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AIP conference proceedings

Alignment effects in Ca-He (51P1-53PJ) energy transfer half-collisions

King‐Chuen Lin, Sharath Ananthamurthy, Paul D. Kleiber, J. X. Wang, W. C. Stwalley, Stephen R. Leone

Published Jan 1, 1989• 0 citations
Publisher

Abstract

We have studied Ca‐He spin changing collisions using far wing laser scattering ‘‘half‐collision’’ techniques. We have observed a strong red wing‐blue wing asymmetry which we interpret in terms of an orbital alignment preference in the energy transfer cross section. This orbital alignment effect approaches 100%.
DOI
10.1063/1.38591

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Physical Review A•Jan 1, 1998•13 citations
The far wing absorption profiles for excitation on the Ca(4s2 1S0–4s5p 1P01) atomic transition, broadened in collisions with He are measured. We observe strong absorption in both wings and a blue wing satellite near Δ∼125 cm−1. We tentatively identify this satellite as due to a maximum in the CaHe(4s2 1∑+–4s5p 1∑+) difference potential. These line-broadening techniques are used to study electronic energy transfer in the spin-changing collisions of Ca with He: Ca(5p 1P01) +He→Ca(5p 3P0J)+He+ΔE. Measurements of the ‘‘single collision’’ triplet–singlet branching ratio as a function of laser detuning from the atomic resonance frequency indicate a clear red wing/blue wing asymmetry. We interpret this asymmetry in terms of a preferential orbital alignment effect in the energy transfer process [Phys. Rev. Lett. 53, 2296 (1984)]. No clear structure is observed in the range of detunings probed that might indicate the curve crossing responsible for the energy transfer.
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.
Rank-6 symmetry in energy transfer collisions is observed. A Ca(4s4f,1{,}^{1},1F3{\mathit{F}}_{3}F3​) state is prepared by three-photon excitation. The angular wave function of the Ca(1{(}^{1}(1F3{\mathit{F}}_{3}F3​) state depends on the angles of the three laser polarizations with respect to each other. The alignment effect for energy transfer, Ca(4s4f,1{,}^{1},1F3{\mathit{F}}_{3}F3​)+He\ensuremath{\rightarrow}Ca(4p2{\mathit{p}}^{2}p2,1{,}^{1},1S0{\mathit{S}}_{0}S0​)+He, is determined for two different initial angular wave functions. Each alignment curve can be analyzed in terms of the same set of relative cross sections for the individual Ca(1{(}^{1}(1F3{\mathit{F}}_{3}F3​) magnetic sublevels, providing an independent verification of the data.
The quantum theory of atomic collisions in the presence of a weak radiation field is used to describe the process of redistribution of light in Ca–He collisions. This work is devoted to the simulation of the recent experiment of Lin and co-workers [J. Chem. Phys. 89, 4771 (1988)] in which, the absorption profile of the (4s2 1S→4s5p 1P) transition and the dependence on the detuning of the spin-changing (4s5p 1P→4s5p 3P) transition were determined. Close-coupled calculations are carried out, based on model interaction potentials for the ground and the excited states of the Ca–He system. The calculated absorption profile for the production of Ca atom in the (4s5p 1,3P) states is in qualitative agreement with the experimental results. The blue wing/red wing asymmetry in the relative transfer to the 3P state, is interpreted in terms of flux transfer among the molecular states, which occurs when the Ca atom is approached by the closed-shell partner.
Retention of orbital alignment during near-resonant energy transfer collisions of rare-gas projectiles with excited alkaline-earth-metal atoms is explored theoretically for scattering of Xe from Rydberg states of Ca. For such collisions, conventional interpretations of alignment phenomena, which are based on potential curves for a transient molecule thought to form during the collision, are not relevant. Theoretical partial magnetic sublevel cross sections for ${\mathrm{Ca}}^{**}(4s17d{}^{1}{D}_{2}\ensuremath{\rightarrow}4s18p{}^{1}{P}_{1})$ transitions confirm the existence of alignment effects, as demonstrated experimentally by Spain et al. [J. Chem. Phys. 102, 9532 (1995)], at a mean relative velocity of 914 m/s. Theory further predicts heretofore unobserved oscillatory structures in these cross sections as a function of relative velocity and that these oscillations depend strongly on the initial magnetic quantum number of the Rydberg electron.