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

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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.