Measuring photoionization cross-sections of excited atomic states

A new method of photoionization cross-section measurement based on the observation of saturation in the ion yield as a function of radiation intensity has been proposed. The photoionization cross-sections for the 6²P_1/2 and 6²P_3/2 levels of Rb atoms of the fundamental and second harmonics of ruby laser radiation are measured, with a tunable pulsed dye laser used for excitation. The following values of cross-sections are obtained: (1.7±0.4)·10⁻¹⁷ cm² and (1.5±0.4)·10⁻¹⁷ cm² for the levels 6²P_3/2 and 6²P_1/2, respectively, ionized by radiation of v₂=14403 cm⁻¹, and (1.9±0.5)·10⁻¹⁷ cm² for the 6²P_3/2 level ionized by v₂=28806 cm⁻¹ radiation.     

Theoretical studies on the photoionization cross-sections of solid silver

An alternative expression for photoionization cross-section of atoms or molecules and a dielectric influence function （DIF） in a high-density system proposed recently are used to study the photoionization cross-sections of solid silver. It is suggested that a density turning point （DTP） of a photoionized system may be viewed as the critical point where the photoionization properties of atoms in a real system may have a notable change. The results show that the present theoretical photoionization cross-sections are in good agreement with the experimental results of a silver crystal both in structure and in magnitude.     

Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV

The results of calculations of photoelectric cross-sections for the Kα lines of magnesium at 1254 eV and of aluminum at 1487 eV are presented. All of the subshell cross-sections are given for Z values up to 96. The calculations were carried out relativistically using the single-potential Hartree-Slater atomic model.     

Molecular photoionization cross-sections and their dependence on excitation energy

A general formula for the photoionization cross-section of a linear molecule in terms of atomic subshell cross-sections and diffraction effects is given. Example calculations are carried out for line intensities of CO and N₂ as a function of incident photon energy from threshold up to the soft X-ray region. The importance of associating CNDO coefficients to orthogonal Slater AO's is pointed out. General rules for dependence of photoionization cross-sections on excitation energy are discussed.     

Relative partial photoionization cross-sections and photoelectron branching ratios of ethylene

Relative partial photoionization cross-sections and photoelectron branching ratios of the valence bands (~10–25 eV binding energy) of ethylene are reported over the photon energy ranges 18–100 eV and 21–100 eV, respectively. The four lowest ionization energy bands (1b⁻¹_1u, 1b⁻¹_1g, 3a⁻¹_g, and 1b⁻¹_2u) show monotonic cross-section decreases with photon energy from 33 eV, the 1b⁻¹_1u CC π band showing the least rapid decline. In contrast, the 2b⁻¹_3u and 2a⁻¹_g bands show almost constant cross-sections up to ~50 eV photon energy, followed by similar, although slower, monotonic decreases. This is attributed to the substantial carbon 2s character of the 2b_3u and 2a_g orbitais. The cross-section behaviour of all bands is interpreted with the aid of SCF-LCAO-MO calculations on the neutral molecule using the Gaussian-80 series of programs.     

Positron lifetime measurements in aliphatic hydrocarbons

The positron lifetimes and relative intensities were measured in highly purified liquid and solid hydrocarbons. It was found that most of the lifetime parameters changed at the melting point of the samples and their values were lower in the solid than in the liquid phase. A study of hexane isomers was also performed.     

Partial photoionization cross-sections and branching ratios of CO from 750 to 304 Å

The absolute values of the partial photoionization cross-sections and branching ratios for producing carbon monoxide ions in their X²Σ⁺, A²Π, B²Σ⁺ and ²Σ⁺(σ2s) states have been obtained as a function of wavelength from the A²Π ionization threshold to 304 Å. Results have been obtained within autoionizing resonances as well as within the photoionization continuum.     

Subshell photoionization cross-sections,electron mean free paths and quantitative X-ray photoelectron spectroscopy

Subshell photoionization cross-sections (SPC) for elements of atomic number in the range 3?, z ? 82 have been determined from measurement of relative photoelectron intensities at a photon energy of 1486.6 eV (Al Kα). A correction procedure has been developed which permits such determinations even when sample surfaces are not atomically clean. The results obtained suggest that Scofield's SPC calculations [2] are reasonably reliable for 1s, 2p and 3d subshells but that the present, experimentally derived data are to be preferred for quantitative use. As a result of this work, a rapid method of quantitative surface-constituent analysis, accurate to ± 20%, has also been developed, which is expected to be of value particularly in industrial situations. A semi-empirical method of extracting electron mean free paths from measurements of relative photoelectron intensities is also illustrated.     

Ultrasonic acceleration of iodination of unactivated aliphatic hydrocarbons

Iodination of unactivated aliphatic hydrocarbons with iodoform (CHI3) and solid NaOH were greatly accelerated under ultrasonic irradiation. The mechanism of the sonochemical acceleration was studied.     

The determination of subshell photoionization cross-sections in some solids and gases using photoelectron spectroscopy

Relative subshell photoionization cross-sections have been measured directly for solid samples of Sodium Fluoride and Sodium Chloride by the technique     

Total and partial photoionization cross-sections of O2 from 100 to 800 Å

The branching ratios and absolute values of the total and partial photoionization cross-sections for producing O₂⁺ in its various electronic states are given as a function of wavelength in the range 100–800 Å. Cross-sections for dissociative ionization are given based on the premise that some of the bound ionic states predissociate. Photoelectron spectroscopy and absorption techniques utilizing a double ionization chamber were used to obtain the data.     

1，4-二甲氧基-2，5-二乙烯基苯系列衍生物的双光子吸收截面

在HF水平上，利用解析响应函数方法，研究了1，4_二甲氧基_2，5_二乙烯基苯系列衍生物 分子的单光子和双光子吸收特性. 实验测量了反，反_2，5_双(4′_N,N_二丁胺基苯乙烯)_1 ，4_二甲氧基苯分子的单光子和双光子荧光谱. 研究结果表明该系列分子具有较强的双光子 吸收特性. 在低能量范围内，对于D_π-A型分子，分子的单光子吸收强度和双光子吸收截面 最大值都发生在分子的第一激发态. 对于D_π_D型分子，单光子吸收强度最大值出现在分子 的第一激发态，而最大双光子吸收值则对应于分子的第二激发态. 分子的单和双光子吸收强 度和分子官能团的电性有关. 对于由该类π中心部分构成的分子，其对称型不一定比不对称型更有利于提高双光子吸收截面. 分子基态与电荷转移态的电荷转移过程定性地解释了双光 子聚合反应的聚合机理. 关键词： 双光子吸收 响应函数方法 非线性光学     

Determination of charm production cross-sections for neutrinos and antineutrinos

It is shown that existing data for production rates of opposite-sign dileptons by neutrinos and antineutrinos are mutually consistent when acceptance corrections are calculated using a standard four-quark parton model, and a flat fragmentation function for charmed quarks. Together with a corrected cross-section, model calculations which adequately describe the data are presented.     

Influence of the photoionization process on the fragmentation of laser desorbed polycyclic aromatic hydrocarbons

Characterization of polycyclic aromatic hydrocarbons (PAHs) samples has been performed by laser desorption combined with multi-photon ionization technique using two different geometries of the ionization laser beam. This comparative study evidences the strong influence of ionization laser fluence on PAH fragmentation. Through a ∼10³ enlargement of the ionization probe volume and 10⁴ reduction of laser fluence over previous studies, fragment free mass spectra are obtained with higher sensitivity and selectivity. The ability to measure fragment free PAH mass spectra is a very important step in the end goal of measuring complex unknown mixtures of PAH desorbed from solid surface such as soot samples.     

Comparison of calculated photoionization cross-sections of diamond and silicon in the approximations of plane wave and orthogonalized plane wave

In the approximation of the orthogonalized plane wave the shape of X-ray photoelectron spectra of the valence band in diamond and silicon has been calculated. It is shown that consideration of orthogonality terms influences greatly the value of the ratio between photoionization cross-sections of s- and p-electrons. X-ray emission and photoelectron spectroscopy allow us to define the density of states for silicon valence electrons.X-ray photoelectron spectroscopy is at present widely used for the study of the electronic structure of solids. The photoelectron spectra of crystals clearly reveal all the variations in the density of states. However the curves for the photoelectron energy distribution may be different from the calculated density of states for valence electrons. This indicates the importance of the transition probability for the formation of a photoelectron spectrum. In refs. 1 and 2 the X-ray photoelectron spectra obtained with high resolution for diamond and silicon crystals are compared with the density of states and the conclusion made that the s-states lie higher than the p-states. Densities of states and X-ray photoelectron spectra for diamond and silicon have been calculated³. The wave functions of the valence electrons were found by the tight-binding method⁴ while plane waves were used as wave functions for the excited electrons. From a theoretical point of view it is more reasonable to use orthogonalized plane waves to describe the electron states in the conduction band. Both the core and valence states are to be orthogonalized.The present work reports calculation of the shape expected for X-ray photo-electron spectra of diamond and silicon valence electrons in the approximation of orthogonalized plane wave. Investigation was also made of the influence caused by the orthogonality terms on the ratio of photoionization cross-sections of s- and p-electrons. The electronic structure of diamond and silicon was calculated also by the tight-binding method with the use of the parameters from ref. 3. X-ray photo-electron spectra of valence electrons were calculated over 5230 points in 1/48 part of the Brillouin zone. For simplicity it was assumed that the polarization vector of the electromagnetic wave A is directed along the crystal Z-axis. As was done in ref. 3, the X-ray photoemission intensity was averaged over angular variables. As an example we shall give the formula used for calculation of the X-ray photoelectron spectrum for diamond I(ω, E) ～ ∝E σ_n,k [($\text{1}\text{3}$ET_2s² + $\text{1}\text{3}$T_2p²U_2p?2s² - $\text{2}\text{3}$ ∝ET_2sT_2pU_2p?2s)|C_sⁿ(K|² + $\text{1}\text{5}$ET_2p² (|C_xⁿ(K|² + $\text{3}\text{5}$ET_2p² + $\text{1}\text{3}$T_1sU_1s-2p + T_2sU_2s-2p)² + $\text{2}\text{3}$ √ET_2p(T_1sU_1s-2p + T_2sU_2s-2p) √C_zⁿ(k)|²] where T_nl(E) = fx_O^∞ r²R_nl(tr)_jl(∝Erdr and U_ij = (E_i-E_j fx8r³R_iR_j(r)R_j(r)dr R_i(r) is the radial part of the atomic wave function, C_iⁿ(k), is found from the Schrödinger equation by the tight-binding method. A similar formula is valid for silicon but the number of integrals T_nl and U_ij will be larger owing to the fact that there are more electron states in the silicon atomic core. In eqn. (1) the terms | C_xⁿ|²,|C_yⁿ|² and |C_zⁿ|² have different factors because the A vector is directed along the crystal Z-axis. These factors will be the same when the A vector is directed along (111). Therefore the contribution of p-electrons to the photoelectron spectrum will be proportional to the partial density of p-states.The formula (1) is simplified in the approximation of a plane wave I(ω, E ～ E^{$\text{3}\text{2}$} Σ_n,k[T_s²$\text{1}\text{3}$|C_sⁿ(k)|² + T_p² ($\text{1}\text{5}$|C_xⁿ(k)|² + C_yⁿ(k)|² + C_zⁿ(k)|²)] (2) Figures 1 and 2 show the results obtained from eqns. (1), (2). Here both the spectra calculated in the plane wave approximation and those found experimentally are given. The ratio between maximum III (p-states prevail) and maximum I (s-states dominate) is given in Table 1.As can be seen from Table 1 and Figs. 1 and 2, the spectra calculated in the

4. Relationship between maximum III and maximum I and between photoionization cross-sections of s- and p- electrons in diamond and silicon

     

18.

Total photoionization cross-sections of Ar and Xe in the energy range of 2.1–6.0 keV     

Lei Zheng  Mingqi CuiYidong Zhao  Jia ZhaoKai Chen 《Journal of Electron Spectroscopy and Related Phenomena》2006

Total photoionization cross-sections of Ar and Xe have been accurately measured in the photon energy region of 2.1–6.0 keV, using a four-electrode ion chamber with monochromatized synchrotron radiation. Experimental data are presented in both graphical and tabular forms and compared with previous data reported in the literatures.     

19.

Ground-state inversion method applied to calculation of molecular photoionization cross-sections by atomic extrapolation: Interference effects at low energies     

P.R. Hilton  S. Nordholm  N.S. Hush 《Journal of Electron Spectroscopy and Related Phenomena》1980,18(1):101-119

The ground-state inversion method, which we have previously developed for the calculation of atomic cross-sections [ 29 ], is applied to the calculation of molecular photoionization cross-sections. These are obtained as a weighted sum of atomic subshell cross-sections plus multi-centre interference terms. The atomic cross-sections are calculated directly for the atomic functions which when summed over centre and symmetry yield the molecular orbital wave function. The use of the ground-state inversion method for this allows the effect of the molecular environment on the atomic cross-sections to be calculated. Multi-centre terms are estimated on the basis of an effective plane-wave expression for this contribution to the total cross-section. Finally the method is applied to the range of photon energies from 0 to 44 eV where atomic extrapolation procedures have not previously been tested. Results obtained for H₂, N₂ and CO show good agreement with experiment, particularly when interference effects and effects of the molecular environment on the atomic cross-sections are included. The accuracy is very much better than that of previous plane-wave and orthogonalized plane-wave methods, and can stand comparison with that of recent more sophisticated approaches. It is a feature of the method that calculation of cross-sections either of atoms or of large molecules requires very little computer time, provided that good quality wave functions are available, and it is then of considerable potential practical interest for photoelectron spectroscopy.     

20.

Theoretical investigation of the energy dependence of photoionization cross-sections and angular distributions of photoemission of CH4 and CF4     

《Journal of Electron Spectroscopy and Related Phenomena》1986,41(2):439-452

     

	Diamond	Silicon
IIII/II
Density of states of valence electrons	2.64	2.29
XPS (in PW-approximation)	0.27	1.65
XPS (in OPW-approximation)	0.41	1.12
XPS (experimental)	0.44	1.40
σs/σp
PW-approximation	25.0	1.7
OPW-approximation	13.6	2.4
Free atom	12.0	3.4

Total photoionization cross-sections of Ar and Xe in the energy range of 2.1–6.0 keV

Total photoionization cross-sections of Ar and Xe have been accurately measured in the photon energy region of 2.1–6.0 keV, using a four-electrode ion chamber with monochromatized synchrotron radiation. Experimental data are presented in both graphical and tabular forms and compared with previous data reported in the literatures.     

Ground-state inversion method applied to calculation of molecular photoionization cross-sections by atomic extrapolation: Interference effects at low energies

The ground-state inversion method, which we have previously developed for the calculation of atomic cross-sections [ 29 ], is applied to the calculation of molecular photoionization cross-sections. These are obtained as a weighted sum of atomic subshell cross-sections plus multi-centre interference terms. The atomic cross-sections are calculated directly for the atomic functions which when summed over centre and symmetry yield the molecular orbital wave function. The use of the ground-state inversion method for this allows the effect of the molecular environment on the atomic cross-sections to be calculated. Multi-centre terms are estimated on the basis of an effective plane-wave expression for this contribution to the total cross-section. Finally the method is applied to the range of photon energies from 0 to 44 eV where atomic extrapolation procedures have not previously been tested. Results obtained for H₂, N₂ and CO show good agreement with experiment, particularly when interference effects and effects of the molecular environment on the atomic cross-sections are included. The accuracy is very much better than that of previous plane-wave and orthogonalized plane-wave methods, and can stand comparison with that of recent more sophisticated approaches. It is a feature of the method that calculation of cross-sections either of atoms or of large molecules requires very little computer time, provided that good quality wave functions are available, and it is then of considerable potential practical interest for photoelectron spectroscopy.