Probing Attosecond Electron Dynamics with X-Ray Free-Electron Lasers*

We present recent experimental results using attosecond x-ray free electron laser pulses and pulse pairs to probe electron dynamics in small molecular systems.

Observing electron motion in molecules on its natural attosecond timescale is a frontier challenge for experimental science. Such measurements require attosecond duration light sources and atomic-site resolution, to localize the electronic charge as it moves around a molecule. Soft x-rays address transitions between core and valence electron orbitals in most of the abundant small atoms in living organisms (carbon, nitrogen, oxygen). Because core orbitals are well localized to their respective nuclei, these transitions offer atomic site specificity for creating and probing valence electron motion. X-ray free-electron lasers offer continuous wavelength tunability across the soft x-ray region and have recently produced attosecond pulses, close to the Fourier limit, with single-shot pulse energies up to hundreds of microjoules. We present time-resolved measurements of coherent electron dynamics and electron correlation effects in small molecules made with single attosecond x-ray pulses. The coherent evolution of a wavepacket of core-excited states in nitric oxide is probed by time-resolving the Auger-Meitner emission rate using angular streaking. Angular streaking is also used to measure the attosecond photoemission delay from the oxygen 1s orbital in nitric oxide. X-ray pulse pairs with controllable delay can also be produced and are used to probe coherent valence electron motion in the para-aminophenol molecule with attosecond time resolution.