DETERMINATION OF THE SPECTROSCOPIC CONSTANTS OF A DARK VIBRATIONAL-STATE - FERMI AND CORIOLIS PERTURBATIONS IN THE NU-2 BAND OF FORMYL CHLORIDE

The nu(2) infrared fundamental of the transient molecule formyl chlori de, HCOCl, has been recorded at high resolution. Detailed assignments of the rotational structure have been made for both HCO Cl-35 and (HCO Cl)-Cl-37. The nu(2) band, whose origin is at 1784.1377 cm(-1) in (HCO Cl)-Cl-35, is predominantly A type with a very weak B-type component. Numerous excited state perturbations occur throughout the band, caused mainly by \Delta K-a\=2 Fermi resonance and \Delta K-a\=1 Coriolis in teractions with the 3(1)5(1) vibrational state. Even though no ''extra '' lines resulting from the perturbations could be assigned, a complet e analysis of the perturbed rotational structure has been possible. Ac curate molecular constants for both the ''bright'' perturbed 2(1) stat e and the ''dark'' 3(1)5(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 K-a are assigned to Coriolis interaction s with the 5(2)6(1) level.