Photodissociation of the ethyl bromide cation at 355 nm by means of TOE-MSand ion velocity imaging techniques

Photodissociation of ethyl bromide cation, C2H5Br+, as well as its isopomer s CH3CD2Br+ and CD3CH2Br+ has been studied at 355nm by means of time of fli ght mass spectrometry (TOF-MS) and ion velocity imaging techniques. The TOF mass spectrum of fragment ions from C2H3Br++hv(355 nm) shows three peaks a t m/e of 29(C2H5+), 28(C2H4+) and 27(C2H3+). The TOF spectra for the two is otopomers CH3CD2Br+ and CD3CH2Br+ clearly show that all possible H-loss pat terns are operative. The observation indicates that both three- and four-ce nter intermediates are: involved for both C2H4+ and C2H3+ channels. Images are recorded for C2H5+, C2H5+ + C2H4+ and C2H3+ ions, from which translatio nal energy and angular distributions are derived using back-projection algo rithm. The C2H5+ + Br channel could be well described by a parallel excitat ion followed by fast fragmentation with beta similar to 1.7. C2H4+ + HBr ch annel has a very small translational energy release and a nearly isotropic angular distribution. Most of the energy went into the vibrational degrees of freedom of both C2H4+ and HBr. The anisotropy parameter beta similar to 1.0 is found for C2H3+, whose translational energy is slightly higher than C2H4+, but much lower than C2H5+. From these results, we can rule our the p ossibility of the secondary dissociation of C2H5+ or C2H4+. Reaction C2H5Br + + h nu (355 nm) --> C2H3+ + H-2 + Br could be considered as a concerted t hree-body dissociation process. Before Br atom moves out of the interaction range the H-2 molecule is already formed. More work is being done to revea l each of the H-loss channels. By measuring negative ions from C2H5Cl and C H3Br we show that the previously proposed ion-pair mechanism for the format ion of C2H5+ from ethyl bromide at 118 nm is wrong [J. Phys. Chem. A 101 (1 997) 1222].