Determination of the spectroscopic constants of a dark vibrational state: Fermi and Coriolis perturbations in the [nu][sub 2] band of formyl chloride

The [nu][sub 2] 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 [sup 35]Cl and HCO [sup 37]Cl. The [nu][sub 2] band, whose origin is at 1784.1377 cm[sup [minus]1] in HCO [sup 35]Cl, is predominantly [ital A] type with a very weak [ital B]-type component. Numerous excited state perturbations occur throughout the band, caused mainly by [vert bar][Delta][ital K][sub [ital a]][vert bar]=2 Fermi resonance and [vert bar][Delta][ital K][sub [ital a]][vert bar]=1 Coriolis interactions with the 3[sup 1]5[sup 1] 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 2[sup 1] state and the dark'' 3[sup 1]5[sup 1] 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 [ital K][sub [ital a]] are assigned to Coriolis interactions with the 5[sup 2]6[sup 1] level.