Calculation of the differential cross sections of excitation and ionization of a helium atom by electrons

Equations for the amplitudes and differential cross sections of electronic excitation and ionization of a helium atom are derived in the approximation of a frozen ion core. The wave functions of the discrete states are chosen in the form of generalized hydrogenlike orbitals. The radial wave functions of the continuous spectrum are determined by solving the equation of motion numerically. The differential excitation cross sections of excitation of the 2p, 3p, and 4p levels and ionization of a helium atom by electrons are calculated in the energy range up to 50 eV. Estimates are obtained for the nonorthogonal wave functions in the amplitudes of the excitation and ionization processes. It is shown that the given method is more compatible with experiment than the Born method.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 48–57, December, 1994.     

Theoretical study of the <Emphasis Type="Italic">K</Emphasis>-photoabsorption cross section of carbon in the acetylene molecule

The magnitudes and the shape of the K-absorption spectrum of carbon in the C₂H₂ molecule are found by the one-center method. Within this method, one-electron wave functions of occupied and unoccupied states are calculated in the approximation of nonorthogonal orbitals. The processes of single and double excitation/ionization with additional excitation of an electron from the external 1π_u shell to the 1π_g state are taken into account. The mechanism of formation of a fine structure in the photoionization spectrum is refined. The calculated cross sections are compared with experimental and theoretical results obtained by other researchers. The results of calculation of the oscillator strengths of single photoexcitation are predictive.     

Calculations of total ionization cross sections of Ne,Ar, Kr,and Xe atoms in the born approximation allowing for electron perturbations in the continuous spectrum

Electron impact ionization cross sections of Ne, Ar, Kr, and Xe atoms are numerically calculated. The electron perturbation in the continuous spectrum in the target field is taken into account. The matrix elements are calculated in many-electron, nonrelativistic approximation with allowance for the superposition of configurations and relaxation effect. The radial part of the electron wave function in the continuum is calculated by solving the single-configuration Hartree-Fock equation. The wave functions of electrons in the continuum are orthogonalized with respect to the core orbital functions. The calculated cross sections are compared to the experimental data and calculations made using the plane-wave Born approximation and that involving generalized oscillator strengths.     

Spin polarization and angular distribution of Auger electrons formed as a result of decay of the 3<Emphasis Type="Italic">d</Emphasis><Superscript>−1</Superscript>5<Emphasis Type="Italic">p</Emphasis> state in the Kr atom

The results of numerical calculations of the energies of Auger transitions, as well as the angular distribution (α₂) and spin polarization (β₂) anisotropy parameters, are presented for transitions in a photoexcited Kr* atom with two open shells. Matrix elements are calculated by the multiconfigurational Fock-Dirac relativistic method using an intermediate type of coupling. The wavefunctions of the initial and final states of the Auger transition are calculated with allowance for relaxation effects. The one-electron wavefunction of the continuous spectrum for an Auger electron is obtained using the single-configuration Fock-Dirac method. The results are compared with experiment and a new experiment is proposed for identifying the Auger state not only from the energy, but also from the total angular momentum of the Auger state.     

Calculation by plane wave Born approximations of electron-impact ionization of silver and copper

The plane wave Born approximation is used to calculate total electron impact ionization cross section of silver and copper. Wavefunctions of the target and residual ions were modeled by non orthogonal Hartree-Fock and Dirac-Fock orbitals. The wave functions of the atom and residual ion are calculated with allowance for relaxation effects. The one-electron wavefunction of the continuous spectrum for the ejected electron is obtained using single-configuration Hartree-Fock and Dirac-Fock method. The orthogonalization of the ejected electron wave functions to all occupied orbitals of the target atom is performed. Results of calculations are compared to available experimental measurements and theoretical calculations performed by non relativistic one-electron PWBA, where the ejected electrons is modeled by the hydrogenic Coulomb wave function.     

Single electron loss by fast ions in helium

The single electron loss cross sections for fast B, N, and Ne ions colliding with He atoms are calculated. In the calculation of the ionization amplitude, the He nucleus screening is taken into account by both the sum rule and the summation over a finite number of excited 1snl (n < 5, l < 2) states and states of the continuous spectrum. The calculated results agree satisfactorily with experimental data.     

L-shell ionization in proton-atom collisions

The cross section of ionization of atoms by protons is calculated in the plane-wave approximation. It is shown that the wave function of the ejected electron must be orthogonal to all wave functions of the atom. A method taking into account a finite number of wave functions in the calculation of the ionization amplitude is proposed. The calculated cross sections of ionization of nitrogen, oxygen, and neon atoms by protons agree well with the experimental data and results of calculations carried out by other authors.     

The effect of wave function orthogonality on the simultaneous ionization and excitation of helium

Within the framework of the first-order Born approximation, the triple differential cross sections （TDCSs） for simultaneous ionization and excitation of helium are calculated. The wave function of the ejected electron is chosen to be orthogonal or non-orthogonal to the wave function of the bound electron before ionization. It is found that the orthogonality has a strong effect on the TDCS, especially when plane waves and Coulomb waves are used to describe the projectile and the ejected electron.     

Al~(10+)的电子碰撞电离及有关过程

用扭曲波方法计算Al~(10+)基态在Is~22s的Is,2s电子和第一激发态Is~22p的2p电子的电离截面以及基态的总电离截面(包括内壳层激发自电离).入射电子能量(?)(即(?)以2s电离能I_(2s)为单位)为1.05,1.125,l.25,1.5,2.25,4……同时计算出了电离阈值以下的激发截面(乘以所谓态密度),实现了在电离阈值处与阈值以上电离能量微分截面光滑联接;计算了光电离截面积分值,实现了在入射电子能量趋于无穷时与电子碰撞电离截面衔接.计算结果与其他方法进行了比较. 关键词：     

Ionization of ytterbium atoms from an excited state

The cross sections of the photoionization and the electron impact-induced ionization of Yb atoms from the excited 6s6p(³ P ₁) state are numerically calculated. Matrix elements are computed in multielectron relativistic and nonrelativistic approximations with allowance for the superposition of configurations and a relaxation effect. The radial part of the electron wavefunction in a continuous spectrum is calculated using the solutions to one-configuration Hartree-Fock and Dirac-Fock equations. The cross sections calculated by a relativistic method are compared to those for a nonrelativistic approximation. The ratios of the radiation reduced matrix elements and the phase shifts of the wavefunctions of a continuous spectrum calculated for the 6p ɛs and 6p → ɛd transitions are compared to the values obtained by approximating the experimental dependences of the angular distribution of photoelectrons for the photoionization by ultraviolet radiation from an oriented excited state.     

Angular distribution and polarization of photons during resonant above-threshold scattering on an atom

The effect of the resonant structure in the continuous spectrum induced by an external laser field due to the coupling of continuum states with the 1s4s ¹ S level in helium on cross sections of elastic and inelastic scattering of probe radiation on an atom in the metastable 1s2s ¹ S state at an energy above the ionization threshold is theoretically studied. Explicit expressions for the angular distribution and the degree of polarization of scattered photons as functions of probe and laser radiation characteristics are derived in the rotating wave approximation. Frequency, angular, and polarization dependences of the cross sections of scattering into final 1s ^{2 1} S, ¹ s2s ¹ S, and 1s4s ¹ S atomic states are calculated.     

Ionization of diatomic molecules by electron impact

An analytical expression for the amplitude of ionization of the hydrogen molecule by electron impact in the first Born approximation with a one-center Coulomb continuum wave function is derived. The case where the incident electron energy is much greater than the ejected electron energy is considered. The molecular wave functions were constructed in the approximation of linear combination of atomic orbitals with overlapping configurations. The role of the orthogonalization of the initial and final wave functions of the active electron of the target is elucidated. The triple differential ionization cross sections for the different orientations of the molecular axis and that averaged over all orientations are calculated. The secondary electron angular distribution is represented in the form of three-dimensional images. A comparison with the results of other theoretical calculations and experimental data is performed.     

Semiclassical fully differential ionization cross sections of helium with negatively charged fast projectiles

Fully differential cross sections are calculated for the ionization of helium by negatively charged fast projectiles using a semiclassical model developed previously for the ionization of atoms by positive projectiles. The method is tested in the case of 1 keV electron and 500 keV antiproton projectiles. The semiclassical results show reasonable agreement with the experiments and other theories. The origin of the obtained structures has been investigated by a partial wave analysis.     

Electron Transitions during the Capture of an Electron by a He<Superscript>2+</Superscript> Ion at the Argon Atom

We have measured the absolute values of the total cross section of the one-electron capture by He²⁺ ions in the kinetic energy range 2–30 keV at the Ar atoms. The absolute values of the differential scattering cross sections of He⁺ ions formed during the one-electron capture and the electron capture with ionization at energies of 2.2, 5.4, and 30 keV have been determined. The electronic states of the formed ions have been determined using collision spectroscopy based on analysis of the change in the kinetic energy of He⁺ after the interaction. We have measured doubly differential (with respect to the kinetic energy and the scattering angle) cross sections of the formation of free electrons. The free electron formation channels (direct ionization and electron capture with ionization) have been analyzed by calculating the electron terms of the (HeAr)²⁺ system. The calculated cross section of capture with ionization is in conformity with the cross section measured using collision spectroscopy.     

Destructive interferences results in bosons anti bunching: refining Feynman’s argument

The photo-absorption cross sections for the dissociative ionization and dissociative excitation of H ₂ ⁺ are calculated for photon polarization parallel and perpendicular to the internuclear axis. The wavefunctions for the initial and final states are prepared using the Born-Oppenheimer approximation. For dissociative ionization, the cross section and angular distribution of photo-electrons are compared with those calculated with the fixed-nuclei approximation. For dissociative excitation, the cross sections for H?(N = 1～4) productions are shown.     

Nonperturbative treatment of ionization with excitation of helium by electron impact

We present cross sections for electron-impact ionization and simultaneous ionization plus excitation of helium by electron impact. The results are obtained from a fully nonperturbative close-coupling formalism using our B-spline R-matrix approach. A large number of pseudostates in the expansion of the wave function represent the coupling to the ionization continuum. We obtain excellent agreement with the directly measured experimental cross section ratios (Bellm et al., Phys. Rev. A 75, 042704 (2007)) for ionization leaving the residual He? ion in either the 1s ground state or the n = 2 (2s + 2p) excited states.     

Inelastic scattering of fast electrons from metastable states of helium

Inelastic scattering cross sections for collisions of electrons with helium atoms in metastable 2 ^1,3 S states are calculated for some final states of S and P symmetry by using the First Born Approximation and the Random Phase Approximation. The results for the total cross sections compare reasonably well with previous accurate values in the case of singlet-singlet transitions.     

Theoretical study of processes of multiple excitation/ionization in K-photoabsorption of carbon and nitrogen in CO and N2 molecules

Spectra of multiple excitation/ionization in the region of photoabsorption K-edge of carbon and nitrogen of CO and N₂ molecules in the gas phase are studied theoretically. Processes of additional excitation/ionization from the 1π-and 5σ shells are taken into account. Single-electron wave functions in the excited state are calculated allowing for the effect of core wave functions in the field of a vacancy by the single-center method in the Hartree-Fock approximation. Cross sections for photoabsorption are calculated using the theory of nonorthogonal orbitals. The influence of many-electron correlations on the extension of the multiplet structure of certain configurations of multiple excitation is studied.     

Electron impact ionization cross sections for Li-like Al ion in ground and excited states

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