Molecular Rydberg transitions. XVIII. Vibronic doubling in methyl iodide

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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.

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