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

Abstract

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.

Improved direct diabatization and coupled potential energy surfaces for the photodissociation of ammonia

“…The photodissociation patterns of the excited NH 3 (Ã 1 A 2 ) strongly depend on the vibrational mode being excited [43]. Figure 7 shows one-dimensional cuts through the potential surfaces for slightly nonplanar geometries with a trisector angle of 77.4 • in both C s and C 1 symmetries; U 22 is dashed.…”

First principles determination of the NH2/ND2($\skew4\tilde A, \skew3\tilde X$Ã,X̃) branching ratios for photodissociation of NH3/ND3 via full-dimensional quantum dynamics based on a new quasi-diabatic representation of coupled ab initio potential energy surfaces

“…28,29 Strong mode and bond selectivity has been found. 24,[30][31][32][33][34][35][36][37] One of the most intriguing findings in the vibrationally mediated ammonia photodissociation is the strong mode selectivity of the NH 2 (Ã 2 A 1 )/NH 2 (X 2 B 1 ) branching ratio. 34,37 Unlike the commonly observed dominance of the ground state NH 2 (X 2 B 1 ) channel, excitation in the antisymmetric stretching mode of NH 3 prior to photolysis resulted in mostly adiabatic dissociation to NH 2 (Ã 2 A 1 ).…”

Vibrationally mediated photodissociation of ammonia: product angular distributions from adiabatic and nonadiabatic dissociation

“…There is a growing body of experiments and calculations on molecules with conical intersections in which the influence of one or more conical intersections on internal conversion or decomposition pathways is apparent [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. We have demonstrated that vibrational excitation can alter the probability of a molecule passing through a conical intersection by changing the trajectory that carries the system into the intersection [20,21]. In general, preparing molecules in a selected vibrational state prior to electronic excitation can alter dissociation dynamics and pathways by allowing the photoexcitation step to reach normally inaccessible regions of the excited-state potential.…”

“…The photolysis of ammonia is a prototypical example of dissociation through a conical intersection between two electronic states, and its study has shaped many ideas about the dissociation involving a conical intersection [20][21][22][23][24][25][26][27][28][29]. The adiabatic decomposition of electronically excited NH 3 (Ã 1 A 00 2 ) produces a hydrogen atom and an electronically excited fragment NH Ã 2 (H þ NH 2 ( 2 A 1 )), and the nonadiabatic decomposition *Corresponding author.…”

“…Nonadiabatic dissociation dominates at low excitation energies and adiabatic dissociation is more important at higher energies. We have previously used sequential vibrational and electronic excitation to prepare selected vibrations in electronically excited ammonia and have shown that H atoms from decomposition of the antisymmetric N-H stretching vibrational state (3 1 ) have very little kinetic energy while those from decomposition of the symmetric stretching state (1 1 ) have recoil energies ranging up to the energetic limit [20,21]. Velocity map ion imaging is able to resolve rotational structure of the partner NH 2 fragments in the total recoil energy distribution and, thus, identify the products as either NH 2 or NH Ã 2 .…”