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

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(2P3/2) and to Br*(2P1/2). 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 C3v geometry in the interaction region.