Coherent Electron Dynamics Probed by XFELs

The question we address is the dynamical behaviour of quantum superpositions of electronic states in molecules; their formation, decoherence and coupling to the nuclear part of wavefunction. We concentrate upon electron hole superpositions created by a "fast" photoionization event that will result in the migration of charge through the formation of hole state superpositions (1h states) and superpositions of hole states and correlation satellites (1h-2h1p states). We employ an X-ray pump – X-ray probe methodology using both X-ray absorption spectroscopy (XAS) and X-ray electron emission spectroscopy (XPS) as a probe which circumvents the limits of previous methods. Earlier work to measure ultrafast electronic dynamics of such states has been centred around HHG spectroscopy and XUV pump – IR probe methods, but these are predominately confined to probing superpositions of outer valence hole states, do not give state and site specific resolved information and the optical fields used can significantly perturb the electron dynamics being studied. We will report the results of several recent experiments that capture the ultrafast dynamics of an electron hole in small molecular targets using few-femtosecond XFEL based pulses in a pump-probe measurement. An XAS measurement performed at LCLS studied isopropanol and identified charge migration dynamics in the form of a "breathing mode" in a superposition of inner valence states below the double ionization potential and so undergoing frustrated Auger-Meitner decay. Coherent hole dynamics was found in an XPS experiment at FLASH in glycine where the oscillatory character of an inner valence superposition with ~ 20 fs periodicity is captured.  These results will be discussed, and a progress report will be given of measurements using attosecond XFEL pulses at LCLS ("Attosecond Campaign") investigating hole dynamics in the para-aminophenol molecule with extraordinary temporal resolution.