The semiclassical close-coupling description of atomic collisions: Recent developments and results

http://www.iap.tuwien.ac.at/www/_media/atomic/publications/sc_andt99.pdf

Database for inelastic collisions of lithium atoms with electrons, protons, and multiply charged ions

In this paper we show that circular dichroism in photoelectron angular distributions (CDAD) can be used to probe atomic and molecular alignment in the gas phase. Careful choice of photon (left or right circularly polarized) propagation and photoelectron collection directions breaks the cylindrical symmetry of the target, giving rise to dichroic effects. CDAD exists in the electric dipole approximation. We illustrate the sensitivity of CDAD to alignment by considering photoionization of the A 2Sigma+ state of NO. Most of the cases of alignment we consider are created by multiphoton absorption while the others, more general, might be created in fragmentation, desorption, etc. The alignment created by n-photon absorption quickly reaches a classical limit which is reflected in the CDAD spectrum. Finally, we show that CDAD is also a sensitive probe of gas phase atomic state alignment by considering photoionization of the 7P3/2 state of cesium created by single photon absorption from the ground state.

/pdf/circular-dichroism-in-photoelectron-angular-distributions-as-11dwnsh8nz.pdf

Circular dichroism in photoelectron angular distributions as a probe of atomic and molecular alignment

Orientation effects for electron capture and excitation in singly-charged ion-atom collisions are analysed using the atomic basis impact parameter method with full inclusion of electron translational factors. The authors find that the orientation preferences previously predicted for excitation in terms of propensity rules may still be observed when capture is present in ion-atom collisions. Furthermore, in spite of intricate behaviour of the direct capture couplings during the collision, they draw some parallel conclusions for the orientation of the capture states. They illustrate these perturbative predictions by close-coupling calculations for H+-Na (3s) collisions where clear propensity for orientation of the H(2p) capture state is demonstrated in impact parameter and velocity dependences. Finally the authors predict pronounced orientation effects for H(2s) and H(2p) capture in collisions of H+ with initially oriented Na(3p) states.

https://iopscience.iop.org/article/10.1088/0953-4075/23/15/025/pdf

Propensity rules for orientation in singly-charged ion-atom collisions

Recent developments in the field of alignment and orientation studies of electron impact excitation of atoms are discussed. The article reviews experiments which have a well defined planar symmetry, i.e. in which the initial and final momenta of electrons are determined. The experimental results obtained by different groups during the last four years are presented after a brief theoretical introduction followed by a description of typical experimental arrangements.

https://iopscience.iop.org/article/10.1088/0953-4075/23/2/004/pdf

Coherence and correlation in electron-atom collisions

The proton-induced 3s-3p excitation of spin-polarised Na atoms has been studied both experimentally and theoretically. The total excitation cross section as well as the Stokes' polarisation parameters of the fluorescence light were measured in the regime of intermediate impact energies (0.8-6 keV). The measured circular polarisation P3 is in accordance with conservation of spin orientation during the collision. The linear polarisation P1 reflects the collisional induced alignment of the 3p state. The measured total cross section and the alignment are compared with various calculations. The p+Na collision system can be treated as a single active outer electron in the field of two inert cores.

https://iopscience.iop.org/article/10.1088/0022-3700/19/15/011/pdf

Excitation of the Na 3p state by proton impact

The 3s-3p impact excitation of spin-polarised Na atoms by unpolarised electrons has been studied. The fluorescence light emitted in the decay of the excited state exhibits linear polarisation caused by the collisional induced 3p alignment, as well as circular polarisation resulting from the spin orientation of the 3p electron at the time of the decay. Since the exchange interaction causes a spin flip, the degree of circular light polarisation is a measure for the contribution of the exchange channel to the total excitation cross section. At small impact energies where exchange becomes important, the authors found a decrease of the circular polarisation. They analysed the hyperfine coupling between the excited electron and the polarised nucleus, and calculated the light polarisation from theoretical excitation cross sections. Comparison of experiment and theory shows reasonable agreement.

http://iopscience.iop.org/article/10.1088/0022-3700/17/9/027/pdf

Electron exchange in the Na 3p electron impact excitation

The 3s-3p excitation of spin-polarized Na atoms by Ne/sup +/ ions has been studied for impact energies E/sub lab/ = 200 eV to 6 keV, ie, in the adiabatic regime The total excitation cross section and the three Stokes polarization parameters of the fluorescence light have been measured The linear polarization of the light shows a preferential excitation of the chemically bondm/sub l/chemically bond = 1 magnetic substates The circular polarization probes the spin orientation of the excited 3p state At the highest impact energies investigated the experimental data are compatible with conservation of spin orientation during the collision With decreasing impact energy, the spin polarization of the final 3p state becomes smaller than the spin polarization of the initial 3s state This apparent spin depolarization is attributed to the exchange interaction between the Na valence electron and the unfilled Ne/sup +/ 2p/sup 5/ core in the quasimolecule formed during the collision

Spin exchange in the excitation of spin-polarized Na atoms by Ne + -ion impact

The 3{ital s}-3{ital p} excitation of Na atoms by He{sup +} ions in the impact energy range 0.6--10 keV has been studied both experimentally and theoretically. In the experiment, the total excitation cross section, the alignment of the Na 3{ital p} state, and the collision-induced change of spin polarization of the Na valence electron were investigated. For the latter purpose, the Na atoms were polarized by a hexapole state selector, and a magnetic field was applied to the collision region, resulting in a spin polarization of the Na ground-state atoms of {ital P}{sub {ital S}}{approx}0.74. The spin polarization of the final 3{ital p} excited state was derived from the measured circular polarization of the fluorescence light. The experimental results are compared to published calculations in the Coulomb approach and to our own calculations in the molecular approach using a 12-state molecular expansion and employing a common translation factor. The molecular approach gives satisfying agreement with all experimental data. In particular, the measured increase of spin depolarization with decreasing collision energy is well reproduced. The depolarization originates from the exchange interaction between the 1{ital s} electron of He{sup +} and the valence electron of Na during the collision.

S Osimitsch

Papers

Excitation of the Na 3p state by proton impact

Electron exchange in the Na 3p electron impact excitation

Spin exchange in the excitation of spin-polarized Na atoms by Ne + -ion impact

Alignment and spin exchange in the Na 3p excitation by He+-ion impact

Collisions of spin-polarized atoms and ions in magnetic fields