The vibrational relaxation of I2 (X 1Σg+) in mesitylene

Transient absorption measurements between 400 nm and 570 nm are used to extract information on the vibrational relaxation of iodine in the complexing solvent mesitylene. The well characterized nature of the I2-arene complex makes it an excellent prototype for the study of relaxation processes in the presence of weak interactions. The data and analysis presented here demonstrate the rapid nonexponential vibrational relaxation of I2 in the interacting solvent mesitylene. The peak of the population distribution has dropped below n=10 by 11 ps and n=7 by 15.5 ps. The energy relaxation is characterized by a biexponential decay with time constants of 4.41±0.08 ps and 20.3±0.7 ps. Quantitative comparisons of relaxation in a variety solvents are made by using a simple time-delay to peak absorption characterization of the relaxation. The initial 4.4 ps decay in mesitylene is significantly faster than the time scales for relaxation in noninteracting hydrocarbon solvents. The difference in the relaxation rate cannot be attributed to a change in vibrational frequency as the vibrational frequency of I2 has only a small dependence on the solvent. It is suggested that the vibrational relaxation of I2 in mesitylene through the high-lying levels is better characterized as an “intramolecular” vibrational energy redistribution process than relaxation to a solvent bath. The ultrafast vibrational relaxation occurs via the anharmonic coupling of the I–I stretching coordinate and the I-MST stretching coordinate of an I2-MST complex.