The ultraviolet two-photon absorption spectrum of the lowest excited singlet state of jet-cooled cyclohexane reveals a highly perturbed system of very sharp bands in the 55 000-58 000 cm-1 region, which we assign as transitions to vibronic elements of the 3s 1Eg Rydberg state. On the basis of polarization, isotope, and variable-temperature data and by use of vibronic coupling calculations, we construct a detailed assignment of the vibronic level structure which demonstrates that the proliferation of low-energy states is a direct result of the introduction of an entirely new type of low-frequency motion (150 cm-1) in the excited state, that of the Jahn-Teller vibronic pseudorotation. We observe moderate to high activity in five of the eight possible Jahn-Teller modes, and estimate a total Jahn-Teller stabilization energy of 2550 cm-1. Importantly, we find large splittings (15% of the zeroth-order frequency) of states which would remain degenerate given only linear Jahn-Teller activity (free pseudorotation), and interpret these in terms of nonlinear vibronic coupling as the first experimental evidence for significant quenching of vibronic angular momentum in an isolated molecule.
ASJC Scopus subject areas
- General Physics and Astronomy
- Physical and Theoretical Chemistry