Investigations of Ultrafast Dynamics of Photoexcited Hemeproteins and Iodomethane in Condensed Phase Using Transient Raman and Absorption Spectroscopies.

Abstract

This dissertation describes the development and application of transient Raman and absorption spectroscopies using 8 ps laser pulses for the study of photoexcitation dynamics of hemeproteins and CH$\sb3$I in the condensed phase. Transient Raman spectroscopy is a powerful ultrafast technique. Hemeproteins have been studied using this technique. The dynamics of vibrational cooling, geminate recombination, and structural change upon the photoexcitation of hemeproteins have been unambiguously separated. The vibrationally hot deoxyhemoglobin, oxyhemoglobin and photoproduct deoxy-liked hemoglobin have been directly detected. The deconvolved time constant of vibrational cooling for the species is about 2-5 ps. Direct evidence for the lack of geminate recombination and structural change occurring on a 2-100 ps time scale has been shown. The slow geminate recombination between the heme and ligand has been observed; the time constant is about 1000 $\pm$ 500 ps. The reaction coordinate for ligand rebinding has been addressed. Transient absorption spectroscopy is another useful technique. For the first time, CH$\sb3$I has been studied in solution using this technique. The dynamics following the photodissociation of CH$\sb3$I have been observed. The dynamics consist of two components. The fast recovery is assigned to the vibrational cooling of hot CH$\sb3$ and the deconvolved time constant is 14 ps, 25 ps, and 27 ps in cyclohexane, heptane, and hexane, respectively. The vibrationally hot CH$\sb3$ directly couples with the lower-lying vibrational modes of the solvent molecules and the relaxation pathway is most likely by V $\to$ V. The geminate recombination between CH$\sb3$ and I is too fast to be detected using our 8 ps laser pulse. The geminate recombination of CH$\sb3$ and I* is too slow to affect the observation of vibrational cooling dynamics of CH$\sb3.$ Other reactions are responsible for the slow recovery dynamics.