Anisotropy measurements of solvated HgI2 dissociation: Transition state and fragment rotational dynamics

Pump-probe anisotropy measurements of HgI2 photodissociation in ethanol were carried out. The reactive motion from electronically excited HgI2 toward HEI and I is clearly evident in the signal. The subsequent randomization of the HgI orientational distribution occurs in the biphasic manner typical for some other diatomic molecules, with a 500-fs component that has an inertial motion contribution and a slow (8-10-ps) diffusive component. Interpretations of these regions were assisted by molecular dynamics simulations, which also were used to show that the rotational energy relaxation would be very fast in this system. The magnitudes of the anisotropies at different pump wavelengths show that the near-UV absorption of HgI2 involves overlapping (1) Sigma(+) --> (1) Sigma(-) and (1) Sigma(+) --> (1) Pi transitions, consistent with magnetic circular dichroism (MCD) measurements. The anisotropy decay shows oscillations which an proposed to arise from oscillations in the volume and moment of inertia as the vibrational wavepacket moves in the anharmonic HEI potential.