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Photoionization dynamics and ion state distributions in single-photon and resonance enhanced multiphoton ionization of molecules

Citation

Braunstein, Matthew (1991) Photoionization dynamics and ion state distributions in single-photon and resonance enhanced multiphoton ionization of molecules. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-06182007-133055

Abstract

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This thesis presents results of theoretical studies of single-photon ionization and resonance enhanced multiphoton ionization (REMPI) of several small molecules. These studies parallel several recent experiments which use tunable sources of radiation to photoionize molecules and measure the resulting electronic, vibrational, and rotational population of ions via photoelectron spectroscopy. The objective of this thesis is to examine the underlying dynamics between the ion and the ejected electron and to understand how these interactions affect photoion state distributions. In particular, this work focuses on the presence and influence of localized quasi-bound states of the photoelectron called "shape resonances" which significantly influence these ion distributions. A key feature of these studies is the use of accurate Hartree-Fock photoelectron wave functions determined from the Schwinger variational method. This method provides a photoelectron wave function determined in the static-exchange and non-central field of the molecular ion. Use of such photoelectron wave functions is crucial in an accurate determination of transition moments and ion state populations.

The first part of the thesis examines shape resonances in the photoionization of O2. Studies reported here include investigations of branching ratios of electronic multiplet states in the [...] and [...] photoionization of O2 and a comparison of photoionization of the singlet states, a [...] and [...], with that of the ground state of O2. These studies show that the electronic exchange interaction between the ion core and the photoelectron in shape resonant energy regions profoundly affects the electronic state distributions of the molecular ion. We also report vibrational branching ratios in the single-photon ionization of O2, and in REMPI of O2 via the [...] Rydberg state. In these studies, we find that a shape resonance causes a dependence of the electronic transition moment on the molecular geometry leading to non-Franck-Condon ion vibrational distributions and a dependence of the rotational branch intensity on the ion vibrational state.

The second part of this thesis examines shape resonances in other molecules, focusing on the more general aspects of the photoionization dynamics. Here we present studies of the vibrational state distributions in the [...] photoionization of the polyatomic N2O, where a shape resonance causes non-Franck-Condon vibrational state distributions, the degree of which depends on the nuclear displacements involved and whether the shape resonance is localized on a particular bond. We also study the photoionization dynamics of the valence shell of Cl2, where a shape resonance is also seen. In contrast to what has been seen in other molecules, this shape resonance is not oriented along the molecular axis, but perpendicular to it. The shape resonance therefore is less sensitive to vibrational motion and does not influence vibrational distributions. Finally, we present studies of the K-shell ionization of CO. Studies in this energy region have assumed a new importance with the development of tunable X-ray synchrotron soures. Here, electronic relaxation in the production of a K-shell hole is seen to significantly influence the photoionization cross section in shape resonant energy regions. The mathematical framework is given to separate and analyze the effects of electronic relaxation in the photoionization cross section and calculate K-shell satellite spectra from first principles.

The results of examination of all these photoionization processes are discussed in the context of recent experiments.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • McKoy, Basil Vincent
Thesis Committee:
  • Unknown, Unknown
Defense Date:9 July 1990
Record Number:CaltechETD:etd-06182007-133055
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-06182007-133055
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2641
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:11 Jul 2007
Last Modified:26 Dec 2012 02:53

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