Competition between Adiabatic and Nonadiabatic Pathways in the Photodissociation of Vibrationally Excited Ammonia

Vibrationally mediated photodissociation combined with Doppler spectroscopy and time-of-flight detection of H-atoms provides information on the photofragmentation dynamics from selected rovibrational states of à 1 A 2 ‘ ‘-state ammonia. The competition between adiabatic dissociation forming excited-state NH 2 ( 2 A 1 ) + H and nonadiabatic dissociation leading to ground-state NH 2 ( 2 B 1 ) + H products changes drastically for dissociation from different parent levels prepared by double-resonance excitation. The H-atom speed distributions suggest that the nonadiabatic dissociation channel is the major pathway except for dissociation from the antisymmetric N−H stretching (3 1 ) parent level, which forms exclusively NH 2 ( 2 A 1 ) + H. The energy disposal depends strongly on the parent state with as little as 2% of the available energy channeled into translational energy for dissociation from the 3 1 state.