Multi-photon dissociation of CHBr 3 at 248 and 193 nm: observation of the electronically excited CH( [formula omitted]) product

Abstract

The CH(A 2 Δ ) multi-photon dissociation channel of CHBr 3 following the excitation at 193 and 248 nm has been studied. Two experimental techniques are applied: time-resolved Fourier transform emission spectroscopy (TR-FTES) and two-laser experiments. The rovibrational state distribution of the nascent CH(A 2 Δ ) fragments generated by multi-photon dissociation of CHBr 3 in a supersonic jet is determined through an analysis of the CH(A 2 Δ –X 2 Π ) emission spectrum obtained by TR-FTES with resolutions between 0.5 and 4.0 cm −1. From the power dependence of the fluorescence the order of the multi-photon dissociation is determined to be two for 193 nm and three for 248 nm radiation. At both wavelengths the rovibrational distributions are power dependent but the effect is most pronounced for 193 nm excitation. At the lowest power investigated, the rotational temperatures are T=1450(70) K for 193 nm and T=2090(90) K for 248 nm multi-photon dissociation. While only v′=0 is observed in the two-photon dissociation at 193 nm, the vibrational temperature at low laser energy is 2400(200) K for three-photon dissociation at 248 nm. In experiments with two spatially separated volumes of the supersonic jet excited by two delayed laser pulses, the dissociation mechanism is shown to be a sequential fragmentation for 193 nm and to involve at least one intermediate species for 248 nm multi-photon dissociation. The possible dissociation channels for the production of CH(A 2 Δ ) by multi-step dissociation of CHBr 3 are discussed with respect to the observed rovibrational distributions.