Photoionization Dynamics of Glycine: The First 10 Picoseconds

Abstract

Single photon ionization dynamics of glycine is studied by classical trajectory simulations using the semiempirical PM3 potential surface in “on the fly” calculations. The glycine conformer is assumed to be in the vibrational ground state prior to ionization. Initial conditions for the trajectories are weighted according to the Wigner distribution function computed for that state. Vertical ionization in the spirit of the classical Franck−Condon principle is assumed. The main findings are as follows: (1) The photoionization triggers a fast internal rotation about the C−C bond, with the NH2 group rotating in one direction, and the COOH group rotating in the opposite direction. For the trajectories where the fast rotation occurs, it persists till the end of the simulation (10 ps). The yield for this process is about 6%, suggesting it may be experimentally observable. (2) For many of the trajectories, the photoproduced glycine ion exhibits “hops” between two conformer structures. The rates computed from the dynamics for these conformational transitions differ considerably from RRK predictions. (3) Different behavior of vibrational energy flow is found for different types of modes. There is no significant approach to statistical distribution of the energy throughout the first 10 picoseconds. (4) The preferred dissociation channel is the C−C bond cleavage. Indeed, fragmentation is observed for a few trajectories, one of them shows H atom hopping from the amino group to the carbonyl group prior to dissociation. Another trajectory shows only this hydrogen transfer and the transfer back. Possible experimental implications of some of the findings are briefly discussed.