Line-shape broadening of an autoionizing state in helium at high XUV intensity

We study the interaction of intense extreme ultraviolet (XUV) light with the 2s2p doubly excited state in helium. In addition to previously understood energy-level and phase shifts, high XUV intensities may lead to other absorption line shape distortions. Here, we report on experimental transient-absorption spectroscopy results on the 2s2p line width modification in helium in intense stochastic XUV fields. A few-level model simulation is realized to investigate the origins of this effect. We find that the line shape broadening is connected to the strong coupling of the ground state to the 2s2p doubly excited state which is embedded in the ionization continuum. As the broadening takes place for intensities lower than for other strong-coupling processes, e.g. observing asymmetry changes of the absorption profile, this signature can be identified already in an intermediate intensity regime. These findings are in general relevant for resonant inner shell transitions in nonlinear experiments with XUV and x-ray photon energies at high intensity.

The multi-photon induced Fano effect

The ordinary Fano effect occurs in many-electron atoms and requires an autoionizing state. With such a state, photo-ionization may proceed via pathways that interfere, and the characteristic asymmetric resonance structures appear in the continuum. Here we demonstrate that Fano structure may also be induced without need of auto-ionization, by dressing the continuum with an ordinary bound state in any atom by a coupling laser. Using multi-photon processes gives complete, ultra-fast control over the interference. We show that a line-shape index q near unity (maximum asymmetry) may be produced in hydrogenic silicon donors with a relatively weak beam. Since the Fano lineshape has both constructive and destructive interference, the laser control opens the possibility of state-selective detection with enhancement on one side of resonance and invisibility on the other. We discuss a variety of atomic and molecular spectroscopies, and in the case of silicon donors we provide a calculation for a qubit readout application.

The effect of exchange and absorption potential in the distorted wave calculation of electron impact excitation of the autoionizing state of caesium

We have calculated the differential and integral cross sections for electron impact excitation of the lowest autoionizing level of caesium atoms in the energy range 13 – 500 eV using the distorted wave method. In this study we have used a complex distortion potential as the distortion potential both at the initial and the final channels. Our results are compared amongst themselves to establish the effect of exchange and imaginary absorption potentials. We also compared with other available theoretical and experimental results to determine the suitability of our present method. From the results it is seen that when an absorption potential is included in the distortion potential, the integral cross section moves closer to the experimental result in the intermediate energy range 19 – 40 eV.

Bound-State Electron Dynamics Driven by Near-Resonantly Detuned Intense and Ultrashort Pulsed XUV Fields

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.