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(X̃(1)A1), full-dimensional state-to-state quantum dynamics of symmetric or antisymmetric stretch excited NH3(X̃(1)A1) 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(Ã(2)A1)/NH2(X̃(2)B1) branching ratio is found to be small regardless of the initial preparation of NH3(X̃(1)A1), while the internal state distribution of the preeminent fragment, NH2(X̃(2)B1), is found to depend strongly on the initial vibrational excitation of NH3(X̃(1)A1). The slow H atoms in photodissociation mediated by the antisymmetric stretch fundamental state are due to energy sequestered in the internally excited NH2(X̃(2)B1) fragment, rather than in NH2(Ã(2)A1) as previously proposed. The high internal excitation of the NH2(X̃(2)B1) 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.
Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Institute of Theoretical and Computational Chemistry, Nanjing University, Nanjing 210093, China.
Changjian
C
Xie
Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA.
Xiaolei
X
Zhu
Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, Sichuan 610065, China.
Jianyi
J
Ma
Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA.
David R
DR
Yarkony
Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Institute of Theoretical and Computational Chemistry, Nanjing University, Nanjing 210093, China.
Daiqian
D
Xie
Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA.
Hua
H
Guo