Direct observation of nonlinear Jahn-Teller effects in the 1 ¹A_1g→3s ¹E_g two-photon spectrum of cyclohexane

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 ¹E_g 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.