation 4. By setting and / or varying the delay with the delay stage 23, the detector device 40 provides time resolved images of the sample 1.Figures 5 and 6 illustrate variants of the embodiment of the electron microscopy apparatus 100 of Figure 1, wherein a static electron mirror is used for pulse compression. In particular, the electro- optical system 60 additionally includes an electro-optical compressor prism 65 and a mirror compressor 66. The electro-optical compressor prism 65 is arranged for electro-optical relaying the source electrons 2 from the electron source device 10 to the mirror compressor 66 and from the mirror compressor 66 to the electro-optical sample prism 62. The mirror compressor 66 may be configured as described in

[0019] , and it comprises two pairs of electrodes and a reflector electrode (as shown in Figure 6).The mirror compressor 66 acts as an electron reflector for shortening the pulses of source electrons 2 at the sample position. Furthermore, the mirror compressor 66 is capable of compensating a linear part of chirp accumulated by the electron pulses upon propagation to the sample, thus further improving an efficient compression of the electron pulse deeply into the femtosecond regime, in particular down to 100 to 200 fs.The electron microscopy apparatus 100 of Figure 5 additionally includes an auxiliary electron source device 80, which comprises for instance a cold field electron gun. The auxiliary electron source device 80 may be arranged for imaging static structures, as illustrated by the solid electron beam path from the cold field emission gun through the electro-optical compressor prism 65 and the electro-optical sample prism 62 to the sample 1, scattering from the sample 1, back through electro-optical sample prism 62, towards projector optics 64 and the detector device 40. Alternatively or additionally, the auxiliary electron source device 80 may be arranged for adjustment purposes.As a further optional feature of the invention illustrated with the embodiment of Figure 5, the pulsed laser beam output by the radiation source device 20 is directed via a frequency shifting element 22 for creating the emitter excitation radiation 3 as the sequence of photoemission pulses towards the photo-emitter source 11. Advantageously, the frequency shifting element 22 allows that the emitter excitation and the sample excitation are provided with different wavelengths, so that an adaptation of the emitter excitation to the tip material of the photo-emitter source 11 and the sample excitation to the sample 1 can be optimized. The frequency shifting element 22 comprises e. g. a second harmonic generation (SHG) crystal, made of e. g. Beta barium borate, p- BaBjC .Figure 5 further shows details of adjusting the mutual synchronization of the source electrons 2 received by the sample 1 and the optical sample excitation 4. The pulsed laser beam output by the radiation source device 20 is directed via a first delay stage 23 as the optical sample excitation 4 (sequence of excitation pulses) to the sample 1. The sample excitation 4 is deflected onto the surface of the sample 1 with a time delay At between the emitter excitation 3 (determining the temporal structure of the source electrons 2) and the optical sample excitation 4, which is set by the first delay stage 23.The first delay stage 23 comprises e. g. a reflector path with changeable path length. At least one of the reflectors of the first delay stage 23 may be moved with a drive device (not shown) for setting the time delay At via the path length. The drive device may be coupled with the control device 70 for setting a predetermined path length (time delay At) and / or for controlling a periodic path length change.Besides the pulse duration, a further concern for the practically resolvable contrast and achievable spatial resolution is the total brightness of the source electron pulses that can be achieved. The duty cycle in time-resolved experiments is limited by the fact, that the pulse repetition time, i.e., the time between consecutive pulses, cannot exceed the required relaxation time of the surface. Although relatively high repetition rates of e. g. 100 kHz to 400 kHz can be employed, the average electron dose is a limiting factor for contrast. Therefore, to maximize the detector signal by contrast enhancement, the short-pulsed nature of the electron beam can be used to establish a type of laser phase plate. To this end, an additional laser pulse may be spatially and temporally overlapped in vacuum with part of the sample electron beam in a back focal plane of the projec-tor optics 64 (see

[0022] ). Those parts of the diffraction pattern traversing the light field will experience a ponderomotive phase shift proportional to the light intensity (see

[0023] ), which can be used to create a Zernike phase contrast.Thus, as a further modification compared with the other embodiments shown in the Figures, a portion of the pulsed laser beam output by the radiation source device 20 is directed as a beam of electron phase manipulation pulses 6 towards the beam path of the sample electrons 5 emerging at the sample 1. The electron phase manipulation pulses 6 are provided for contrast enhancement in the detector signal. For setting a time delay of the electron phase manipulation pulses 6 relative to the emitter excitation 3 and the optical sample excitation 4, a second delay stage 24 is provided.In addition or as an alternative to the laser phase plate created by the electron phase manipulation pulses 6, the contrast of the detector signal can be further improved by providing a tailored dark-field mask 65 in the projector optics 64. The dark-field mask 65 transmits a series of spots of interest, while blocking others.In operation, the electro-optical compressor prism 65 is switched on for incoupling the pulsed beam of source electrons 2 derived from the photo-emitter source 11. In a stroboscopic imaging mode, time-dependent changes to the sample 1 are induced by a short sample pump pulse exciting the sample 1 (e. g., red laser beam), and images are recorded with the detector device 40 as a function of the controlled time delay At between the laser-pump beam and the probe beam of source electrons 2. Alternatively to the time-resolved investigation, the electro-optical compressor prism 65 is switched for relaying electrons from the auxiliary electron source device 80 to the sample 1, thus providing the function of a LEEM for the imaging of static sample features.While the above embodiments of the invention are configured for a direct optical sample excitation, the electron microscopy apparatus 100 according to a modified embodiment of the invention includes an electrical sample excitation, as shown in Figure 7. The electrical sample excitation may be preferred if electrical surface states of the sample 1 are to be excited directly via at least one contact electrode. Additionally or as an alternative to the electrical sample excitation, a magnetic sample excitation can be provided in an analogue manner as described with reference to Figure 7, wherein the at least one contact electrode is replaced by at least one magnetic excitation coil.According to the embodiment shown in Figure 7, the sample excitation device 30 is provided by the radiation source device 20, a radiation sensor 31, like a photodiode, and a waveform generator 32. In operation, the radiation source device 20 creates the pulsed emitter excitation radiation 3. A portion of the emitter excitation radiation 3 is deflected via a beam splitter to the radiation sensor 31. A pulsed electrical signal created by the radiation sensor 31 is used as an input of the waveform generator 32. The waveform generator 32 creates the electrical sample excitation 4 as a sequence of current pulses with free selectable waveforms. The current pulses are applied to the sample 1 via the at least one contact electrode (not shown). Synchronization of the pulses of source electrons 2 with the current pulses of the electrical sample excitation 4 is obtained as the radiation source device 20 is the common source of both pulse sequences. A delay between both pulse sequences can be set and / or varied by introducing an optical delay unit (not shown) in the beam path of the emitter excitation 3 and / or by controlling the waveform generator 32.As an alternative to the embodiment of Figure 7, the sample excitation device 30 may be provided by an additional, separate sample excitation radiation source (not shown), like a fs laser system, which is synchronized with the radiation source device 20.The electron microscopy apparatus 100 according to the above embodiments can be employed for executing various imaging modes with time resolution. For example, bright field imaging provides contrast for variations in backscattering diffraction efficiency, while defocus phase contrast visualizes step edges, and dark-field imaging provides for high contrast in mapping different structural domains. As a further imaging mode, LEEM potentiometry gives access to local variations in electrostatic potential, e.g. in a structure carrying electrical currents.Further applications include microbeam diffraction, in which diffraction patterns from micrometer or sub-micrometer regions of the sample yield quantitative information on local orientation and structure. Moreover, the possibility to conduct electron energy-loss spectroscopy within LEEM represents an additional dimension for the exploration of electronic density of states and electronic occupation, as does the intrinsic possibility to conduct photoemission electron microscopy (PEEM) within a LEEM. Generally, the great variability of the incident electron energy and beam properties, as well as the possible choice of contrast apertures, render the inventive ULEEM apparatus an important tool for time resolved investigations of surface structures and domain textures. The ULEEM apparatus can also be implemented at high pressures and temperatures, and it can been used to characterize morphological changes, e.g. during growth, surface chemistry and catalysis, e. g. on a timescale of seconds.Advantageously, various sample features and / or processes can be investigated with the inventive electron microscopy apparatus, like the formation and ordering of metastable states at the sample surface, structural phase transitions at the sample surface, light-induced structural phase tran- sitions, and / or the generation and propagation of collective modes at the sample surface.The features of the invention disclosed in the above description, the drawings and the claims can be of significance both individually as well as in combination or sub-combination for the realization of the invention in its various embodiments. The invention is not restricted to the preferred embodiments described above. Rather a plurality of variants and derivatives is possible which also use the inventive concept and therefore fall within the scope of protection. In addition, the invention also claims protection for the subject and features of the subclaims independently of the features and claims to which they refer.

Claims

Claims1. Electron microscopy apparatus (100), being configured for a time resolved low energy electron microscopy investigation of a sample (1), comprising- an electron source device (10) having a tip-shaped photo-emitter source (11), which is arranged for radiation-induced emission of source electrons (2) towards the sample (1),- a radiation source device (20) being arranged for creating emitter excitation radiation (3) with an emitter excitation waveform and for irradiating the tip-shaped photo-emitter source (11) with the emitter excitation radiation (3),- a sample excitation device (30) being arranged for applying a sample excitation (4) to the sample (1), and- a detector device (40) being arranged for collecting sample electrons emerging at the sample (1) in response to an interaction of the source electrons (2) with the sample (1), wherein- the electron microscopy apparatus (100) is configured for the time-resolved investigation of the sample (1) based on a synchronization of the sample excitation (4) applied to the sample (1) with the source electrons (2) received by the sample (1), characterized in that- the tip-shaped photo-emitter source (11) is configured for creating the source electrons (2) in response to the irradiation with the emitter excitation radiation (3) by an emission process in which the number of emitted electrons is linear with respect to the irradiated power of the emitter excitation radiation (3) on the tip-shaped photo-emitter source (11).

2. Electron microscopy apparatus according to claim 1, wherein- the photo-emitter source (11) comprises at least one of a Schottky-type emitter, a cold-field- type emitter and a thermionic emitter.

3. Electron microscopy apparatus according to one of the foregoing claims, wherein the photo-emitter source (11) comprises- a tungsten tip having a (100) crystal facet covered by a zirconium-oxide thin film, or- a composite tip made from a material having an electron work function equal to or below the electron work function of Lanthanum Hexaboride, LaBs, or Cerium Hexaboride, CeBs, or- a metal tip with a crystal tip facet having a reduced electron work function compared with the remaining tip material, or- a pure metal tip, in particular being made of W, Mo, Re, Ir, Ta, He, Pt, or Ni, or- a transition metal carbide tip, in particular being made of HfC, ZrC, NbC, TaC, TiC, or VC, or- carbon-cone emitter or a single carbon nanotube tip.

4. Electron microscopy apparatus according to one of the foregoing claims,- the radiation source device (20) is configured for creating the emitter excitation waveform as a time-modulated waveform, in particular as a periodic sequence of radiation pulses or as a digital on / off sequence of radiation sections.

5. Electron microscopy apparatus according to one of the foregoing claims, wherein- an emitter excitation modulator device (21) is arranged between the radiation source device (20) and the photo-emitter source (11) for modulating the emitter excitation radiation (3) created by the radiation source device (20), in particular as a periodic sequence of radiation pulses or as a digital on / off sequence of radiation sections.

6. Electron microscopy apparatus according to one of the foregoing claims, wherein- the radiation source device (20) comprises a laser source, and / or- the radiation source device (20) is configured for creating the emitter excitation radiation (3) with a wavelength in a range from 1 nm to 200 nm or from 200 nm to 1500 nm or from 1500 nm to 16 pm, and / or- the radiation source device (20) is configured for creating the emitter excitation radiation (3) with a wavelength of THz, RF, or microwave radiation.

7. Electron microscopy apparatus according to one of the foregoing claims, wherein- the radiation source device (20) is arranged for creating the emitter excitation radiation (3) such that the waveform of the source electrons (2) has an electron density of less than 100 electrons per pulse in temporal average.

8. Electron microscopy apparatus according to one of the foregoing claims, wherein- the radiation source device (20) is further arranged for applying the sample excitation (4) to the sample (1) by creating sample excitation radiation with a sample excitation waveform and by irradiating the sample (1) with the sample excitation radiation, wherein the sample excitation waveform has a predetermined temporal relationship relative to the source electrons (2) received by the sample (1).

9. Electron microscopy apparatus according to one of the foregoing claims, further comprising- an electrical sample excitation device (30) and / or a sample excitation radiation source being arranged for applying the sample excitation (4) to the sample (1).

10. Electron microscopy apparatus according to one of the foregoing claims, further comprising- a delay device (22) being configured for adjusting a temporal relationship between the sample excitation (4) and the source electrons (2) received by the sample (1).

11. Electron microscopy method of investigating a sample (1) by time resolved low energy electron microscopy, comprising the steps of- irradiating a tip-shaped photo-emitter source (11) with emitter excitation radiation (3) being created with a radiation source device (20) and having an emitter excitation waveform,- radiation-induced emission of source electrons (2) from the photo-emitter source (11) towards the sample (1) in response to irradiating the photo-emitter source (11),- applying a sample excitation (4) to the sample (1) with a sample excitation device (30), and- collecting sample electrons emerging at the sample (1) in response to an interaction of the source electrons (2) with the sample (1) with a detector device (40), wherein- the time-resolved investigation of the sample (1) is based on a synchronization of the sample excitation (4) applied to the sample (1) with the source electrons (2) received by the sample (1), characterized in that- the source electrons (2) are created in response to irradiating the photo-emitter source (11) by an emission process in which the number of emitted electrons is linear with respect to the irradiated power of the emitter excitation radiation (3) on the tip-shaped photo-emitter source (11) .

12. Method according to claim 11, wherein- the photo-emitter source (11) comprises at least one of a Schottky-type emitter, a cold-field- type emitter and a thermionic emitter.

13. Method according to one of the claims 11 to 12, wherein- the emitter excitation waveform is created as a time-modulated waveform, in particular as a periodic sequence of radiation pulses or as a digital on / off sequence of radiation sections.

14. Method according to one of the claims 11 to 13, comprising- applying a modulation to the emitter excitation radiation (3), in particular as a periodic sequence of radiation pulses or as a digital on / off sequence of radiation sections, with an emitter excitation modulator device (21) which is arranged between the radiation source device (20) and the photoemitter source (11).

15. Method according to one of the claims 11 to 14, wherein- the emitter excitation radiation (3) is created with a laser source, and / or- the emitter excitation radiation (3) is created with a wavelength in a range from 1 nm to 200 nm or from 200 nm to 1500 nm or from 1500 nm to 16 pm, and / or- the emitter excitation radiation (3) is created with a wavelength of THz, RF, or microwave radiation.

16. Method according to one of the claims 11 to 15, wherein- the emitter excitation radiation (3) is created such that the waveform of the source electrons (2) is set with an electron density of less than 100 electrons per pulse in temporal average.

17. Method according to one of the claims 11 to 16, wherein- the sample excitation (4) is applied to the sample (1) with the radiation source device (20) by creating sample excitation radiation with a sample excitation waveform and by irradiating the sample (1) with the sample excitation radiation, wherein the sample excitation waveform has a predetermined temporal relationship relative to the source electrons (2) received by the sample (1).

18. Method according to one of the claims 11 to 17, wherein- the sample excitation (4) comprises an electrical sample excitation applied to the sample (1) with an electrical sample excitation device (30) and / or a radiation sample excitation applied to the sample (1) with a sample excitation radiation source.

19. Method according to one of the claims 11 to 18, further comprising- adjusting the temporal relationship between the sample excitation (4) and the source electrons (2) received by the sample (1) with a delay device (23).

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