## LSU Historical Dissertations and Theses

1993

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Chemistry

Erwin D. Poliakoff

#### Abstract

We use measurements of dispersed fluorescence from electronically excited photoions to study fundamental aspects of intramolecular dynamics. Our experimental innovations make it possible to obtain highly resolved photoionization data that offer qualitative insights into molecular scattering. In particular, we obtain vibrationally resolved data to probe coupling between the electronic and nuclear degrees of freedom by studying the distribution of vibrational energy among photoions. Vibrationally resolved branching ratios are measured over a broad spectral range of excitation energy and their non-Franck-Condon behavior is used as a tool to investigate two diverse aspects of shape resonant photoionization. First, vibrational branching ratios are obtained for the SiF$\sb4$ 5$a\sb1\sp{-1}$ and CS$\sb2$ 5$\sigma\sb{\rm u}\sp{-1}$ photoionization channels to help elucidate the microscopic aspects of shape resonant wavefunction for polyatomic molecules. It is shown that in such molecules the shape resonant wavefunction is not necessarily attributable to a specific bond in the molecule. Second, the multichannel aspect of shape resonant photoionization dynamics, reflected in continuum channel coupling, is investigated by obtaining vibrational branching ratios for the 2$\sigma\sb{\rm u}\sp{-1}$ and 4$\sigma\sp{-1}$ photoionization of the isoelectronic molecules N$\sb2$ and CO, respectively. These data indicate that effects of continuum coupling may be widespread. We also present the first set of rotationally resolved data over a wide energy range for the 2$\sigma\sb{\rm u}\sp{-1}$ photoionization of N$\sb2$. These data probe the partitioning of the angular momentum between the photoelectron and photoion, and highlight the multicenter nature of the molecular potential. These case studies illustrate the utility of dispersed fluorescence measurements as a complement to photoelectron spectroscopy for obtaining highly resolved data for molecular photoionization. These measurements makes it possible to probe intrinsically molecular aspects, such as the vibration and rotation, of photoionization dynamics over an extended spectral range when used in conjunction with synchrotron radiation as the exciting source. Furthermore, the high resolution made possible by this technique provides high selectivity for accessing weaker ionization channels which are the ones strongly affected by resonant activity, and the present study repeatedly stresses the importance of this capability in discovering and deciphering new trends in resonant molecular ionization dynamics.

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