Determination of the spectroscopic constants of a dark vibrational state: Fermi and Coriolis perturbations in the ν2 band of formyl chloride

Abstract

The ν2 infrared fundamental of the transient molecule formyl chloride, HCOCl, has been recorded at high resolution. Detailed assignments of the rotational structure have been made for both HCO 35Cl and HCO 37Cl. The ν2 band, whose origin is at 1784.1377 cm−1 in HCO 35Cl, is predominantly A type with a very weak B-type component. Numerous excited state perturbations occur throughout the band, caused mainly by ‖ΔKa‖=2 Fermi resonance and ‖ΔKa‖=1 Coriolis interactions with the 3151 vibrational state. Even though no ‘‘extra’’ lines resulting from the perturbations could be assigned, a complete analysis of the perturbed rotational structure has been possible. Accurate molecular constants for both the ‘‘bright’’ perturbed 21 state and the ‘‘dark’’ 3151 perturbing state have been obtained, using a Hamiltonian which includes both the Fermi and Coriolis interactions; only two interaction parameters are needed, one for each mechanism. Further perturbations at high Ka are assigned to Coriolis interactions with the 5261 level.