Rovibrational state mixing in the C–H stretch fundamental region of norbornadiene
Journal Article
Overview
abstract
Infrared laser induced fluorescence has been used to probe the extent of state mixing, as well as its rotational dependence, in the C–H stretch fundamental region of norbornadiene in the electronic ground state. Both total and dispersed IR fluorescence were collected from molecules, cooled, and isolated in a molecular beam, after excitation with an infrared optical parametric oscillator. The dilution factor, which is inversely related to the number of coupled states, has been measured for four C–H stretch bands, and in addition, the rotational dependence of the dilution factor has been investigated by exciting different positions, or average J values, within a single vibrational band. The relative energy contents of various vibrational modes, whose fluorescence was observed, have also been determined. An approximate average coupling width of 0.05 cm−1 can be derived from the dilution factors and the calculated state density of the rovibrational bath states {‖li〉} that are permitted by symmetry and angular momentum to couple to the zeroth order optically active rovibrational state ‖s〉. The rotational dependence of the dilution factor shows that the number of mixed states scales linearly with J, where J is the total angular momentum quantum number for the ‖s〉 state. Allowing for both Coriolis and Fermi couplings within a width of 0.05 cm−1, we can adequately predict the extent of state mixing and the distribution of the excitation energy among the modes of the molecule by using a straightforward statistical model, which assumes equal coupling between all the zeroth order rovibrational states that have the correct energy, symmetry, and J to couple to a given optically active state.
