Ultrafast Photoelectron Spectroscopy of Electron Dynamics in Atoms and Molecules

This dissertation presents experiments studying electron dynamics in atoms and molecules using the tools of ultrafast photoelectron spectroscopy. The first experiment studies extreme-ultraviolet excitation and relaxation dynamics in molecular oxygen and identifies a previously undiscovered excitation and dissociation pathway, filling a hole in the spectroscopic library of oxygen that had existed for almost twenty years. This is in fact a multielectron excitation and would therefore be expected to have a very low excitation cross-section. Fresh analysis of the Fano approach to autoionizing states revealed that this can be explained as a sort of backwards autoionization– “excitation” initially proceeds to the continuum and population is transferred back to the discrete state via the same coupling mechanism as autoionization. The second and third experiments stem serendipitously from the same few sets of exploratory data sets taken in argon when testing a new optical technique. In one an argon atom is excited into a superposition of autoionizing states and this wavepacket is probed with a delayed IR pulse. This has the unintended but beneficial effect of inducing Raman transitions among the wavepacket constituent states and allowing study of its dynamics with unprecedented time and energy resolution simultaneously. Finally, a similar Raman process is used to probe a superposition of bound states, this time leading to rich and unanticipated angular structure in the electron emission. Categorically, the method used across all these experiments is known as “time-resolved photoelectron spectroscopy” and consists of an initial excitation, or “pump,” of an atom or molecule followed by a time-delayed “probe” inducing some measurable effect. Most simply, the probe photoionizes the system, thereby producing a photoelectron with measurable momentum and energy, but more subtle probes can be designed where, for example, it effects the rate of autoionization. The experimental appar