Spectroscopic study of the acetylene species

Abstract

This thesis describes recent spectroscopic studies of acetylene and its HCCD isotopomer. Three different kinds of experiments have been performed: Laser-Induced Fluorescence (LIF), Dispersed Fluorescence (DF), and Stimulated Emission Pumping (SEP). Each experiment has yielded new discoveries in the vibrational and rotational dynamics of acetylene in its ground electronic state (X) state and its first excited singlet state (A) state. The goal of my research at MIT has focused on making contributions to the investigation of isomerization processes in acetylene. These include the linear vinylidene isomerization process on the X state and the trans -+ cis isomerization process on the A state. I have reasons to believe that the local bending motion is the major contributor in both processes. LIF spectroscopic study of the A state of acetylene has led to new assignments of 23' and 13' bands which involve a symmetry. The frequency of the upper state symmetric CH stretch mode normal, v, is determined to be 2880.5 cm-1, 60 cm-llower than the presently-accepted value of 3040.6 cm-1[104, 106]. DF spectroscopic study of the X state of acetylene has revealed a collisional K-changing process in the A state. In order to model theoretically the intensities measured from the present DF dataset, an local mode based Franck-Condon calculation scheme is given, and the results are shown to agree with the nodal structures exhibited in the experimental observations to within two quanta in both the CC stretch and the trans-bending coordinates. Improvements on the FC calculation are expected to aid in the direct assignment of the HCCD DF spectra, which are more complicated than those from the symmetric isotopomer, HCCH, due to reduced symmetry.

(cont.) SEP spectroscopic study of the X state of acetylene has uncovered a Coriolis coupling between the v' + 4v4' and 7v4' vibrational states of acetylene. Modelling this coupling through a least-square fit gives a Coriolis coupling parameter, = 0.00057 cm-1, but such a coupling is expected to have negligible effects on the single-bright-state-per-polyad assumption in the overall vibrational dynamics in the X state, as sampled by DF as well as SEP spectra.