Communication: On the competition between adiabatic and nonadiabatic dynamics in vibrationally mediated ammonia photodissociation in its A band

Non-adiabatic processes play an important role in photochemistry, but the mechanism for conversion of electronic energy to chemical energy is still poorly understood. To explore the possibility of vibrational control of non-adiabatic dynamics in a prototypical photoreaction, namely, the A-band photodissociation of NH3((X) over tilde (1)A(1)), full-dimensional state-to-state quantum dynamics of symmetric or antisymmetric stretch excited NH3((X) over tilde (1)A(1)) is investigated on recently developed coupled diabatic potential energy surfaces. The experimentally observed H atom kinetic energy distributions are reproduced. However, contrary to previous inferences, the NH2((A) over tilde (2)A(1))/NH2((X) over tilde B-2(1)) branching ratio is found to be small regardless of the initial preparation of NH3((X) over tilde (1)A(1)), while the internal state distribution of the preeminent fragment, NH2((X) over tilde B-2(1)), is found to depend strongly on the initial vibrational excitation of NH3((X) over tilde (1)A(1)). The slow H atoms in photodissociation mediated by the antisymmetric stretch fundamental state are due to energy sequestered in the internally excited NH2((X) over tilde B-2(1)) fragment, rather than in NH2((A) over tilde (2)A(1)) as previously proposed. The high internal excitation of the NH2((X) over tilde B-2(1)) fragment is attributed to the torques exerted on the molecule as it passes through the conical intersection seam to the ground electronic state of NH3. Thus in this system, contrary to previous assertions, the control of electronic state branching by selective excitation of ground state vibrational modes is concluded to be ineffective. The juxtaposition of precise quantum mechanical results with complementary results based on quasi-classical surface hopping trajectories provides significant insights into the non-adiabatic process. (C) 2015 AIP Publishing LLC.