Spectroscopic study of the predissociation H(D)CN(C1A′)→ H(D)+ CN(B2Σ+)

Abstract

The predissociation of HCN and DCN has been studied in the wavelength range (130–145) nm by measuring the CN(B²Σ⁺) photofragment excitation spectra, the polarization of the fluorescence, and the relative quantum yields for CN(B²Σ⁺) production from single vibronic levels. By analysing the vibrational structure in the absorption and excitation spectra, and comparing the behaviour of HCN and DCN, two competing predissociation mechanisms have been identified, one of which (quantum mechanical tunnelling) removes all vibrational progressions based on excitation of the C—H stretching mode in HCN. Calculated Franck–Condon factors which reproduce the experimental observations have been obtained, using potential functions for the C—H(D) motion in the intermediate bound and final continuum states which are consistent with the observed frequencies in the intermediate (C¹A′) state and the vibrational energy disposal in the CN(B²Σ⁺) photofragments.