Molecular elimination in photolysis of fluorobenzene at 193 nm: Internal energy of HF determined with time-resolved Fourier-transform spectroscopy

Abstract

Following photodissociation of fluorobenzene (C(6)H(5)F) at 193 nm, rotationally resolved emission spectra of HF(1<or= v <or= 4) in the spectral region 2800-4000 cm(-1) are detected with a step-scan Fourier-transform spectrometer. In the period 0.1-1.1 mus after photolysis, HF(v <or= 4) shows similar Boltzmann-type rotational distributions corresponding to a temperature approximately 1830 K; a short extrapolation from data in the period 0.1-4.1 mus leads to a nascent rotational temperature of 1920+/-140 K with an average rotational energy of 15+/-3 kJ mol(-1). The observed vibrational distribution of (v = 1):(v = 2):(v = 3):(v = 4) = (60+/-7):(24+/-3):(10.5+/-1.2):(5.3+/-0.5) corresponds to a vibrational temperature of 6400 +/- 180 K. An average vibrational energy of 33 +/- 9/3 kJ mol(-1) is derived based on the observed population of HF(1 <or= v <or= 4) and an estimate of the population of HF(v = 0) by extrapolation. The observed internal energy distribution of HF is consistent with that expected for the four-center (alpha,beta) elimination channel. A modified impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts satisfactorily the rotational excitation of HF. We also compare internal energies of HF observed in this work with those from photolysis of vinyl fluoride (CH(2)CHF) and 2-chloro-1,1-difluoroethene (CF(2)CHCl) at 193 nm.