Molecular Rydberg transitions. XVIII. Vibronic doubling in methyl iodide

Abstract

A massive vibronic coupling (dynamic Jahn–Teller) effect has been observed in the first s-Rydberg transition of CD3I. The effect is conditioned more by energy-denominator (i.e., near degeneracy) sizes than by matrix element (i.e., 〈H′〉?15 cm−1) sizes. The effect is observed in four different transitions of CH3I and CD3I where, by virtue of energy-denominator variations, alterations of the effect by 2–3 orders of magnitude are observed and interpreted. A very simple model of the coupling is developed and is shown to be numerically powerful. Since the vibronic coupling takes place in an (Ωc,ω)-spin–orbit coupling regime, the smallness of 〈H′〉 is surprising. It is shown that this small magnitude of 〈H′〉 is attributable to spin-selection breakdown which the vibronic coupling in question must accomplish even in the (Ωc,ω) regime.