Photodissociation dynamics of I−2 in solution

Abstract

We discuss recent studies of the photodissociation, internal conversion, and vibrational relaxation dynamics of I−2 in ethanol [see J. Chem. Phys. 98, 5375 (1993) for further details]. I−2 was photoexcited at 770 nm (1.6 eV) and probed by ultrafast transient‐absorption spectroscopy at 15 wavelengths between 580 and 950 nm. Using a two‐state spectroscopic model to analyze the effect of vibrational excitation on the I−2 absorption spectrum, we conclude that internal conversion and vibrational relaxation at the top of the well are extremely rapid (<0.3 ps), with loss of the final 0.3 eV of energy (v≤20) occurring on a timescale of ∼4 ps. A simple kinetic scheme for the vibrational relaxation is able to qualitatively account for the observed behavior of the transient‐absorption signals. We find good qualitative agreement with recent molecular‐dynamics simulations of I−2 vibrational relaxation [I. Benjamin and R. M. Whitnell, Chem. Phys. Lett. 204, 45 (1993)]. In contrast to experiment, however, the calculated vibrational relaxation rate is independent of vibrational energy; the calculated rate is too slow at the top of the well and too rapid near the bottom.