CORIOLIS PERTURBATIONS TO THE 3ν4 LEVEL OF THE a STATE OF FORMALDEHYDE

Abstract

Formaldehyde is the smallest stable organic molecule containing the carbonyl functional group, and is commonly considered to be a prototype for the study of high-resolution spectroscopy of polyatomic molecules. The a-axis Coriolis interaction between the near-degenerate ν 4 and ν 6 (out-of-plane and in-plane wagging modes, respectively) of the ground electronic state has received extensive attention and is thoroughly understood. In the first excited singletà 1 A 2 electronic state, the analogous Coriolis interaction does not occur, because theà state suffers from a pseudo Jahn-Teller distortion, which causes a double-well potential energy structure in the q 4 out-of-plane coordinate, and which dramatically reduces the effective ν 4 frequency. The ν 4 frequency is reduced by so great an extent in theà state that it is the 3ν 4 overtone which is near degenerate with ν 6. In the current work, we report the precise ν 6 fundamental frequency in theà state, and we determine the strength of the a-axis Coriolis interaction between 3ν 4 and ν 6. We also provide a rotational analysis of the ν 4 + ν 6 combination band, which interacts with 3ν 4 via an additional c-axis Coriolis perturbation, and which allows us to provide a complete deperturbed fit to the 3ν 4 rotational structure. Knowledge of the Coriolis interaction strengths among the lowest-lying levels in theà state will aid the interpretation of the spectroscopy and dynamics of many higher-lying band structures, which are perturbed by analogous interactions.