Geminate recombination and relaxation of molecular iodine

Abstract

Picosecond absorption kinetics of I2 in several room temperature solvents have been obtained. Probe wavelengths of 500, 580, and 640 nm were used following excitation with 30 ps pulses of 532, 630, or 683 nm light. The results indicate that population of the electronically excited A(3π1u) and A′(3π2u) states occurs rapidly (<30 ps) following dissociation. Analysis of transient absorption intensities indicates that repopulation of the ground electronic state requires about 30–50 ps. We propose that the difference in population times may be understood in terms of trapping in weakly bound electronic states (perhaps 3π0−u) which relax to populate the ground electronic state, or in terms of slow atom recombination into the ground state. The results show that for I2 in CCl4, X state repopulation and vibrational relaxation occur on roughly comparable time scales. The results also show that the recombination dynamics are determined primarily by the nature of dissociative state, not the recoil energy. Vibrational relaxation rates through the ground state mainfold were interpreted in terms of vibration to translation energy transfer as given by the Schwartz, Slawsky, Herzfeld theory. This was found to work quite well for weakly interacting solvents such as CFCl3, C2Cl3F3, and CCl4. This simple theory breaks down for more strongly interacting solvents such as CH2Cl2 and CHCl3.