Abstract
We report on numerical results revealing line-shape asymmetry changes of electronic transitions in atoms near-resonantly driven by intense extreme-ultraviolet (XUV) electric fields by monitoring their transient absorption spectrum after transmission through a moderately dense atomic medium. Our numerical model utilizes ultrashort broadband XUV laser pulses varied in their intensity (1014–1015 W/cm2) and detuning nearly out of resonance for a quantitative evaluation of the absorption line-shape asymmetry. It will be shown how transient energy shifts of the bound electronic states can be linked to these asymmetry changes in the case of an ultrashort XUV driving pulse temporally shorter than the lifetime of the resonant excitation, and how the asymmetry can be controlled by the near-resonant detuning of the XUV pulse. In the case of a two-level system, the numerical model is compared to an analytical calculation, which helps to uncover the underlying mechanism for the detuning- and intensity-induced line-shape modification and links it to the generalized Rabi frequency. To further apply the numerical model to recent experimental results of the near-resonant dressing of the 2s2p doubly excited state in helium by an ultrashort XUV free-electron laser pulse we extend the two-level model with an ionization continuum, thereby enabling the description of transmission-type (Fraunhofer-like) transient absorption of a strongly laser-coupled autoionizing state.
Highlights
The manipulation of electronic states in atoms with intense electromagnetic fields has been studied theoretically for several decades [1,2]
We computationally model intense-field XUV absorption spectroscopy by directly observing the intense XUV light after its transmission through a resonant medium and use it to explain and predict line-shape asymmetry changes of resonant transitions strongly coupled by XUV light
We consider a two-level system for our numerical model, for which the Hamiltonian H2lvl is described as a matrix with two states at energy Eg = 0 eV and Ee = 60.15 eV, for the ground and the excited state, respectively, which are coupled in dipole approximation by the dipole matrix element dge = deg = −0.035 a.u. (a.u. = atomic units; which are used throughout this work unless stated otherwise)
Summary
The manipulation of electronic states in atoms with intense electromagnetic fields has been studied theoretically for several decades [1,2]. Transient-absorption spectroscopy experiments have been carried out more recently with sensitivity to strong-coupling dynamics of Fano resonances in rare gas atoms [5,6,7,8,9]. These experiments employ broadband extreme-ultraviolet (XUV) attosecond pulses in combination with time-delayed intense femtosecond laser pulses in the near-infra red (NIR) spectral regime. Regarding transient absorption spectroscopy with intense XUV-FEL pulses, signatures of strongly driven XUV resonant transitions have been observed [16,17]. Our numerical findings for the autoionizing 2s2p doubly excited state in helium further support recent experimental results that have been obtained with intense XUV-FEL pulses [16]
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