Influence of vibrations on passage through conical intersections: Velocity map imaging of the photodissociation model of ammonia and phenol.

摘要

We study the vibrationally mediated photodissociation of ammonia and phenol detecting the resulting H atoms using velocity map imaging (VMI). For ammonia, we compare the energy distributions that result from two initially prepared vibrational modes followed by subsequent photodissociation to the A state. Initial preparation of the symmetric N-H stretching state (nu1) followed by excitation to the nu1 in the excited state (11) leads to a bimodal energy distribution forming products up to the energetic limit. Initial preparation of the antisymmetric N-H stretching state (nu3) followed by excitation to the 3 1 state leads to a narrow energy distribution at low translational energies. The H-atom images obtained with VMI resolve the rotational structure of the partner NH2 fragment. We use term values of NH2 to determine that the symmetric N-H stretching state forms ground state products and the antisymmetric N-H stretching state forms nearly exclusively excited state NH2.;For the photodissociation of phenol to the 1pipi* state, we find a change in the H-atom images following the initial excitation of the fundamental of the O-H stretch. In these experiments, we use phenol-d 5 instead phenol to reduce H-atom background signal. At excitation energies greater than 45 000 cm-1, we find that a higher energy peak at 12 000 cm-1 in the total kinetic energy release spectrum decreases. This peak is due to direct dissociation after passage through a conical intersection to the 1pisigma* state and subsequent dissociation through a second conical intersection with the ground state forming H and ground state phenoxyl radicals. We assign this change to production of excited state phenoxyl as the phenol molecule stays on the adiabatic curve when in reaches the second conical intersection. At lower excitation energies ( 45 000 cm-1), we find that the high energy peak in the energy release spectra remains, but shifts down in energy to about 9000 cm -1. We assign this feature to internal conversion to high overtones of the O-H stretch in the ground state followed by dissociation.