Abstract
The Fano absorption line shape of an autoionizing state encodes information on its internal atomic structure and dynamics. When driven near-resonantly with intense extreme ultraviolet (XUV) electric fields, the absorption profile can be deliberately modified, including observable changes of both the line-shape asymmetry and strength of the resonance, revealing information on the underlying dynamics of the system in response to such external driving. We report on the influence of the XUV pulse parameters at high intensity that can be achieved with a free-electron laser (FEL) with statistically broadened spectra based on self-amplified spontaneous emission (SASE). More specifically, the impact of the FEL pulse duration is studied for the example of the doubly excited 2s2p resonance in helium, where line-shape modifications have been measured with XUV transient absorption spectroscopy in Fraunhofer-type transmission geometry. A computational few-level-model provides insight into the impact of different average pulse durations of the stochastic FEL pulses. These findings are supported by measurements performed at the Free-Electron Laser in Hamburg (FLASH) and provide further insight into XUV strong-coupling dynamics of resonant transitions driven by intense high-frequency FEL sources.
Highlights
If one excites an atomic or molecular system with more energy than it needs to be ionized, phenomena like fluorescence, Auger decay or semi-stable resonance states embedded in the continuum—so-called autoionizing states—come into play
We report on the influence of the XUV pulse parameters at high intensity that can be achieved with a free-electron laser (FEL) with statistically broadened spectra based on self-amplified spontaneous emission (SASE)
The impact of the FEL pulse duration is studied for the example of the doubly excited 2s2p resonance in helium, where line-shape modifications have been measured with XUV transient absorption spectroscopy in Fraunhofer-type transmission geometry
Summary
If one excites an atomic or molecular system with more energy than it needs to be ionized, phenomena like fluorescence, Auger decay or semi-stable resonance states embedded in the continuum—so-called autoionizing states—come into play. We present a study on the FEL pulse-length-dependent modification of the 2s2p autoionizing absorption line shape in helium when driven directly by intense and partially coherent XUV fields which are generated through self-amplified spontaneous emission (SASE) This is performed within the TAS scheme, an all-optical method that works in transmission mode and is sensitive to the interference of the incoming and scattered light in forward direction, which recently has been experimentally realized with intense FEL pulses [24, 28]. We find that with increasing FEL pulse duration, but fixed average bandwidth, the magnitude of observable line-shape asymmetry changes decreases significantly These model predictions can be intuitively understood through the peak intensity distribution of individual temporal SASE spikes contained in each pulse, and are further confirmed by transmission-mode TAS measurements of the 2s2p resonance in helium at the Free-Electron Laser in Hamburg (FLASH). FEL pulse-length effects are expected to become important whenever the resonance lifetime and FEL pulse duration are comparable
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
