Ultrafast Measurement of Chemical Dynamics in the Condensed Phase Using Transient Vibrational Spectroscopy.

Geminate recombination and vibrational energy relaxation of iodine in the condensed phase are investigated using picosecond transient Raman spectroscopy. The advantages of transient Raman in measuring vibrational energy relaxation and separating different reaction dynamics are demonstrated in this dissertation. Using transient Raman spectroscopy we have for the first time directly observed geminate recombination and vibrational energy relaxation of I$\sb2$ on both X and $A\sp\prime$ $\sp3\Pi\sb{2u}$ states. For the X state, energy relaxation from vibrational levels ranging from $\upsilon$ = 52 to $\upsilon$ = 1 have been observed. These levels correspond to absolute energies of 9,300 to 210 cm$\sp{-1}$ above the zero point in the X potential. The importance of vibrational to vibrational (V-V) energy transfer pathways has been studied and the V-V transfer efficiency is found to be very solvent dependent. The results suggest that the vibrational coupling between the excited iodine oscillator and the solvent is drastically affected by the nature of the normal mode character of the solvent vibration. For the $A\sp\prime$ state, geminate recombination rate is found to be significantly slower than the X state. Evidence is reported which suggests the possible role of iodine atom-solvent complexes in the reaction step leading from photodissociated atoms to formation of the excited state. Electronic relaxation is investigated and is found to be largely the result of the formation and decay of I$\sb2\cdot$Solvent exciplexes. Geminate recombination between iodine radical and iodine molecule is directly observed by dissociating iodine dimer. The geminate recombination time of I + I$\sb2$ in cyclohexane is measured to be 50-100 ps. The slower recombination rate of I + I$\sb2$ in comparison with the fast geminate recombination of I + I provides direct experimental evidence for the existence of an activation barrier for the former reaction. Recombination rates in different solvents decrease in the order n-hexane $>$ neo-hexane $\ge$ deuterated cyclohexane $\approx$ cyclohexane. The lifetime of I$\sb3$ is determined to be greater than 500 ps.