Creating and probing coherent wavepackets of valence electrons
Scattering of powerful LCLS attosecond pulses (100 GW peak power, with several eV of coherent bandwidth) from nitric oxide (NO) molecules creates an electronic excitation (coherent wavepacket) near a single atom [O’Neal]. Impulsive stimulated X-ray Raman scattering (ISXRS) involves a single impulsive interaction to excite a coherent superposition of electronic states. This is a powerful new approach to create and probe valence electron motions with high temporal and spatial resolution. Observing these electronic motions is crucial to understand the role of electronic coherence in chemical processes. Electronic population transfer via ISXRS using broad bandwidth (5.5 eV FWHM) attosecond X-ray pulses is demonstrated for the first time. The impulsive excitation is resonantly enhanced by the oxygen 1s→2π* resonance of (NO), and excited state neutral molecules are probed with a time-delayed UV laser pulse.
Tracking the Attosecond Time-Energy Structure of FEL Pulses
Angular streaking has been developed as a powerful method for characterizing attosecond X-rays from LCLS.[Hartmann] In this approach, X-ray photoelectrons (e.g. from the neon core shell) are dressed with the electric field of a circularly polarized IR laser pulse. This encodes characteristic angle and frequency-dependent modulations of the resulting photoelectron spectrum, with sub-optical-cycle precision.
Ultrafast Molecular Dynamics
Chirality is a fundamental phenomenon that determines everybody’s all-day-life to a large extent. A versatile tool for investigating enantiomers in the gas phase in a controlled and theoretically well accessible way is circularly polarized light, since it possesses a handedness as well. Short-wavelength free-electron lasers with their ultrashort pulses at high intensities have originated new approaches for tracking molecular dynamics from the vista of specific sites. X-ray pump/X-ray probe schemes even allow to address individual atomic constituents with a ‘trigger’-event that preludes the subsequent molecular dynamics while being able to selectively probe the evolving structure with a time-delayed second X-ray pulse. We use a linearly polarized X-ray photon to trigger the photolysis of a prototypical chiral molecule, namely trifluoromethyloxirane (C3H3F3O), at the fluorine K-edge at around 700 eV. The created fluorine-containing fragments are then probed by a second, circularly polarized X-ray pulse of higher photon energy in order to investigate the chemically shifted inner-shell electrons of the ionic mother-fragment for their stereo-chemical sensitivity. We experimentally demonstrate and theoretically support how two-color X-ray pump X-ray probe experiments with polarization control enable XFELs as tools for chiral recognition.
Understanding atoms and molecules in extreme light fields
Understanding the fundamental interaction of atoms and molecules in extreme light fields is one of the earliest drivers of XFEL science. This area is of great interest, not only from a basic science perspective, but also for its relevance to molecular astrophysics and for exploiting intense XFEL pulses for ultrafast atomic imaging. Early LCLS research uncovered new phenomena in the reaction of atoms and small molecules to intense X-ray fields, such as rapid sequences of inner-shell photoionization and Auger ionization. More recent studies of the larger molecular model, buckminsterfullerene (C60), examined the role of chemical effects, such as chemical bonds and charge transfer, on the fragmentation following multiple ionization of the molecule via ultrafast soft X-ray pulses.[Berrah,] Femtosecond X-ray pump/X-ray probe measurements, accompanied by advanced simulations, reveal that despite substantial ionization induced by the ultrashort (20 fs) X-ray pump pulse, the fragmentation of C60 is considerably delayed. This work uncovers the persistence of the molecular structure of C60, which impedes fragmentation to occur over a timescale of hundreds of femtoseconds. Furthermore, a substantial fraction of the ejected fragments are shown to be neutral carbon atom which thwart radiation damage. These findings provide insights into X-ray free-electron laser-induced radiation damage in large molecules, including biomolecules.