Photodissociation of the Ethyl Bromide Cation at 355 nm by Means of TOF-MS and Ion Velocity Imaging Techniques

Photodissociation of ethyl bromide cation, C₂H₅Br⁺, as well as its isotopomers CH₃CD₂Br⁺ and CD₃CH₂Br⁺ has been studied at 355 nm by means of time of flight mass spectrometry (TOF-MS) and ion velocity imaging techniques. The TOF mass spectrum of fragment ions from C₂H₅Br⁺+hν_355 nm shows three peaks at m/e of 29 (C₂H₅⁺), 28 (C₂H₄⁺) and 27 (C₂H₃⁺). The TOF spectra for the two isotopomers CH₃CD₂Br⁺ and CD₃CH₂Br⁺ clearly show that all possible H-loss patterns are operative. The observation indicates that both three- and four-center intermediates are involved for both C₂H₄⁺ and C₂H₃⁺ channels. Images are recorded for C₂H₅⁺, C₂H₅⁺ + C₂H₄⁺ and C₂H₃⁺ ions, from which translational energy and angular distributions are derived using back-projection algorithm. The C₂H₅⁺ + Br channel could be well described by a parallel excitation followed by fast fragmentation with β∼1.7. C₂H₄⁺+HBr channel 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 β∼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 out the possibility of the secondary dissociation of C₂H₅⁺ or C₂H₄⁺. Reaction C₂H₅Br⁺+hν_355 nm → C₂H₃⁺+H₂+Br could be considered as a concerted three-body dissociation process. Before Br atom moves out of the interaction range the H₂ molecule is already formed. Morework is being done to reveal each of the H-loss channels. By measuring negative ions from C₂H₅Cl and CH₃Br we show that the previously proposed ion-pair mechanism for the formation of C₂H₅⁺ from ethyl bromide at 118 nm is wrong [J. Phys. Chem. A 101 (1997) 1222].