Collision-induced transitions between A ¹Σ _u⁺ and b ³Π_u states of Na ₂: The "gateway" effect of perturbed levels

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 ¹Σ_u⁺ ν'=26≃b ³Π_2u ν'=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 ¹Σ_u⁺←X ¹Σ_g⁺ (26,4) band and the 2 ³Π_2g←b ³Π_2u (28,28) subband. The qualitative observation of the gateway effect is that whenever an a-symmetry A ¹Σ_u⁺ ν'=26 parent level is selected, b ³Π_2u ν=28 daughter and granddaughter levels are observably populated, but when an s-symmetry A ¹Σ _u⁺ ν'= 26 parent is selected, essentially no population is detected in b ³Π_2u ν' = 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.