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Title: 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 NH{sub 3}(X{sup ~1}A{sub 1}), full-dimensional state-to-state quantum dynamics of symmetric or antisymmetric stretch excited NH{sub 3}(X{sup ~1}A{sub 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 NH{sub 2}(A{sup ~2}A{sub 1})/NH{sub 2}(X{sup ~2}B{sub 1}) branching ratio is found to be small regardless of the initial preparation of NH{sub 3}(X{sup ~1}A{sub 1}), while the internal state distribution of the preeminent fragment, NH{sub 2}(X{sup ~2}B{sub 1}), is found to depend strongly on the initial vibrational excitation of NH{sub 3}(X{sup ~1}A{sub 1}). The slow H atoms in photodissociation mediated by the antisymmetric stretch fundamental state are due to energy sequestered in the internally excited NH{sub 2}(X{sup ~2}B{sub 1}) fragment, rather than in NH{sub 2}(A{sup ~2}A{sub 1}) as previously proposed. The high internal excitation of the NH{sub 2}(X{sup ~2}B{sub 1}) fragment is attributed to the torques exerted on the molecule as it passes through themore » conical intersection seam to the ground electronic state of NH{sub 3}. 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.« less
Authors:
 [1] ; ;  [2] ;  [3] ;  [1] ;  [4] ;  [5]
  1. Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Institute of Theoretical and Computational Chemistry, Nanjing University, Nanjing 210093 (China)
  2. Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218 (United States)
  3. Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, Sichuan 610065 (China)
  4. (China)
  5. Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131 (United States)
Publication Date:
OSTI Identifier:
22416195
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 142; Journal Issue: 9; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ADIABATIC PROCESSES; AMINO ACIDS; AMMONIA; ATOMS; BRANCHING RATIO; CONVERSION; DISSOCIATION; EXCITATION; GROUND STATES; KINETIC ENERGY; MOLECULES; PHOTOCHEMISTRY; PHOTOLYSIS; POTENTIAL ENERGY; QUANTUM MECHANICS; SURFACES; TRAJECTORIES