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Physical Review A

Quasiclassical study of differential inelastic scattering of oriented Ca(4s5p,1P1) atoms on He

Alexander Z. Devdariani, E. A. Tchesnokov, E. I. Dashevskaya, E. E. Nikitin

Published Jun 1, 1998• 3 citations
Publisher

Abstract

A quasiclassical analysis of the differential inelastic scattering of oriented $\mathrm{Ca}{(4s5p,}^{1}{P}_{1})$ atoms on He at a collision energy of 1 eV is presented. Inelastic channels correspond to spin-changing events populating different Zeeman sublevels of three fine-structure states of $\mathrm{Ca}{(4s5p,}^{3}{P}_{j}),$ $j=0,$ 1, and 2. The magnitude of the right-left scattering asymmetry in the helicopter plane is shown to depend on the locking dynamics. The relative values of different Zeeman-specific cross sections are determined by the diffraction scattering on the attractive ${}^{1}\ensuremath{\Pi}$ state in the entrance channels, by the Coriolis mixing between degenerate components of the repulsive ${}^{3}{\ensuremath{\Sigma}}^{+}$ intermediate state, and by the radial coupling between 1-1 and ${0}^{\ensuremath{-}}{\ensuremath{-}0}^{\ensuremath{-}}$ fine-structure components of the ${}^{3}{\ensuremath{\Sigma}}^{+}{\ensuremath{-}}^{3}{\ensuremath{\Pi}}_{\ensuremath{\Omega}}$ states in the exit channels. The predicted large right-left scattering asymmetry suggests that it can be measured experimentally.

DOI
10.1103/physreva.57.4472

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A quasiclassical analysis of the differential inelastic scattering of oriented Ca(4s5p,1P1)\mathrm{Ca}{(4s5p,}^{1}{P}_{1})Ca(4s5p,1P1​) atoms on He at a collision energy of 1 eV is presented. Inelastic channels correspond to spin-changing events populating different Zeeman sublevels of three fine-structure states of Ca(4s5p,3Pj),\mathrm{Ca}{(4s5p,}^{3}{P}_{j}),Ca(4s5p,3Pj​), j=0,j=0,j=0, 1, and 2. The magnitude of the right-left scattering asymmetry in the helicopter plane is shown to depend on the locking dynamics. The relative values of different Zeeman-specific cross sections are determined by the diffraction scattering on the attractive 1\ensuremathΠ{}^{1}\ensuremath{\Pi}1\ensuremathΠ state in the entrance channels, by the Coriolis mixing between degenerate components of the repulsive 3\ensuremathΣ+{}^{3}{\ensuremath{\Sigma}}^{+}3\ensuremathΣ+ intermediate state, and by the radial coupling between 1-1 and 0\ensuremath−\ensuremath−0\ensuremath−{0}^{\ensuremath{-}}{\ensuremath{-}0}^{\ensuremath{-}}0\ensuremath−\ensuremath−0\ensuremath− fine-structure components of the 3\ensuremathΣ+\ensuremath−3\ensuremathΠ\ensuremathΩ{}^{3}{\ensuremath{\Sigma}}^{+}{\ensuremath{-}}^{3}{\ensuremath{\Pi}}_{\ensuremath{\Omega}}3\ensuremathΣ+\ensuremath−3\ensuremathΠ\ensuremathΩ​ states in the exit channels. The predicted large right-left scattering asymmetry suggests that it can be measured experimentally.
We apply a new method [M. H. Alexander, J. Chem. Phys. 94, 8931 (1991)] for the study of the mechanism of inelastic collisions, to the analysis of spin-changing collisions of Ca atoms in the 4s5p Rydberg state. The method involves the determination of the current density associated with, separately, the incoming and outgoing scattering wave functions in a locally adiabatic basis. This yields a picture of how the incoming flux, initially associated with a given internal state, redistributes itself as a function of the interparticle separation both as the particles approach, and, subsequently, as the particles recede. By proper selection of the initial state, we explore the dependence on orbital orientation of the probability for the spin-changing Ca(4s5p 1P)→Ca(4s5p 3P) process. Further, we show how the distribution of population among the fine-structure levels of the 3P state depends on final-state interactions in the exit channel.
Differential cross sections for the transition K(4P3/2{P}_{3/2}P3/2​\ensuremath{\rightarrow}4P1/2{P}_{1/2}P1/2​) induced by He and Ar have been studied both experimentally and theoretically. Calculations were performed using a full quantal close-coupling formalism. The angular distribution of the scattered atoms is measured by Doppler shift (ADDS method). The comparison with the theoretical calculations is performed on the basis of thorough kinematic analysis. In addition, the polarization effects corresponding to the experimental conditions are taken into account carefully. This allows one to test the accuracy of various interaction potentials available for the K-He and -Ar systems. The most interesting result concerns the K-He l-dependent pseudopotentials, which are found accurate to a sufficiently high level to allow good predictions of the observed differential cross-section patterns. In particular the positions and amplitudes of the cross-section oscillations at large center-of-mass scattering angles are well reproduced. The results for Ar show that the K-Ar potentials must be improved. Finally, taking the K(4P)-He fine-structure transition as an example, it has been shown how polarization effects may be used in ADDS measurements to obtain information on the dynamics of the collision process.
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Results of the quantum close-coupling calculations for the spin-changing transition in Ba(6s6p, 1 P 1 → 3 P 2 ) induced by collisions with argon are presented. The dependence on angular and collision energy of the calculated multi-structure differential cross-sections are discussed. The backward scattering is most prominent in a small energy range, particularly in the case of Π preparation of the initial Ba( 1 P 1 )-Ar molecular state. The similar characteristic features of differential cross-sections are observed in the experiment by Visticot et al (Visticot J P et al 1992 Phys. Rev. A 45 6371).
A laser pump–probe experiment is used to study the orbital alignment effects, orientation effects and vector correlations of collisional transfer of the Ca (4s)1(4p)1, 3P1 state to the Ca (4s)1(4p)1, 3P2, 0 levels. The experiment is configured in a single-collision crossed-beam arrangement between Ca and He, and multi-structure cross-sections are determined using appropriate combinations of linear and circular laser light for the pump/probe steps. Real and imaginary parts of coherence cross-sections are obtained along with the conventional population cross-sections for the m1→m2 magnetic sublevel transitions into the 3P2 level. The total relative cross-section ratio for the perpendicular (m1=±1) to parallel (m1= 0) polarization preparation of 3P1 transferring to 3P2 is 1.46 ± 0.15. For initial 3P1 preparation with laser light linearly polarized perpendicular to the initial relative velocity vector, the transfer into the m2-sublevels of the 3P2 state show a distinct preference for the sign-changing m1=+ 1 →m2=–1 transition. Preparation of Ca 3P1 with laser light linearly polarized parallel to the initial relative velocity vector produces population transfer into the 3P2 level that is completely aligned in the ±1 and ±2 sublevels, consistent with symmetry considerations. The magnitudes of the coherence cross-sections range from a few percent to greater than 100% of some of the population transfer conventional cross-sections. Study of the alignment effect into the final 3P0 state found a very large observed effect (σ|m|= 1/σ|m|= 0) of 23 ± 0.9. Interpretation of the energy transfer results indicates that the energy transfer obeys symmetry rules and follows predictions of curve crossings between the Σ and Π potentials, where for the transfer into 3P2 only indirect coupling can occur.