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 A'A(2)-state ammonia. The competition between adiabatic dissociation forming excited-state NH2((2)A(1)) + H and nonadiabatic dissociation leading to ground-state NH2(B-2(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 NH2((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.