Initial and final orbital alignment probing of the fine-structure-changing collisions among the Ca (4s)1(4p)1, 3PJ states with He: determination of coherence and conventional cross-sections

Abstract

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, ³P_{2, 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.