UC Berkeley

The ultrafast photodissociation dynamics of gas phase CH3Br molecules in the red wing of the A-band, i.e. the first molecular adsorption continuum, is investigated by pump-probe spectroscopy. The experiment employs femtosecond laser pulses at 266 nm in the ultraviolet to dissociate the molecule and high order harmonic extreme ultraviolet pulses in conjunction with time-of-flight mass spectrometry to ionize and detect the fragments. A dissociation time of 116 ± 25 fs is obtained from the pump-probe risetime of the Br+ ion signal, which originates from formation of either or both Br(2P3/2) or Br(2P1/2). The timescale is in a good agreement with the previously calculated A-band dissociation time of CH3Br using the anisotropy parameter β deduced from angular photodissociation experiments. Based on classical molecular dynamics simulations and previous spectroscopic information, the most likely pathway is dissociation of the CH3Br molecule via the 3Q0 state of the A-band, which correlates with the formation of the spin-orbit excited bromine fragment.