(σ3s) Rydberg states of cyclohexane, bicyclo[2.2.2]octane, and adamantane

Abstract

In this effort the effect of Rydberg electronic excitation on the structure of cyclic and polycyclic alkanes is investigated. Two-photon resonant, one-photon ionization, mass-resolved excitation spectroscopy is employed to observe the (σ3s)←(σ)2 Rydberg transitions of cyclohexane, bicyclo[2.2.2]octane, and adamantane cooled in a supersonic jet expansion. Rydberg spectra of these three molecules display sharp, well-resolved vibronic structure. Analysis of the spectra is assisted by isotopic substitution, circular/linear polarization, vibronic feature widths (rotational selection rules), as well as comparison to the ground-state vibrational energies. A significant reduction of vibrational energies in the excited electronic state and a 381 cm−1 blue shift of the transition origin upon deuterium isotope substitution for cyclohexane are interpreted as due to the promotion of an electron from a σ-bonding orbital to a nonbonding Rydberg orbital upon optical excitation. Extensive vibronic coupling is observed for both cyclohexane and adamantane in their excited (σ3s) Rydberg electronic states. Jahn–Teller splitting is small for adamantane but quite substantial for cyclohexane. This difference is attributed to the basic stability difference for the two different ring systems (mono- and tri-cyclic). A progression in a nontotally symmetric mode is observed in the Rydberg spectrum of bicyclo[2.2.2]octane suggesting a change in the geometry of this molecule upon (σ3s)←(σ)2 excitation.