Abstract

Vibrational branching ratios serve as sensitive experimental probes of resonance phenomena in molecular photoionization.¹ In the absence of resonant excitation, the Franck-Condon approximation holds and vibrational branching ratios are independent of photon energy. Conversely, deviation from Franck-Condon behavior is an indicator of resonance excitation, such as shape resonant excitation. We have studied vibrationally resolved aspects of shape resonant excitation. This work is summarized briefly, followed by a discussion of new directions that will be made possible by intense free electron laser (FEL) sources. Shape resonant excitation occurs at relatively high incident photon energies, and for lack of a better source, is best studied using synchrotron radiation. This experiment is performed by generating dispersed fluorescence spectra from electronically excited photoions. The results described here are the first vibrationally resolved results on a polyatomic shape resonance and they illustrate the utility of extending this work, which should be possible for a wide variety of systems, given FEL sources. A central motivation for studying polyatomic shape resonances is that alternative vibrational modes may be explored, revealing facets that are nonexistent for diatomic systems, which have only a single vibrational degree of freedom.

© 1988 Optical Society of America