LSU Historical Dissertations and Theses

1994

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Erwin D. Poliakoff

Abstract

Highly resolved molecular photoionization is a topic of intense current interest due to insights it provides into fundamental scattering processes. Rotationally resolved data provide a window on the angular momentum composition of the photoelectron and the partitioning of angular momentum between the ion and photoelectron. Since the earliest rotational measurements via photoelectron spectroscopy (PES) of H$\sb2$ using resonance lamp excitation, studies have accelerated as a result of experimental innovations. However, all of these recent developments have been limited to threshold or near-threshold ionization phenomena. In order to bypass this constraint, we have employed dispersed fluorescence measurements. This strategy allows us to exploit the broad tunability of synchrotron radiation because the detection bandwidth is uncoupled from the excitation bandwidth. The isoelectronic target molecules N$\sb2$ and CO are rotationally cooled by a supersonic expansion, and ionized by synchrotron radiation from a 6-m Plane Grating Monochromator (PGM), at the Louisiana State University Center for Advanced Microstructures and Devices (CAMD). The rotational fluorescence spectra are obtained over the energy range from $\rm20\ \le\ h\nu\sb{exc}\ \le\ 220$ eV for $\rm N\sb2\ 2\sigma\sb{u}\sp{-1}$ photoionization and from $\rm23\ \le\ h\nu\sb{exc}\ \le\ 145$ eV for CO 4$\sigma\sp{-1}$ photoionization. The results of $\rm N\sb2\sp+(B\sp2\Sigma\sb{u}\sp+$) and $\rm CO\sp+(B\sp2\Sigma\sp+$) rotational distributions show strikingly different energy dependences. Detailed calculations reveal that the dramatically changing $\rm N\sb2\sp+(B\sp2\Sigma\sb{u}\sp+$) rotational distributions arise from Cooper minima of g- and d-waves in the k$\sigma\sb{\rm g}$ photoelectron continuum, while the flatness of $\rm CO\sp+(B\sp2\Sigma\sp+$) rotational distributions results from a delicate balance between the $4\sigma\ \to\ {\rm k}\sigma\ f$-wave shape resonance and the broad f-wave enhancement in k$\pi$ photoelectron continuum. Cooper minima for $\rm N\sb2\ 2\sigma\sb{u}\sp{-1}$ ionization lead to small $\Delta N$ transitions at lower energies and large $\Delta N$ transitions at higher energies, while the CO $\rm 4\sigma\ \to\ k\sigma$ shape resonance results in larger $\Delta N$ transitions over the whole energy range studied. The present studies of $\rm N\sb2\sp+(B\sp2\Sigma\sb{u}\sp+$) and $\rm CO\sp+(B\sp2\Sigma\sp+$) rotational distributions illustrate effects of Cooper minima, shape resonances, and l mixing in photoelectron continua at energies well beyond the reach of conventional photoelectron spectroscopy. Moreover, these results also demonstrate that rotationally resolved fluorescence is suitable for studies of molecular photoionization dynamics over a broad spectral range.

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