Dissociation of internal energy-selected methyl bromide ion revealed from threshold photoelectron-photoion coincidence velocity imaging

摘要

Dissociative photoionization of methyl bromide (CH_3Br) in an excitation energy range of 10.45-16.90 eV has been investigated by using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. The coincident time-of-flight mass spectra indicate that the ground state X~2E of CH_3Br~+ is stable, and both A~2A_1 and B~2E ionic excited states are fully dissociative to produce the unique fragment ion of CH_3~+. From TPEPICO 3D time-sliced velocity images of CH_3~+ dissociated from specific state-selected CH_3Br+ ion, kinetic energy release distribution (KERD) and angular distribution of CH_3~+ fragment ion are directly obtained. Both spin-orbit states of Br(~2P) atom can be clearly observed in fast dissociation of CH_3Br~+(A~2A_1) ion along C-Br rupture, while a KERD of Maxwell-Boltzmann profile is obtained in dissociation of CH_3Br~+(B~2E) ion. With the aid of the re-calculated potential energy curves of CH_3Br~+ including spin-orbit coupling, dissociation mechanisms of CH_3Br+ ion in A2A1 and B2E states along C-Br rupture are revealed. For CH_3Br~+(A~2A_1) ion, the CH_3~+ + Br(~2P_(1/2)) channel is occurred via an adiabatic dissociation by vibration, while the Br(~2P_(3/2)) formation is through vibronic coupling to the high vibrational level of X~2E state followed by rapid dissociation. C-Br bond breaking of CH_3Br~+(B~2E) ion can occur via slow internal conversion to the excited vibrational level of the lower electronic states and then dissociation.