Photofragmentation of CH3Br in the red wing of the first continuum absorption band

Abstract

Photodissociation dynamics of CH₃Br following excitation in the red wing (240–280 nm) of the Ã-band absorption are investigated by state-selective resonance enhanced multiphoton ionization probing and photofragment time-of-flight spectroscopy of Br and CH₃ products. Evidence of significant methyl fragment rotational excitation is found in the decay channels leading to Br(²P_3/2) and to Br*(²P_1/2). Vibrational excitation is found in both channels, but is greater, with a population inversion, in that accompanying formation of ground state Br. This latter channel displays an unexpectedly isotropic fragment angular distribution, while the Br* channel has, overall, a parallel angular distribution especially for formation of the lowest v₂=0, 1 levels of CH₃. These observations are shown to be consistent with a strong non-adiabatic coupling between spin–orbit states of CH₃Br with a distortion from C_3v geometry in the interaction region.