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Physical Review A

Differential cross sections of fine-structure transitions in K(4P)-He and -Ar collisions

J.‐M. Mestdagh, P. de Pujo, J. Pascale, J. Cuvellier, J. Berlande

Published Feb 1, 1987• 10 citations
Publisher

Abstract

Differential cross sections for the transition K(4${P}_{3/2}$\ensuremath{\rightarrow}4${P}_{1/2}$) 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.

DOI
10.1103/physreva.35.1043

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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.
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).
Optical collisions are studied in a crossed beams experiment. Differential cross sections of K-Ar collisions are measured and are used to derive the repulsive parts of the XΣ and BΣ potential curves. The achieved accuracy has not been realized with scattering experiments before. A collision energy resolved analysis of the final state probes the relative population of the fine-structure states K(4p1/2) and K(4p3/2) which depends on details of the outer part of the potentials. Calculations from the determined potentials are in concordance with the experimental results. The relative population of the Na fine-structure states after Na-N2 and Na-O2 collisions is determined similarly. The results for N2 are in very good agreement with the theory. Differential cross sections of Ca-Ar optical collisions are measured for an asymptotically forbidden transition. The spectral dependence of the signal intensity shows a characteristic maximum. The experimental data are in good agreement with ab initio calculations. Wires on a microchip create a magnetic trap that is used to obtain a Rb-87 Bose-Einstein condensate. The roughness of the magnetic potential is characterized by the measured density of a cold atom cloud. The measured roughness is compared to the roughness that is calculated from the geometry of the microwire.
The cross sections for the excitation energy transfer in the collisional processes Na*(3P 1 2/)+M(nS 1 2/) to or from Na*(3p 3 2/)+m(Ns 3 2/), m being ground-state Rb (n=5) or Cs (n=6) atom, have been measured by laser fluorescence method. With either of the two sodium 3P fine-structure states optically excited, the intensity ratio of the sensitized to direct fluorescence was measured. The polarization effects due to partially polarized and anisotropic direct fluorescence were investigated. The measured cross sections (T=530 K) for the sodium 3P fine-structure excitation transfer induced by collisions with Rb are: sigma ( 1 / 2 to 3 / 2 )=170 AA 2 , with an overall accuracy of 20%. The corresponding values in the Cs case are: sigma ( 1 / 2 to 3 / 2 )=140 AA 2 , sigma ( 3 / 2 to 1 / 2 )=83 AA 2 . The theoretical calculations reveal the decisive role of the dipole-quadrupole interaction. The obtained values are in satisfactory agreement with the experiment only if both the dipole-dipole and dipole-quadrupole interactions are taken into account, while the calculations involving the dipole-dipole term alone yield the cross section values which are as much as one order of magnitude lower than the experimentally observed ones.
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 ${}^{1}\ensuremath{\Pi}$ 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.
The cross sections for the excitation energy transfer induced by collisions with He, Ar and Na ground-state atoms have been measured. The sodium atoms in the 4D state have been produced by two-photon excitation using cw laser radiation at 578.89 nm. Within a model which takes into account collisional population and depopulation of the 4F level as well as radiative relaxation (direct or cascade) of the 3D, 4P, 4D and 4F states ending in the 3P level, the rate for the collisional transfer turns out to be related to the ratio of the steady-state populations created in 3D and 4D states. They have been determined by measuring the fluorescence intensities of the and transitions. The measurements yield the following cross section values for those processes investigated: , and , at .