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

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.