Above-threshold effects in the photodissociation and photoionization of iodobenzene

The method of velocity map imaging is used to investigate the photoinduced dynamics of iodobenzene in weak laser fields as well as in fields of sufficient intensity to produce multiphoton excitation without dressing the potential energy surfaces. The speed and angular distributions of the recoiling iodine atoms observed with weak fields at 266 and 303 nm are in good agreement with previous studies, showing evidence of dissociation via n,sigma* and pi,pi* states. Density functional theory calculations are used to determine the energies of the excited states. Ion and electron images produced with intense visible radiation (532-609 nm, 3 x 10(10) to 5 x 10(12) W/cm(2)) show both two-photon threshold dissociation and higher order (four- to seven-photon) ionization processes. A unique characteristic of the Ii images obtained with intense visible radiation is a continuous, hourglass-shaped feature peaked at zero kinetic energy, displaying highly nonstatistical speed and angular distributions. A mechanism that is consistent with all of the observations is dissociative autoionization of a superexcited repulsive Rydberg state.