## LSU Historical Dissertations and Theses

1992

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

Chemistry

John B. Hopkins

#### Abstract

In this dissertation ultrafast photochemistry of Cr(CO)$\sb6$ using picosecond resonance Raman spectroscopy is studied. The advantage of the transient resonance Raman spectroscopy for this study is demonstrated along with a Raman differencing technique. This study is divided into two main parts. The first part deals with the property of the electronic excited state of the Cr(CO)$\sb6.$ The attempt to reveal the excited electronic structure of Cr(CO)$\sb6$ is tried with the help of Albrecht vibronic theory and wave packet description. Reaction coordinate and Jahn-Teller effects have been investigated in detail. Two possible reaction coordinates are found in contrast to the results reported by other group. The $\nu\sb2$ overtones are observed for the first time with both 213 nm and 266 nm excitations. The observation of the $\nu\sb2$ overtones implies that the $\nu\sb2$ can be a reaction coordinate. The appearance of two even overtones of the $\nu\sb7$ also implies that this mode can be a reaction coordinate along with the $\nu\sb2.$ The singlet excited state is regarded as a photodissociative state as a consequence of the appearance of the overtones. The Jahn-Teller distortion study shows that the structure of the excited Cr(CO)$\sb6$ is not a perfect octahedron. The second part deals with the vibrational relaxation of the solvated Cr(CO)$\sb5.$ The transient band is found at 380 cm$\sp{-1}.$ The vibrational relaxation time of this mode is about 80 ps in both cyclohexane and tetrahydrofuran solvents. Vibrational relaxation is confirmed with the help of the complementary dynamics of the stokes and anti-stokes scattering. No direct evidence for the dynamics of solvent coordination, reorientation, and electronic relaxation is observed. However, the initial internal temperature of the molecule was found to be exceedingly hot. This non-equilibrium temperature persists for an unusually long time and is likely to play a major role in solvent coordination and solvent reorientation processes.

120

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