Molecular dynamics of chemical reactions in solution: Experimental picosecond transient spectra for I2 photodissociation

Abstract

Picosecond transient electronic absorption spectroscopy is used to investigate the molecular dynamics of I2 reaction in solution following dissociative excitation into the A state. Our aim is to provide a sufficiently detailed set of experimental observations so that theoretical treatments of the molecular dynamics of a solution reaction can be rigorously tested. We report here measurements obtained in 13 solvents, pumping at 680 nm with four probe wavelengths of 840, 690, 630, and 595 nm. The time scale of the transient electronic spectral response varies considerably among the different solvents. For a given solvent, the response in general varies strongly with probe wavelength, being slower for higher probe photon energy. This indicates that the spectral response times are not due to recombination times, but are more likely due to the time for vibrational or perhaps electronic decay of already recombined molecules. This conclusion is reinforced by the presence of transient absorption in the near infrared, where vibrationally excited molecules would be expected to absorb. The connection to vibrational decay is supported by the correlation for the different solvents between observed transient response times and the presence of solvent vibrational modes in the frequency range needed to vibrationally relax hot I2.