Real-Time Observation and Analysis of Coherence and Alignment in Molecular Systems: Isolated Molecules and Chemical Reactions

Citation

Baskin, John Spencer (1990) Real-Time Observation and Analysis of Coherence and Alignment in Molecular Systems: Isolated Molecules and Chemical Reactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/H8TF-EE69. https://resolver.caltech.edu/CaltechETD:etd-11252003-112746

Abstract

Picosecond time-resolved rotational coherence spectroscopy is developed as a probe of excited state rotational level structure and alignment. The measurement technique employs a combination of supersonic cooling by molecular beam expansion, coherent picosecond pulsed laser excitation, and time-resolved and polarization-analyzed detection of spectrally dispersed fluorescence. The requisite measurement system response time of approximately 50 picoseconds is attained using time-correlated single photon counting and a microchannel plate detector.

In the case of purely rotational coherence (PRC), i.e., when rotation may be treated in the rigid rotor approximation, analysis of the polarization-analyzed fluorescence provides direct information about the rotational constants and structure of the molecule's excited vibronic state. This method of structural determination of excited states has the inherent advantages over conventional frequency-domain spectroscopy of sub-Doppler resolution and insensitivity to ground state structure. As a result, it is particularly valuable in investigations of large molecules and complexes. Analyses of PRC measurements on eight different molecular systems are detailed in this thesis. These provide illustrative examples of various aspects of the technique while permitting the derivation of new information about the excited states of six of the eight molecules or complexes studied. Principal among the findings are values of the sum of rotational constants B' and C' of the t-stilbene S₁ electronic state (B'+ C' = 0.5132 ± .0008 GHz) and of all three S₁ rotational constants of anthracene.

We also report measurements of time-resolved and polarization-analyzed fluorescence as a function of excess vibrational energy in the S₁ electronic states of both t-stilbene and anthracene. We are able to distinguish the contribution of purely rotational coherence from the contributions of purely vibrational (or rovibrational) coherence to the evolution of fluorescence from the vibrationally excited molecule. Our results provide a test of the extent of coupling between vibrational and rotational motion and its influence on intramolecular vibrational energy redistribution.

Measurements of polarization-analyzed fluorescence of dissociation products demonstrate that rotational coherence of the reagent can be transferred to its fragments. In order to interpret the results of these and related experiments, a classical model of fluorescence anisotropy in prompt, impulsive dissociation reactions is developed.

Thesis Files