Abstract
Three-photon double excitation of He in intense EUV FEL fields is studied by the shot-by-shot photoelectron spectroscopy, revealing the enhancement by resonances to the doubly excited states converging to the He+ N =3 level.
Highlights
The recent advances in free electron laser (FEL) technology have enabled us to generate intense ultrashort laser fields in the extreme ultraviolet (EUV)
Three-photon double excitation of He in intense EUV FEL fields is studied by the shot-by-shot photoelectron spectroscopy, revealing the enhancement by resonances to the doubly excited states converging to the He+ N=3 level
The photoelectron spectroscopy is powerful in elucidating the non-linear processes in EUV-FEL fields, because (i) electronic states involved in the ionization process can be identified directly from the photoelectron energy and (ii) the spectral properties of fluctuating FEL pulses can be evaluated on the shot-by-shot basis
Summary
The recent advances in free electron laser (FEL) technology have enabled us to generate intense ultrashort laser fields in the extreme ultraviolet (EUV). Atoms exposed to intense laser fields typically undergo multiphoton multiple ionization. In our previous studies [1], we have demonstrated that single-shot photoelectron spectroscopy provides detailed information on the multi-photon processes of simple atoms and molecules utilizing the inherent fluctuation of FEL pulses. We apply the single shot photoelectron spectroscopy to double excitation of He in ultrashort pulse duration of EUV-FEL pulses [2]. The present study provides a route towards an efficient creation of doubly excited states of He, making it possible to investigate dynamics of strongly correlated electrons in real time
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