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Journal of Physics B Atomic Molecular and Optical Physics

Quantum close-coupling calculations of the differential cross-sections for Ba(1P1) + Ar→ Ba(3P2) + Ar inelastic collisions

E. Paul-Kwiek

Published Jan 18, 2001• 2 citations
Publisher

Abstract

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).

DOI
10.1088/0953-4075/34/3/307

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•Jan 1, 2000•0 citations
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).
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.
The results of a joint experimental and theoretical study concerning elastic electron scattering by laser-excited 138 Ba( ... 6s6p 1 P 1 ) atoms are described. These studies demonstrate several important aspects of elastic electron collisions with coherently excited atoms, and are the first such studies. From the measurements, collision and coherence parameters, as well as cross sections associated with an atomic ensemble prepared with an arbitrary in-plane laser geometry and linear polarization (with respect to the collision frame), or equivalently with any magnetic sublevel superposition, have been obtained at 20 eV impact energy and at 10°, 15° and 20° scattering angles. The convergent close-coupling (CCC) method was used within the non-relativistic LS -coupling framework to calculate the magnetic sublevel scattering amplitudes. From these amplitudes all the parameters and cross sections at 20 eV impact energy were extracted in the full angular range in 1° steps. The experimental and theoretical results were found to be in good agreement, indicating that the CCC method can be reliably applied to elastic scattering by 138 Ba( ... 6s6p 1 P 1 ) atoms, and possibly to other heavy elements when spin-orbit coupling effects are negligible. Small but significant asymmetry was observed in the cross sections for scattering to the left and to the right. It was also found that elastic electron scattering by the initially isotropic atomic ensemble resulted in the creation of significant alignment. As a byproduct of the present studies, elastic scattering cross sections for metastable 138 Ba atoms were also obtained.
The close coupling calculation of rotationally inelastic collision of He with Na2 is first performed by employing a recently computed ab initio potential energy surface. The process of rovibrational energy excitation and relaxation of Na2 in collision with He has been studied in detail based on the single and double exitations coupled cluster with a noniterative perturbation treatment of triple excitations CCSD （T） potential energy surface. Our results are in good agreement with the experimental results. Our results show that: （1）The elastic cross section（Δj=0） is much larger than the inelastic cross section. （2）The largest amplitude occurs at small scattering angles. At low Δj transition, the He projectile is predominantly forward scattering. Backward scattering occurs as Δj increases. （3）For inelastic cross section, the most effective transition is that with Δj=2.（4）The fast narrow diffraction oscillations in the cross sections are the obvious characteristics for the low Δj inelastic transitions, which decreases with increasing j. （5）The elastic differential-cross-sections（DCS）provides a useful visualization of rotational rainbows at low translational energy. It is beneficial to reducing the loss in molecular cooling and trapping.
The spin-changing (6s)1(6p)1, 1P1→(6s)1(6p)1, 3P2 transition of barium induced by single collisions with H2 has been studied using a crossed beam apparatus. An mj-selective preparation and detection of barium was achieved using the pump and probe technique in the presence of a magnetic field that breaks the degeneracy of the barium magnetic sublevels. The probe laser was scanned across the (6s)1(6p)1, 3P2,mj′→(6s)1(7s)1, 3S1, mj″ multiplicity. Analysis of the Doppler profiles of each line reveals the angular distribution of scattered barium atoms, the polarization of which is selected both before and after the collision. This is therefore a four-vector correlation experiment, and it is the first to document spin-changing transitions of alkaline-earth-metal atoms induced by a molecular target. It must be emphasized that all four vectors in the present experiment do not refer to the same quantization axis, whereas in the standard notion of n-vector correlation experiments, the n-vectors refer to the same axis. The results of the present work are discussed in the light shed by other results obtained on the same experiments with only two and three vectors controlled. A collision mechanism is proposed that accounts for all the existing experimental results on this process.
Experimental studies concerning elastic and inelastic electron scattering by coherently ensembles of Ba (...6s6p (sub 1)P(sub 1)) atoms with various degrees of alignment will be described. An in-plane, linearly-polarized laser beam was utilized to prepare these target ensembles and the electron scattering signal as a function of polarization angle was measured for several laser geometries at fixed impact energies and scattering angles. From these measurements, we derived cross sections and electron-impact coherence parameters associated with the electron scattering process which is time reverse of the actual experimentally studied process. This interpretation of the experiment is based on the theory of Macek and Herte. The experimental results were also interpreted in terms of cross sections and collision parameters associated with the actual experimental processes. Results obtained so far will be presented and plans for further studies will be discussed.