THE VIBRATIONAL-RELAXATION OF I-2 (X-1-SIGMA(+)(G)) IN MESITYLENE

Transient absorption measurements between 400 nm and 570 nm are used t o extract information on the vibrational relaxation of iodine in the c omplexing solvent mesitylene. The well characterized nature of the I-2 -arene complex makes it an excellent prototype for the study of relaxa tion processes in the presence of weak interactions. The data and anal ysis presented here demonstrate the rapid nonexponential vibrational r elaxation of I-2 in the interacting solvent mesitylene. The peak of th e population distribution has dropped below n=10 by 11 ps and n=7 by 1 5.5 ps. The energy relaxation is characterized by a biexponential deca y with time constants of 4.41+/-0.08 ps and 20.3+/-0.7 ps. Quantitativ e comparisons of relaxation in a variety solvents are made by using a simple time-delay to peak absorption characterization of the relaxatio n. 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 chan ge in vibrational frequency as the vibrational frequency of I-2 has on ly a small dependence on the solvent. It is suggested that the vibrati onal relaxation of I-2 in mesitylene through the high-lying levels is better characterized as an ''intramolecular'' vibrational energy redis tribution process than relaxation to a solvent bath. The ultrafast vib rational relaxation occurs via the anharmonic coupling of the I-I stre tching coordinate and the I-MST stretching coordinate of an I-2-MST co mplex. (C) 1998 American Institute of Physics.