Atomic scale simulation method for ultraviolet effect

Abstract

The invention discloses an atomic scale simulation method for an ultraviolet effect, relates to the technical field of an ultraviolet radiation simulation method, and aims to solve the problem that a fracture result of a molecular chain on the surface of a polymer is influenced as a spectrum of a part of wavebands cannot be effectively simulated during ground ultraviolet radiation simulation at present. The method is technically characterized by comprising the following steps: step 1, constructing a chemical bond model of a polymer and the number of bonds, determining irradiation time and time step length, and performing ultraviolet irradiation of different wavelengths on the chemical bond model; 2, the absorption probability of the continuous energy spectrum is calculated according to the absorption cross section, whether photons are absorbed or not is judged, and the wavelength of the photons is selected randomly; 3, photon energy Euv and quantum yield are calculated based on the selected photon wavelength, and the chemical bond state is judged; 4, calculating the lifetime of an excited state, and updating the states of all keys; and step 5, repeating the steps 2-4 until the number of iterations or the irradiation time is reached.

Description

Technical Field

[0001] This invention relates to the field of ultraviolet irradiation simulation methods, and more specifically, to an atomic-scale simulation method for ultraviolet effects. Background Technology

[0002] Currently, research on the space environment is fundamental to ensuring the long-term operation of spacecraft and has gradually entered the realm of competition among major powers. Space environment effects encompass a variety of factors, such as the atomic oxygen effect, proton / electron irradiation, magnetic field environment, and ultraviolet (UV) irradiation. While UV irradiation with wavelengths of 115-400 nm accounts for only about 1% of the total solar irradiance, it has a significant impact. Under UV irradiation, the chemical bonds of polymers are broken, leading to a decrease in molecular weight, material decomposition, fragmentation, discoloration, and a reduction in elasticity and tensile strength. Research on the effects of UV irradiation has become an important research direction.

[0003] Currently, space ultraviolet irradiation experiments involve establishing solar simulators on the ground to simulate the space ultraviolet irradiation process under vacuum. NASA in the US, ESA in Europe, and the Beijing Satellite Equipment Research Institute in China have all established equipment for space ultraviolet irradiation experiments on spacecraft materials, obtaining data on the breaking of different chemical bonds under ultraviolet irradiation. This provides important support for studying the effects of ultraviolet irradiation and the coupling effects of ultraviolet radiation with other space environments. However, current experimental equipment and systems used for ultraviolet irradiation have the following main drawbacks: 1) It is difficult to simulate ultraviolet irradiation in the 110-400 nm wavelength range; 2) The intensity of mercury lamp sources is uncontrollable and cannot reproduce the continuous spectral characteristics of solar ultraviolet radiation. Xenon lamp sources have extremely low energy in the 115-200 nm band, while ultraviolet irradiation in this band is key to causing the breakage of polymer surface molecular chains; 3) Each space irradiation experiment typically lasts 2-3 months, resulting in high time and financial costs. Summary of the Invention

[0004] The technical problem to be solved by this invention is:

[0005] Currently, some wavelengths of the spectrum cannot be effectively simulated in ground-based ultraviolet irradiation simulations, which in turn affects the results of polymer surface molecular chain breakage.

[0006] Therefore, this invention provides an atomic-scale simulation method for ultraviolet effects.

[0007] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0008] This invention provides an atomic-scale simulation method for ultraviolet effects, comprising the following steps:

[0009] Step 1: Construct a chemical bond model of the polymer and the number of bonds, determine the irradiation time and time step, and irradiate the chemical bond model with ultraviolet light at different wavelengths;

[0010] Step 2: Calculate the absorption probability of the continuous energy spectrum based on the absorption cross section, determine whether a photon is absorbed, and randomly select a photon wavelength;

[0011] Step 3: Calculate the photon energy E based on the selected photon wavelength. uv And quantum yield, to determine the state of chemical bonds;

[0012] Step 4: Calculate the excited state lifetime and update the state of all bonds;

[0013] Step 5: Repeat steps 2-4 until the required number of iterations or irradiation time is reached.

[0014] Furthermore, step 2 includes the following steps:

[0015] Step 2.1: Calculate the ultraviolet spectral flux based on the distance z from the polymer surface. ;

[0016] Step 2.2: Calculate the photon absorption probability P within one step based on the absorption cross-section. abs ;

[0017] Step 2.3: Assign a random number r to the chemical bond. The random number r is uniformly distributed in the range [0,1). If If the photon is absorbed, it is absorbed; otherwise, no photon is absorbed. Then a random number is selected again until the photon is absorbed.

[0018] Step 2.4: Randomly select the photon wavelength based on the distribution of the probability f of chemical bonds absorbing photons per unit time, where f = .

[0019] Furthermore, in step 2.1, the ultraviolet spectral flux... The calculation formula is:

[0020]

[0021] Where α(λ) is the absorption coefficient of the irradiated material to ultraviolet light, λ is the wavelength, Φ(λ,0) is the initial photon flux, i.e. the ultraviolet spectral flux when z is 0, Φ(λ,0)=W·λ / hc, W is the ultraviolet irradiance at wavelength λ, h is Planck's constant, and c is the speed of light.

[0022] Furthermore, in step 2.2, the photon absorption probability P within a step size is calculated based on the absorption cross-section. abs Specifically:

[0023]

[0024] Where σ(λ) is the absorption cross section and Δt is the time step.

[0025] Further, step 2.4 includes the following process: the probability f of a chemical bond absorbing a photon per unit time is... Integrate the wavelength to construct a wavelength-probability linear curve; assign a random number u to the chemical bond, which is a random number uniformly distributed in [0,1). Take the wavelength corresponding to the probability equal to the random number u or the first probability greater than the random number u, and select it as the photon wavelength λ.

[0026] Furthermore, step 3 includes the following steps:

[0027] Step 3.1: Calculate the photon energy E based on the selected photon wavelength. uv ;

[0028] Step 3.2: Assign a random number R to the chemical bond. The random number R is uniformly distributed in [0,1). If R < φ(λ), and E uv ≥E bond Then the chemical bond breaks, where E bond Let λ be the bond energy and φ(λ) be the quantum yield.

[0029] Step 3.3: If R ≥ φ(λ), or E uv <E bond If the chemical bond is not broken, depending on the bonding type and the wavelength / energy of the absorbed photon, the electron enters a singlet or triplet excited state with a certain probability.

[0030] Furthermore, the photon energy E mentioned in step 3.1 uv The calculation formula is:

[0031] 。

[0032] Furthermore, the process of constructing the quantum yield φ(λ) described in step 3.2 is as follows:

[0033] The relative quantum yield function is determined using the probability density function of a normal distribution: φ l = normpdf(λ, μ,σ l ); where μ is the peak wavelength and σl is the standard deviation;

[0034] A scaling factor k is used to adjust this value to be within a reasonable range relative to the quantum yield. The scaling factor k is determined by the ratio of the highest quantum yield value at the existing wavelength μ to the peak value of the relative quantum yield.

[0035] The quantum yield φ(λ) is calculated based on the adjusted quantum yield function.

[0036] Furthermore, step 4 includes the following steps:

[0037] Step 4.1: After obtaining the electronic excitation state, determine the excited state bond order and bond energy based on the specific bonding type and excitation state;

[0038] Step 4.2: Obtain the average lifetime of the excited state according to the bonding type and the excited state, and randomly generate the duration of the excited state based on the average lifetime to update the state of all bonds.

[0039] Furthermore, the distribution function of the average lifetime of the excited state mentioned in step 4.2 is:

[0040] p(t) = (1 / τ) * exp(-t / τ)

[0041] in The mean lifetime of the excited state is given by t, which is time.

[0042] Compared with the prior art, the beneficial effects of the present invention are:

[0043] 1) This invention has the capability to conduct ultraviolet irradiation simulation experiments within the wavelength range of 110-400 nm;

[0044] 2) The use of continuous ultraviolet spectroscopy indirectly based on the absorption cross-sections of different bonds improves accuracy compared to ground-based experiments;

[0045] 3) Compared to 2-3 months for ground-based tests, simulations only require a few hours with the same energy and irradiation time, and the calculation speed is faster;

[0046] 4) The method of this invention can be coupled with other environmental effects, further breaking through the limitations of ground-based experimental simulation. Attached Figure Description

[0047] Figure 1 The flowchart of the atomic-scale simulation method for ultraviolet effects in this embodiment of the invention is as follows. Figure 1 ;

[0048] Figure 2 This is a flowchart illustrating the simulation of the effect of ultraviolet photons on polymers in an embodiment of the present invention.

[0049] Figure 3 The flowchart of the atomic-scale simulation method for ultraviolet effects in this embodiment of the invention is as follows. Figure 2 ;

[0050] Figure 4 This is a graph showing the probability results of CN bond breakage or excitation in an embodiment of the present invention. Detailed Implementation

[0051] To enable those skilled in the art to better understand the present invention, exemplary embodiments or examples of the present invention will be described below in conjunction with the accompanying drawings. Obviously, the described embodiments or examples are merely some, not all, of the embodiments or examples of the present invention. All other embodiments or examples obtained by those skilled in the art based on the embodiments or examples of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0052] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0053] Specific Implementation Plan 1: Combining Figures 1 to 3 As shown, this invention provides an atomic-scale simulation method for ultraviolet effects, comprising the following steps:

[0054] Step 1: Construct a chemical bond model of the polymer and the number of bonds, determine the irradiation time and time step, and irradiate the chemical bond model with ultraviolet light at different wavelengths;

[0055] Step 2: Calculate the absorption probability of the continuous energy spectrum based on the absorption cross section, determine whether a photon is absorbed, and randomly select a photon wavelength;

[0056] Step 3: Calculate the photon energy E based on the selected photon wavelength. uv And quantum yield, to determine the state of chemical bonds;

[0057] Step 4: Calculate the excited state lifetime and update the state of all bonds;

[0058] Step 5: Repeat steps 2-4 until the required number of iterations or irradiation time is reached.

[0059] like Figure 2As shown, the effect of ultraviolet light on molecular materials in this invention includes the following processes: First, photons propagate within the material, and after partial absorption, the flux gradually decreases with the depth of incidence. Second, photons of different energies encounter electrons, especially bonding electrons, and have a certain probability of being absorbed. Statistically, this is equivalent to each bond having a certain absorption cross-section σ that dominates photon absorption. The size of the absorption cross-section σ(λ) is related to the photon wavelength. After absorbing photons, bonding electrons are excited. When the absorbed photon energy is greater than the bond energy, there is a certain probability of bond breakage. The probability of a single photon causing breakage is the quantum yield φ. The quantum yield φ(λ) is related to the wavelength and dominates bond breakage. It reaches its peak when the photon energy equals the bond energy. When the photon energy is greater than the bond energy, the yield decreases. The quantum yield is related to the wavelength. The relationship between long and short wavelengths can be approximated by a Gaussian distribution. If a bonding electron absorbs a photon and becomes excited but does not break the bond, the energy of the absorbed photon will result in different transitions and different bond order changes. For short-wavelength photons, the energy is higher, which may cause the electron to enter the antibonding orbital and thus lower the bond order. For long-wavelength photons, the energy is lower, and the electron is not enough to produce a significant energy level transition, so the bond order remains unchanged. When a bonding electron enters an excited state, it can remain in the excited state for a certain period of time, generally in the range of ps to ns, depending on the excited state of the bond and the electron. For example, the triplet excited state can reach the s-level lifetime. In principle, the excited electron can absorb a photon again to be excited to a higher energy level or break the bond, but the probability is very small, and secondary absorption is ignored in the calculation.

[0060] Specific implementation plan two: Step 2 includes the following steps:

[0061] Step 2.1: Define the distance z from the polymer surface, and calculate the ultraviolet spectral flux based on the distance z from the polymer surface. ;

[0062] Step 2.2: Calculate the photon absorption probability P within one step based on the absorption cross-section. abs ;

[0063] Step 2.3: Assign a random number r to the chemical bond. The random number r is uniformly distributed in [0,1) (each number has an equal probability of being drawn). If If the photon is absorbed, then it is absorbed; otherwise, it is not absorbed, and the bond remains unchanged, ending this step of the calculation. Select a new random number until the chemical bond absorbs a photon;

[0064] Step 2.4: Randomly select the photon wavelength based on the distribution of the probability f of chemical bonds absorbing photons per unit time, where f = This implementation plan is otherwise the same as Implementation Plan 1.

[0065] Specific implementation plan three: Ultraviolet spectral flux in step 2.1 The calculation formula is:

[0066]

[0067] Where α(λ) is the absorption coefficient of the irradiated material to ultraviolet light, λ is the wavelength, Φ(λ,0) is the initial photon flux, i.e., the ultraviolet spectral flux when z is 0, Φ(λ,0) = W·λ / hc, W is the ultraviolet irradiance at wavelength λ, h is Planck's constant, and c is the speed of light. Other aspects of this implementation scheme are the same as in specific implementation scheme two.

[0068] Specific implementation plan four: Step 2.2 describes calculating the photon absorption probability P within a step size based on the absorption cross-section. abs Specifically:

[0069]

[0070] Where σ(λ) is the absorption cross section and Δt is the time step. This implementation scheme is otherwise the same as specific implementation scheme three.

[0071] Specific implementation plan five: Step 2.4 includes the following process: the probability f of a chemical bond absorbing a photon per unit time is... Integrate over the wavelength to construct a wavelength-probability linear curve; assign a random number u to the chemical bond, where u is a uniformly distributed random number in [0,1). Select the wavelength λ corresponding to the probability equal to or greater than the probability of the random number u. The rest of this implementation scheme is the same as in specific implementation scheme four.

[0072] Specific implementation plan six: Step 3 includes the following steps:

[0073] Step 3.1: Calculate the photon energy E based on the selected photon wavelength. uv ;

[0074] Step 3.2: Assign a random number R to the chemical bond. The random number R is uniformly distributed in [0,1). If R < φ(λ), and E uv ≥E bond Then the chemical bond breaks, where E bond Let λ be the bond energy and φ(λ) be the quantum yield.

[0075] Step 3.3: If R ≥ φ(λ), or E uv <E bond If the chemical bond remains unbroken, the electron will enter a singlet or triplet excited state with a certain probability, depending on the bonding type and the wavelength / energy of the absorbed photon. This implementation scheme is otherwise identical to specific implementation scheme five.

[0076] Specific implementation plan seven: The photon energy E mentioned in step 3.1 uv The calculation formula is:

[0077] 。

[0078] This implementation plan is otherwise the same as Specific Implementation Plan Six.

[0079] Specific implementation plan eight: The process of constructing the quantum yield φ(λ) mentioned in step 3.2 is as follows:

[0080] First, determine the peak wavelength μ (in nanometers), at which the quantum yield is highest.

[0081] Secondly, determine the standard deviation σ. l (Unit: nanometers) represents the width of the quantum yield distribution. The quantum yield approximates a normal distribution with respect to wavelength, and the standard deviation of the normal distribution for different bonds can be directly specified; this is an empirical parameter. It is used to measure the dispersion of the quantum yield variation with wavelength.

[0082] The relative quantum yield function is determined using the probability density function of a normal distribution: φ l = normpdf(λ,μ,σ l ); where μ is the peak wavelength, σ l Standard deviation;

[0083] A scaling factor k is used to adjust this value to be within a reasonable range relative to the quantum yield. The scaling factor k is determined by the ratio of the highest quantum yield value at the existing wavelength μ to the peak value of the relative quantum yield.

[0084] The quantum yield φ(λ) is calculated based on the adjusted quantum yield function. This implementation scheme is otherwise identical to specific implementation scheme seven.

[0085] Specific implementation plan nine: Step 4 includes the following steps:

[0086] Step 4.1: After obtaining the electronic excitation state, determine the excited state bond order and bond energy based on the specific bonding type and excitation state;

[0087] Step 4.2: Obtain the average lifetime of the excited state based on the bonding type and excitation state, and randomly generate the duration of the excited state based on the average lifetime to update the state of all bonds. This implementation scheme is otherwise the same as specific implementation scheme eight.

[0088] Specific Implementation Scheme Ten: The distribution function of the average lifetime of the excited state mentioned in step 4.2 is:

[0089] p(t) = (1 / τ) * exp(-t / τ)

[0090] in The mean lifetime of the excited state is given by t, which is time.

[0091] This table shows the average lifetime of the excited state obtained through data collection.

[0092] The method for randomly generating the duration of the excited state based on the average lifetime in this implementation scheme is as follows:

[0093] The `exprnd` function is used to generate random numbers from an exponential distribution with an average lifetime of 1e-9 seconds. The probability density function of the exponential distribution is p(t) = (1 / τ) * exp(-t / τ). Accumulating p(t) yields F(t) = 1 - exp(-t / τ) = U, where F(t) takes values ​​from 0 to 1; and τ is the average lifetime (taken as 1e-9 seconds).

[0094] The process of generating a random lifetime is as follows: exprnd(1e-9) generates a random number, which is taken from the above exponential distribution.

[0095] For example, running exprnd(1e-9) once might result in 0.7e-9 seconds. This value is randomly selected and the specific value is determined by the random number generator.

[0096] The principle of generating random numbers with exponential distribution: Let U be a random number uniformly distributed in [0,1). Then, by transforming: t = -τ* ln(U), we can obtain random numbers with exponential distribution.

[0097] Therefore, the value of 0.7e-9 seconds is obtained through the above transformation. For example, if U=0.5, then t = -1e-9 *ln(0.5) ≈ 0.693e-9 seconds.

[0098] Therefore, 0.7e-9 seconds is an example; in practice, different random lifetimes will be generated for each run, but the average of these lifetimes will be close to 1e-9 seconds. This implementation scheme is otherwise the same as specific implementation scheme nine.

[0099] The beneficial effects of the present invention will be described below with reference to specific embodiments.

[0100] Example 1

[0101] First, this embodiment establishes a table containing the bond energy of different chemical bonds, the bond order changes and corresponding bond energies of excited states at different wavelengths, the absorption cross-sections of different bonds at different wavelengths, the wavelength at which the quantum yield peaks, the irradiance at different wavelengths, and the absorption coefficient.

[0102] Enter the type of chemical bond and the time step, for example: select CN bond and time step 10h.

[0103] The flux at zero thickness was calculated using formula (1). Irradiation was performed at 5 solar constants. The calculated flux was multiplied by 5 to obtain the photon flux at the material surface at 5 solar constants. The spectral absorption probability was calculated using formula (4). In this invention, 100,000 bonds were distributed on the surface for ultraviolet irradiation calculations.

[0104] Then, a random number is selected and compared with the absorption probability to determine whether the photon is absorbed. If absorbed, a wavelength is randomly selected according to σ(λ)·Φ(λ).

[0105] Finally, the absorbed photon energy E is calculated. uv The result is compared with the bond energy of the set bond. The quantum yield is calculated, and a random number is generated and compared with the quantum yield to determine whether the bond is broken or in an excited state. The state may enter a singlet excited state S1 or a triplet excited state T1, or remain unchanged. Figure 4 As shown, update the lifetime of the bond and the corresponding bond energy; the calculation ends when the calculation time is reached.

[0106] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. An atomic-scale simulation method for ultraviolet effects, characterized in that, Includes the following steps: Step 1: Construct a chemical bond model of the polymer and determine the number of bonds, and then irradiate the chemical bond model with ultraviolet light at different wavelengths; Step 2: Calculate the absorption probability of the continuous energy spectrum based on the absorption cross section, determine whether a photon is absorbed, and randomly select a photon wavelength; Step 3: Calculate the photon energy E based on the selected photon wavelength. uv And quantum yield, to determine the state of chemical bonds; Step 4: Calculate the excited state lifetime and update the state of all bonds; Step 5: Repeat steps 2-4 until the required number of iterations or irradiation time is reached.

2. The atomic-scale simulation method for ultraviolet effects according to claim 1, characterized in that, Step 2 includes the following steps: Step 2.1: Calculate the ultraviolet spectral flux based on the distance z from the polymer surface. ; Step 2.2: Calculate the photon absorption probability P within one step based on the absorption cross-section. abs ; Step 2.3: Assign a random number r to the chemical bond. The random number r is uniformly distributed in the range [0,1). If If the photon is absorbed, it is absorbed; otherwise, no photon is absorbed. Then a random number is selected again until the photon is absorbed. Step 2.4: Randomly select the photon wavelength based on the distribution of the probability f of chemical bonds absorbing photons per unit time, where f = .

3. The atomic-scale simulation method for ultraviolet effects according to claim 2, characterized in that, Step 2.1 Ultraviolet spectral flux The calculation formula is: Where α(λ) is the absorption coefficient of the irradiated material to ultraviolet light, λ is the wavelength, Φ(λ,0) is the initial photon flux, i.e. the ultraviolet spectral flux when z is 0, Φ(λ,0)=W·λ / hc, W is the ultraviolet irradiance at wavelength λ, h is Planck's constant, and c is the speed of light.

4. The atomic-scale simulation method for ultraviolet effects according to claim 3, characterized in that, Step 2.2 involves calculating the photon absorption probability P within a step size based on the absorption cross-section. abs Specifically: Where σ(λ) is the absorption cross section and Δt is the time step.

5. The atomic-scale simulation method for ultraviolet effects according to claim 4, characterized in that, Step 2.4 includes the following process: the probability f of a chemical bond absorbing a photon per unit time is... Integrate the wavelength to construct a wavelength-probability linear curve; assign a random number u to the chemical bond, which is a random number uniformly distributed in [0,1). Take the wavelength corresponding to the probability equal to the random number u or the first probability greater than the random number u, and select it as the photon wavelength λ.

6. The atomic-scale simulation method for ultraviolet effects according to claim 5, characterized in that, Step 3 includes the following steps: Step 3.1: Calculate the photon energy E based on the selected photon wavelength. uv ; Step 3.2: Assign a random number R to the chemical bond. The random number R is uniformly distributed in [0,1). If R < φ(λ), and E uv ≥E bond Then the chemical bond breaks, where E bond Let λ be the bond energy and φ(λ) be the quantum yield. Step 3.3: If R ≥ φ(λ), or E uv <E bond If the chemical bond is not broken, depending on the bonding type and the wavelength / energy of the absorbed photon, the electron enters a singlet or triplet excited state with a certain probability.

7. The atomic-scale simulation method for ultraviolet effects according to claim 6, characterized in that, The photon energy E mentioned in step 3.1 uv The calculation formula is: 。 8. The atomic-scale simulation method for ultraviolet effects according to claim 7, characterized in that, The process of constructing the quantum yield φ(λ) in step 3.2 is as follows: The relative quantum yield function is determined using the probability density function of a normal distribution: φ l = normpdf(λ, μ, σ l ); where μ is the peak wavelength and σl is the standard deviation; A scaling factor k is used to adjust this value to be within a reasonable range relative to the quantum yield. The scaling factor k is determined by the ratio of the highest quantum yield value at the existing wavelength μ to the peak value of the relative quantum yield. The quantum yield φ(λ) is calculated based on the adjusted quantum yield function.

9. The atomic-scale simulation method for ultraviolet effects according to claim 8, characterized in that, Step 4 includes the following steps: Step 4.1: After obtaining the electronic excitation state, determine the excited state bond order and bond energy based on the specific bonding type and excitation state; Step 4.2: Obtain the average lifetime of the excited state according to the bonding type and the excited state, and randomly generate the duration of the excited state based on the average lifetime to update the state of all bonds.

10. The atomic-scale simulation method for ultraviolet effects according to claim 9, characterized in that, The distribution function of the average lifetime of the excited state mentioned in step 4.2 is: p(t) = (1 / τ) * exp(-t / τ) in The mean lifetime of the excited state is given by t, which is time.

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