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The advent of third generation synchrotron light sources has significantly improved our understanding of the quantum chemistry of small molecules. The present study concentrates on processes following the photoexcitation of dissociative resonance states. The investigations were made using two-dimensional, angle resolved time-of-flight photoelectron spectroscopy. Two-dimensional photoelectron spectra show electron yield as a function of photon and electron energy and the comprehensive nature of the data sets often reveals features that are not apparent in one-dimensional photoelectron spectra. The measurements were carried out at the Advanced Light Source on the undulator beamline 10.0.1, which is used for high-resolution atomic, molecular, and optical physics and photoemission studies of highly correlated materials. In the small molecular systems in question, the molecular dissociation time is comparable with the lifetime of the core hole, created by the resonant excitation, so that there is competition between autoionization and dissociation. The two step process of fast dissociation followed by fragment autoionization was first observed for inner shell excitation in HBr and gives rise to sharp peaks in the photoelectron spectrum, at electron energies that correspond to transitions in the excited fragment. Although the sharp peaks may also contain contributions from resonant Auger decay before the fragmentation is complete if the energy separation between the two potential curves becomes constant before the dissociation limit. Alternatively, broader spectral features are associated with electron emission before or during fragmentation, when there is not a constant separation between the potential curves. The existence of indistinguishable channels gives rise to the possibility of observing interference phenomena in the photoelectron spectrum.
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