Laser spectroscopy of acetylene

Abstract

The purpose of this thesis is to explore recent advances in the spectroscopy of acetylene. Acetylene is among the most-studied molecules, and an astoundingly large volume of work has been done on it. Highly excited S1 acetylene suers from many effects that complicate a thorough understanding of it. For instance, isomerization occurs between trans- and cis-bent geometries. Any models, including the most successful polyad models that have been used to study S1 acetylene for many years, that are based on the more stable trans-bent structure are doomed to failure at the energy of isomerization is approached. A further problem is experimental, rather than theoretical. The "interesting" region of S1 dynamics, namely the energy region in the vicinity of the isomerization barrier, is dissociative. Near the cis-trans barrier, the electronic surface interacts with a nearby dissociative curve, and molecules tunnel through the barrier and dissociate. The lifetime constraints are addressed with a detection technique that, to a certain point, is insensitive to predissociative lifetimes, Photofragment Fluorescence Action Spectroscopy (PFAS). PFAS detection involves the photofragmentation of excited acetylene, at a faster rate than the molecules dissociate. The excited photofragments themselves fluoresce, and this fluorescence is collected as the signal. Using PFAS, the most detailed spectra of high-energy S₁ ever have been collected. The additional insight into the structure and dynamics of acetylene, both that have already been analyzed and that require further work, are discussed in this thesis.