N M Kabachnik

Theoretical study of inner-shell alignment of atoms in electron impact ionisation: angular distribution and polarisation of X-rays and Auger electrons

The alignment of ions with an inner-shell vacancy resulting from ionisation by a fast charged particle is considered. The alignment has been calculated in the Born approximation using the Herman-Skillman model. The angular distribution and polarisation of characteristic X-radiation and Auger electrons from decay of aligned ions are discussed. It is shown that the theoretical anisotropy of the angular distribution is sensitive to the wavefunctions used. The results of calculations using the Herman-Skillman wavefunctions are in much better agreement with experiment than the previous calculations involving the hydrogen-like functions.

Angular distribution and polarization of photoelectrons in the region of resonances

General expressions for the angular distribution and polarization of photoelectrons in the region of autoionizing states have been obtained. It is shown that when an autoionizing state has two or more channels of decay a resonance structure can occur in both the angular distribution and the polarization. For the autoionizing states nsnp6mp of Ne and Ar, possible shapes for the energy dependence of the angular distribution have been discussed. This dependence is sensitive to the relation between the matrix elements which characterize excitation and decay of the autoionizing state through the different channels.

Spin polarisation of Auger electrons following atomic photoionisation by circularly polarised X-rays

Spin polarisation of Auger electrons arising from photoionisation of atoms by circularly polarised photons is studied theoretically. Calculations show that in a wide range of photon energies the polarisation is large and can be measured in experiment. It is shown that the polarisation transfer can be used to study the dynamics of the Auger decay and in particular the influence of many-electron correlations on the Auger transition amplitudes. It is suggested that the polarisation transfer be used for measurements of the circular polarisation of X-rays.

Angular distribution and spin polarisation of Auger electrons

The angular distribution and spin polarisation of Auger electrons following atomic collisions are discussed for the case where both the target atom and the projectile are unpolarised. General expressions for the angular distribution and polarisation of Auger electrons in the (LS)J coupling approximation are obtained and analysed. In the particular case of the p3/2 subshell vacancy the anisotropy coefficient and polarisation are expressed in terms of Auger-decay matrix elements. It is shown that spin-polarised Auger electron spectroscopy of atoms can provide new information on both the Auger process and the structure of atomic hole states, namely, the Auger-decay matrix elements and their relative phaseshifts.

Inner-shell alignment of atoms in ion-atom collisions. I. Impact ionisation

The alignment of ions with an inner-shell vacancy resulting from ion-atom collisions is considered. The calculations are made in the Born approximation for a number of atoms and subshells using the hydrogen-like, screened hydrogen-like and Herman-Skillman models. It is shown that the alignment is a complicated function of the incoming ion velocity and is large in the low-velocity region. Comparison is made with experimental data and with the results of calculations for electron impact ionisation.

Spin-polarised Auger-electron spectroscopy of the d atomic shell. Model calculation for noble-gas atoms

The angular distribution and spin polarisation of Auger electrons of free atoms are discussed. In the (LS) J-coupling approximation the anisotropy coefficient and polarisation are expressed in terms of Auger-decay matrix elements in the particular case of the d3/2 and d5/2 subshell vacancies. For some Auger transitions in Ar, Kr and Xe atoms the anisotropy coefficient and polarisation have been calculated. The Herman-Skillman atomic model was used to calculate the Auger-decay amplitudes. In all cases considered the polarisation is small (less than 20%). However, the anisotropy is large and its measurement may be useful for studying the Auger process.

The theory of coincidence experiments on electron impact ionisation of inner atomic shells

A theoretical treatment is given of possible experiments on electron impact ionisation of inner atomic shells, in which an inelastically scattered electron is detected in coincidence with the decay products of the ion formed, namely with an X-ray photon or with an Auger electron. General expressions for the angular distribution and polarisation of the X-ray photons and Auger electrons are given in terms of the parameters describing the anisotropy of the ionic state, namely statistical tensors which depend on the electron scattering angle and are determined by the ionisation mechanism. The statistical tensors, the asymmetry and the polarisation of the X-rays and the asymmetry of the Auger electrons are calculated for ionisation of the 2p shell of Ar, the 3d shell of Kr and the 4d shell of Xe at different scattering angles of the fast electron.

On spin polarisation of Auger electrons

Conditions for occurrence of spin polarisation of Auger electrons have been analysed. It is shown that if an inner-shell vacancy arising from particle impact is aligned then the Auger electrons can have spin polarisation transverse to the scattering plane. The expressions for the degree of polarisation as a function of the angle of detection of Auger electrons have been obtained. Using the LS coupling approximation it is shown that a significant spin polarisation may be expected for the Auger lines corresponding to the triplet terms of the residual ion if individual components of the fine-structure multiplet are separated out. After the summation over the multiplet the polarisation disappears.

Angular correlation between photoelectron and Auger electron in two-step double photoionization of atoms

A general expression for the angular correlation between two escaping electrons in double photoionization of atoms is derived using the density matrix and statistical tensor formalism. In the framework of the two-step model the angular correlation between a photoelectron and a subsequent Auger electron is considered in detail. Photoionization by linearly polarized and by unpolarized light is considered and the angular correlation patterns are compared.

The effect of many-electron correlations on the angular distribution and spin polarisation of Auger electrons in Kr and Xe

The effect of many-electron correlations on the angular distribution and spin polarisation of Auger electrons is studied theoretically. The correlations are taken into account by two different methods: using the random phase approximation with exchange (RPAE) and using the many-body perturbation theory (MBPT). The calculations were made for some transitions of the M4.5N1N2.3 spectrum in Kr and of the N4.5O1O2.3 and M4.5N4.5N4.5 spectra in Xe. It is shown that in some cases the many-electron correlations considerably change the calculated values of the anisotropy coefficients and polarisation of Auger electrons and can even change their signs. The results of recent experiments are analysed.