Picosecond Excitation and Quantum Beats of Molecules in Supersonic Molecular Beams

Author: Lambert, William Roger

Year: 1983

Degree: Dissertation (Ph.D.)

Advisor: McKoy, Basil Vincent

Committee Members: Zewail, Ahmed H.; Richards, John H.; Janda, Kenneth C.; McKoy, Basil Vincent

Option: Chemistry

DOI: 10.7907/ans8-4e87

Abstract

An understanding of the mechanisms responsible for intramolecular vibrational redistribution in polyatomic molecules has important implications for the efficacy of mode selective laser chemistry. In this thesis the application of picosecond laser excitation and supersonic molecular beam techniques to the investigation of the intramolecular dynamics of large polyatomic molecules is presented. Specifically, the first investigations on quantum interference effects observed in the energy and time resolved fluorescence following coherent picosecond laser excitation to intermediate excess energies in the first excited singlet state manifold are reported. The observation of quantum beats in anthracene and trans-stilbene indicates that at moderate excess energies, the effective density of rotational and vibrational states is quasi-discrete, even though the predominant relaxation dynamics reflect properties associated with the statistical limit. The relationship of internal temperatures and vibronic level structure to the quantum beats is investigated. The potential for using quantum interference effects as a probe of intramolecular energy and phase redistribution is emphasized.

The design and construction of a supersonic molecular beam apparatus developed for the application of pulsed picosecond excitation and fluorescence detection are also presented. In as much as the free jet expansion conditions perturb the internal dynamics of the diluent molecules, an understanding of the hydro- and thermodynamics of the expansion is essential. The influence of the expansion conditions on intermolecular collisional processes complex formation, and internal temperatures is discussed with respect to the intramolecular dynamics of anthracene.

Files