Collision-induced transitions between A 1Σu+ and b 3Πu states of Na2: The ‘‘gateway’’ effect of perturbed levels

Abstract

We present here the best qualitative and quantitative illustration to date of the perturbation ‘‘gateway’’ effect in collision-induced transitions between two mutually perturbing electronic states. The gateway effect, as described by Gelbart and Freed [Chem. Phys. Lett. 18, 470 (1973)], is a suggestion that all collision-induced transfer of population between two electronic states proceeds through a small number of isolated-molecule eigenstates which are of mixed electronic character, the ‘‘gateway levels,’’ and that the rates for such gateway-mediated processes are related to the mixing fractions in the gateway levels. The gateway levels here are the Na2 A 1Σu+ v′=26∼b 3Π2u v′=28 J′=16e,a-symmetry levels which are significantly mixed owing to an extremely small spin–orbit perturbation matrix element (the neighboring J′=15 and 17e,s-symmetry levels are essentially free of mixing). A cw optical–optical double resonance (OODR) scheme is used to PUMP a single parent level and PROBE single daughter and granddaughter levels. The oscillator strengths for the PUMP and PROBE transitions are derived, respectively, from the A 1Σu+←X 1Σg+ (26,4) band and the 2 3Π2g←b 3Π2u (28,28) subband. The qualitative observation of the gateway effect is that whenever an a-symmetry A 1Σu+ v′=26 parent level is selected, b 3Π2u v′=28 daughter and granddaughter levels are observably populated, but when an s-symmetry A 1Σu+ v′=26 parent is selected, essentially no population is detected in b 3Π2u v′=28 daughter and granddaughter levels (i.e., no perturbation, no interelectronic state transfer). The quantitative observation of the gateway effect is that when a J′=12 (or 14)e,a parent is selected, the most efficiently populated rotational levels of the other electronic state are granddaughter levels centered about the J′=16e,a gateway daughter level rather than about the J′ value of (or minimum energy gap relative to) the parent level.