Photodissociation of superexcited states of hydrogen iodide: A photofragment imaging study using resonant multiphoton excitation at 13.39 and 15.59 eV

Jet-cooled HI has been excited using a resonant three-photon excitation scheme to energies corresponding to 13.39 and 15.59 eV. Analysis of velocity mapping images of the iodine atom fragments allowed the identification of the HI excited states at these energies as the (4S-1/2) 6p superexcited state and the repulsive 4S-1/2 state of HI+, respectively. Following excitation at 13.39 eV, we observe formation of iodine atomic fragments through the H(2S) + I[(3PJ) 6p] (J = 0, 1, 2) fragment channels, as well as through the H(2S) + I[(1D2) 6p] channel. This observation is explained by extensive nonadiabatic interactions between the (4S-1/2) 6p state with the repulsive (4P1/2) 6p state and the weakly bound (A 2S+) 6p state. In support for this proposed dissociation mechanism excitation of the corresponding ionic 4S-1/2 state at 15.59 eV also results in formation of comparable quantities of I+ in its 1D2, 3P0,1, and 3P2 levels indicating again extensive nonadiabatic interactions with other repulsive curves. A similar mechanism based on the local interaction of the 4S-1/2 state with the A 2S+ and the 4P1/2 state is proposed.