A laser induced cold electron neutral post ionization source

Abstract

The application relates to the technical field of mass spectrometry, in particular to a laser-induced cold electron neutral post-ionization source. When an ion beam or laser sputters a sample surface, neutral plume is generated. Based on the principle of photoelectric effect, a high-brightness photoelectron is generated by irradiating the cathode surface with a light source. The photoelectron obtains sufficient energy under the acceleration of an electric field, can ionize the neutral gas phase plume, and achieves the purpose of improving sensitivity and eliminating ionization matrix effect. By adjusting the extraction electric field, the distribution of electrons and the extraction efficiency of ions can be optimized, and better ionization efficiency and ion utilization rate can be obtained. Compared with hot electrons, photoelectrons can work under different gas pressures and have the characteristics of cold electron ionization.

Description

Technical Field

[0001] This invention belongs to the field of secondary ion mass spectrometry technology, specifically relating to a laser-induced cold electron neutral post-ionization source. Background Technology

[0002] To address the issues of low sensitivity and matrix effects in secondary ion mass spectrometry (SIMS), post-ionization [16-18] is employed to directly ionize neutral species generated by sputtering, significantly improving ion yield. Furthermore, the difference in sputtering yield between different elements is much smaller than the difference in secondary ion yield, thus avoiding the matrix effect of secondary ion emission. Post-ionization separates the molecular sputtering and ionization processes, making the generation and extraction of secondary ions independent of the sputtering surface. This reduces the requirements for target surface morphology during sample preparation, fundamentally enhancing the stability of the mass spectrometer. In the products of primary ion sputtering, secondary ions (positive and negative) obtained through ionization account for only 0.001–10%. Detecting the 90%–99.9999% neutral particles within these ions has led to the development of secondary neutral mass spectrometry (SNMS), a technique specifically designed for neutral particle ionization. Compared to SIMS, SNMS offers advantages such as high sensitivity, accurate quantitative analysis, and minimal matrix effects.

[0003] In recent years, SNMS has rapidly become a research hotspot for new surface analysis after SIMS. The core of SNMS technology is to ionize these neutral particles using effective post-ionization techniques. Currently, the main post-ionization techniques used in SNMS include "electron gas", "electron beam", and "laser beam". Among them, laser ionization is 1 to 2 orders of magnitude more efficient than the former two and is easy to apply to different SIMS instruments, thus attracting widespread attention. Commonly used laser ionization sources include high-intensity lasers and vacuum ultraviolet lasers (VUV). Since it is difficult to obtain high-intensity VUV light sources, the mainstream post-ionization method is currently femtosecond laser post-ionization technology [18-24]. The pulse width of femtosecond lasers is 10 -15 The power peak intensity is on the order of s, and after focusing, it can easily reach 10. 14 ~10 15 W / cm 2 While saturation ionization can be achieved for most elements, the overall utilization and ionization efficiency of the neutral plume are not high, despite the high ionization probability within the laser focal point during femtosecond strong-field ionization. This is because there are spatial and temporal matching issues between post-laser ionization and the neutral plume, leading to insufficient and fluctuating ionization efficiency for some components. These limitations restrict the sensitivity and quantitative accuracy of secondary ion neutral post-ionization mass spectrometry. Summary of the Invention

[0004] This invention is based on the photoelectric effect, utilizing a light source to irradiate the cathode surface and generate high-brightness photoelectrons. These photoelectrons, accelerated by an electric field, gain sufficient energy to ionize a neutral gas plume, thereby improving sensitivity and eliminating the ionization matrix effect. By adjusting the extraction electric field, the distribution of electrons and the extraction efficiency of ions can be optimized, resulting in better ionization efficiency and ion utilization.

[0005] The specific content includes:

[0006] A laser-induced cold electron neutral post-ionization source comprises a light source, a sample sputtering source, a sample target, a cathode extraction cone, an electrostatic lens, and a vacuum chamber. The cathode extraction cone, sample target, electrostatic lens, and focusing lens are all housed within the vacuum chamber. The light source, sample sputtering source, and mass spectrometer are connected to the vacuum chamber via a window flange, a sputtering source flange, and a mass spectrometer interface flange, respectively. The vacuum chamber maintains a vacuum environment. The cathode extraction cone is positioned directly above the sample target, with its central axis perpendicular to the upper surface of the sample target. A laser or ion beam emitted from the sample sputtering source sputters a neutral gas plume onto the upper surface of the sample target. The central axis of the sample sputtering source forms an acute angle with the normal to the sample target. A voltage V1 is applied to the sample target, cathode extraction cone, and electrostatic lens, respectively. Under the combined action of voltages V1, V2, and V3, an extraction electric field is formed between the sample target and the cathode extraction cone. The light beam emitted by the light source, focused by an optical lens, passes through a window flange and irradiates the surface of the cathode extraction cone, generating photoelectrons. Accelerated by the extraction electric field, these photoelectrons move towards the sample target and collide with and ionize molecules in the neutral gas plume. The ionized ions move towards the cathode extraction cone under the influence of the extraction electric field and enter the mass spectrometer. By adjusting the extraction electric field between the sample target and the cathode extraction cone, the distribution of photoelectrons and the ion extraction efficiency can be optimized, resulting in better ionization efficiency and ion utilization. The light source includes, but is not limited to, one or more of vacuum ultraviolet lamps, lasers, and synchrotron radiation sources. The area of ​​the cathode extraction cone irradiated by the light source is composed of photocathode material, or has a thin layer of photocathode material coated on its surface. The product ions, accelerated by the extraction electric field, are focused by an electrostatic lens and a focusing lens, and finally enter the mass spectrometer to measure the ion mass-to-charge ratio. Attached Figure Description

[0007] Figure 1 It is a laser-induced cold electron neutralization source. Detailed Implementation

[0008] A laser-induced cold electron neutralization post-ionization source comprises a 266nm laser source 9, an ion gun 8, a sample target 1, and a cathode extraction cone 5. The area of ​​the cathode extraction cone 5 irradiated by the light source 1 is composed of a photocathode material Pb. The cathode extraction cone 5 is positioned directly above the sample target 1, with its central axis perpendicular to the sample target 1. The ion beam emitted by the ion gun 8 sputters a neutral gas plume 3 onto the surface of the sample target 1. The central axis of the ion gun 8 forms a 60° angle with the normal to the sample target 1. Voltages of 200V, -300V, and -1500V are applied to the sample target 1, the cathode extraction cone 5, and the electrostatic lens 6, respectively, forming an extraction electric field between the sample target 1 and the cathode extraction cone 5. The laser beam emitted by the laser 9 irradiates the surface of the cathode extraction cone 5, generating photoelectrons 4 which, accelerated by the electric field, move towards the sample target 1 and collide with the neutral gas plume 3, resulting in ionization. By adjusting the electric field between the sample target 1 and the cathode extraction cone 5, the electron distribution and ion extraction efficiency can be optimized, achieving better ionization efficiency and ion utilization. The product ions are accelerated by the extraction electric field and focused by the electrostatic lens 6 and the focusing lens 11, and finally enter the time-of-flight mass spectrometer to measure the ion mass-to-charge ratio.

Claims

1. A laser-induced cold electron neutralization post-ionization source, comprising a light source (9), a sample sputtering source (8), a sample target (1), a cathode extraction cone (5), an electrostatic lens (6), and a vacuum chamber (2); the cathode extraction cone (5), the sample target (1), the electrostatic lens (6), and the focusing lens (11) are all placed inside the vacuum chamber (2); the light source (9), the sample sputtering source (8), and the mass spectrometer (7) are connected to the vacuum chamber (2) through a window flange (13), a sputtering source flange (12), and a mass spectrometer interface flange (14), respectively; the inside of the vacuum chamber (2) is a vacuum environment; The cathode extraction cone (5) is placed directly above the sample target (1), and the central axis of the cathode extraction cone (5) is perpendicular to the upper surface of the sample target (1). The laser or ion beam emitted by the sample sputtering source (8) sputters a neutral gas plume (3) on the upper surface of the sample target (1). The central axis of the sample sputtering source (8) forms an acute angle with the normal of the sample target (1). Voltages V1, V2 and V3 are applied to the sample target (1), the cathode extraction cone (5) and the electrostatic lens (6), respectively. Under the combined action of voltages V1, V2 and V3, an extraction electric field is formed between the sample target (1) and the cathode extraction cone (5). The beam emitted by the light source (9) is focused by the optical lens (10) and passes through the window flange (13), irradiating the surface of the cathode extraction cone (5) to generate photoelectrons (4). The photoelectrons (4) move toward the sample target (1) under the acceleration of the extraction electric field and collide with the molecules of the neutral gas plume (3) to ionize.

2. The ionization source according to claim 1, characterized in that: By adjusting the extraction electric field between the sample target (1) and the cathode extraction cone (5), the distribution of photoelectrons (4) and the extraction efficiency of ions can be optimized, resulting in better ionization efficiency and ion utilization.

3. The ionization source according to claim 1, characterized in that: The light source (9) includes, but is not limited to, one or more of the following: vacuum ultraviolet lamp, laser, and synchrotron radiation light source.

4. The ionization source according to claim 1, characterized in that: The area of ​​the cathode extraction cone (5) irradiated by the light source (9) is made of photocathode material or has a thin layer of photocathode material coated on its surface.

5. The ionization source according to claim 1, characterized in that: The product ions are accelerated by the extraction electric field and focused by the electrostatic lens (6) and the focusing lens (11), and finally enter the mass spectrometer (7) to measure the ion mass-to-charge ratio.

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