Polarised photofluorescence excitation spectroscopy. Part 2.—Vacuum ultraviolet photodissociation of HCN and BrCN

Abstract

The new technique of polarised photofluorescence excitation spectroscopy has been applied to the vacuum u.v. photodissociations HCN, BrCN [graphic omitted] H, Br + CN(B²∑⁺) and a theoretical analysis of the technique has been developed. By monitoring the polarised excitation spectrum of the fluorescent and rotationally excited CN fragments the following conclusions have been reached. (i) The fluorescence excitation spectra closely resemble the absorption spectra. (ii) The primary excited states of HCN and BrCN that predissociate to form CN(B²∑⁺) are all non-linear and have the symmetry A′: transitions into the predissociating levels of HCN(C¹A′) are predominantly type B, while those into the “B” and “C” states of BrCN are type A. (iii) The predissociative lifetimes of the “B” and “C” states of BrCN are 0.6 ps, and vibronic levels in which the C—Br stretching mode is excited are shorter lived than those where the C—N mode is excited. (iv) The slight non-linearity of the predissociated Rydberg states in BrCN lying at λ > 125 nm may be caused by a Renner–Teller interaction.