Methyl iodide A-band decomposition study by photofragment velocity imaging

The methyl iodide A-band photodissociation process CH3I+h nu-->CH3(upsilon,N,K)+I(P-2(3/2)), I*(P-2(1/2)) has been studied in a cold molecular beam. Full three-dimensional state-specific speed and angular distributions of the nascent fragments were recorded using (2+1) resonance-enhanced multi-photon ionization (REMPI) and velocity imaging, a new variant of ion imaging. By combining the I* quantum yield and anisotropy parameters for both I and I* channels, the relative absorption strength to the contributing electronic states ((3)Q(0), (3)Q(1) and (1)Q(1)) as well as the probability for curve crossing ((3)Q(0)-->(1)Q(1)) are determined for excitation wavelengths across the full A band (240-334 nm). Parallel excitation to the (3)Q(0), state turns out to dominate the A band even more than previously thought. (C) 1998 American Institute of Physics.