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Doubly-Excited Helium Strongly Driven with Short and Long Wavelength Pulses

Alexander Magunia

Published Jan 1, 2018• 0 citations
Publisher

Abstract

Within this work, electron dynamics of autoionizing, doubly excited states in helium are investigated. Therefore, strong field couplings of these states are studied in two spectral regimes: in the infrared and in the extreme ultraviolet. A laser system capable of tuning its output-pulses' central wavelength within the infrared spectral regime is installed and characterized by a FROG setup. Furthermore, high harmonics of these output pulses are generated and discussed. First intensity dependent and time resolved experiments in helium with this laser system are presented and supported by numerical calculations. The transition from an unperturbed light induced state, which is generated from the 2p4p state, to a resonant Autler-Townes splitting of the sp2;3+ state is observed. Furthermore, strong field couplings between the ground and a doubly-excited state are studied in the extreme ultraviolet only regime. With the help of simulations Rabi-like couplings and Stark shifts for ultrashort pulses leading to a manipulated absorption line shape are investigated. The same study is carried out for autoionizing states. This theory part is used to explain experimentally measured line-shape changes of the 2s2p resonance in helium interacting with FEL pulses.

DOI
10.17617/2.3026021

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•Jan 1, 2021•0 citations
Within this work, electron dynamics of autoionizing, doubly excited states in helium are investigated. Therefore, strong field couplings of these states are studied in two spectral regimes: in the infrared and in the extreme ultraviolet. A laser system capable of tuning its output-pulses' central wavelength within the infrared spectral regime is installed and characterized by a FROG setup. Furthermore, high harmonics of these output pulses are generated and discussed. First intensity dependent and time resolved experiments in helium with this laser system are presented and supported by numerical calculations. The transition from an unperturbed light induced state, which is generated from the 2p4p state, to a resonant Autler-Townes splitting of the sp2;3+ state is observed. Furthermore, strong field couplings between the ground and a doubly-excited state are studied in the extreme ultraviolet only regime. With the help of simulations Rabi-like couplings and Stark shifts for ultrashort pulses leading to a manipulated absorption line shape are investigated. The same study is carried out for autoionizing states. This theory part is used to explain experimentally measured line-shape changes of the 2s2p resonance in helium interacting with FEL pulses.
We report on the experimental observation of a strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of Microjoules. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, evidences strong-field-induced energy and phase shifts of the doubly excited state, which are extracted from the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of a few meV, induced by strong XUV fields. These results open up a new way of performing nonperturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths.
We discuss the dynamics of doubly resonant ionization in helium involving the strong coupling of two doubly excited autoionizing resonances, under the influence of XUV radiation of short duration (DESY FEL) and a synchronized optical laser. We explore the possibility of using the photoelectron spectra to extract information about the pulse shape of the FEL. We present in addition new calculations of the expected AC Stark splitting due to the strong coupling of the two resonances and show that, within the range of intensities and pulse durations expected in the experiments, additional structure, beyond the usual doublet, is to be expected.
We report time-resolved photoelectron spectroscopy of the 3s3p3s3p3s3p doubly excited states of helium dressed by an intense near-infrared (NIR) laser field. Using synchronized XUV free-electron-laser and 800-nm NIR laser pulses, we observe a pronounced delay-dependent shift of resonance-related spectral minima together with the emergence of additional structures around the NIR sideband energy. \textit{Ab initio} theoretical calculations support these observations and identify the features as signatures of NIR-induced coupling of the bright (3s3p1Po3s3p {}^{1}P^{o}3s3p1Po) autoionizing state to nearby dark (1De^{1}D^{e}1De and 1Se^{1}S^{e}1Se) resonances below the N=3N = 3N=3 threshold. A multichannel Fano resonance analysis of the measured spectra yields delay-dependent line-shape parameters and resonance energies, establishing a quantitative route to characterize and control correlated two-electron resonances in strong laser 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.
We discuss the dynamics of the two-photon resonant ionization of helium involving two autoionizing states in the presence of two-color laser fields. The first source is tuned around the transition from the ground state to the 2s2p 1Po autoionizing state, and the second couples the state 2s2p 1Po to the 2p2 1S autoionizing state. The laser coupling between the doubly excited states is shown to lead to modifications of the Beutler-Fano profile and the appearance of an Autler-Townes doublet. This double resonance effect between autoionizing states can be observed at moderate laser intensities easily attainable by currently operated sources.
In this work, the strong-field-modified dipole response at the ionization threshold of helium is studied. The dipole response is induced by an attosecond pulse in the extreme ultraviolet spectral range and is manipulated by an ultrashort and strong femtosecond pulse in the near-infrared. To probe the response, the transient absorption spectrum of helium is recorded for different time delays between both pulses and different intensities of the femtosecond pulse. From the spectra, the dipole response of the ionization threshold is reconstructed, which is linked to the dynamics of excited electrons with energies in the transition region from bound to free. To identify the underlying processes of light-matter interaction leading to the observed structures in the time and spectral domain, different quantum-mechanical model simulations are conducted. As a result, the measured dipole response reveals light-induced energy shifts of the photoelectron’s kinetic energy close to the parent ion, signatures for field-driven recollisions of a photoelectron into the parent ion, and a temporal amplitude and phase gating mechanism. With the latter, the build-up dynamics of complex spectral structures are temporally resolved, which are the time-dependent separation and line-shape modification of the doubly excited Rydberg series as well as the temporal build-up of the ionization threshold.