Time resolved photoelectron spectroscopy with ultrafast soft x-ray light

<p>Understanding the nature of chemical bonding in molecules has long been of fundamental interest, and manipulating specific chemical bonds has found practical potential in many areas, such as polymer chemistry, biophysics, and genetics. Photoelectron spectroscopy (using photons to eject electrons from a molecule to determine electron energies and bonding properties) has been a proven technique for gaining insight into the electronic structure of the simplest atoms to large molecules. Here, I describe experiments designed to investigate bonding in a simple molecule (Br2) by measuring electronically excited molecular states and dissociative transient electronic states via time-resolved photoelectron spectroscopy with soft x-ray light. High-harmonic generation produces a novel, femtosecond source of soft x-rays by focusing an ultrafast 800 nm Ti:Sapphire laser into a rare gas jet. I outline the details of the experimental apparatus, including the optical layout, two-grating separation and compression of the highharmonic pulses, and the magnetic bottle photoelectron spectrometer. The feasibility of using the generated soft x-ray pulses for photoelectron spectroscopy is established, and the spectral and temporal nature of the pulses are determined. The photoelectron spectrum of the bound excited B state of neutral Br2 is measured and the issues involved in ionization from excited electronic states is discussed. The time-resolved dissociation of the excited C state of Br2 is observed using a visible pump (400 nm) and a soft x-ray probe. Key results point to challenging new problems involving the cross-sections and shapes of photoelectron features arising from dissociating states, as well as understanding the role of cross-correlation processes versus dissociative wavepacket signals. UV-pump/soft x-ray probe photoelectron spectroscopy promises to offer a unique and powerful way to probe excited electronic states and dissociation dynamics of neutral molecules in the gas phase on ultrafast time scales.</p>
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University of Colorado Boulder
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