Title

Femtosecond Dynamics of Small Polyatomic Molecules in Solution: A Combined Experimental and Computational Approach

Date of Award

2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Alexander Tarnovsky, PhD (Committee Chair)

Second Advisor

Massimo Olivucci, PhD (Committee Co-Chair)

Third Advisor

R. Marshall Wilson, PhD (Committee Member)

Fourth Advisor

H. Peter Lu, PhD (Committee Member)

Fifth Advisor

Rex Lowe, PhD

Abstract

A detailed understanding of condensed-phase ultrafast photo-induced chemical reaction dynamics is still sought after. This is because of the intrinsic complexity of liquid-phase photophysical and photochemical phenomena arising from competing intra- and intermolecular processes. Such processes often take place on a timescale of a few femtoseconds to several tens of picoseconds. In this work, the model photochemical processes used to investigate ultrafast photo-induced reaction dynamics in solution are bond-breaking and bond making reactions. The model compounds are di- and poly-halogenated methanes. The gas-phase photochemistry of these small molecules is thoroughly investigated, which enables to draw a direct comparison to the photophysical and photochemical dynamics in solution. Moreover, in contrast to the thoroughly investigated di- and triatomic molecular systems, more vibrational degrees of freedom are available both to the model parent molecules and nascent polyatomic radical species. Thus, a detailed mapping of the photochemical reaction paths of these systems develops into a comparative advantage, revealing different couplings between the reactive modes and other dark states in a far-from-equilibrium situation. The complexity of the encountered ultrafast phenomena requires the use of several experimental and computational approaches. Results of femtosecond transient absorption, picosecond transient resonance Raman, and matrix isolation experiments in concert with ground and excited state ab initio calculations are discussed in this context. The findings from this work illustrate the power of a solvent environment in (i) altering the topology of ground and excited state potential energy surfaces, and (ii) leading to different photoproducts through intermediates otherwise absent from gas-phase studies.