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arXiv:2604.03943 · Physical Review A

Photoelectron spectroscopy of 3 s 3 p doubly excited helium dressed with strong near-infrared laser fields

Mizuho Fushitani, Chien-Nan Liu, Yuki Ono, Shunsuke Amaike, Wataru Yamazaki, Keiko Kato, Akitaka Matsuda, Shigeki Owada, Makina Yabashi, Y. Hikosaka, Tōru Morishita, Akiyoshi Hishikawa

Published Jun 17, 2026• 0 citations
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Abstract

We report time-resolved photoelectron spectroscopy of the $3s3p$ 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 ($3s3p {}^{1}P^{o}$) autoionizing state to nearby dark ($^{1}D^{e}$ and $^{1}S^{e}$) resonances below the $N = 3$ 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.

DOI
10.1103/d8qn-8y4v

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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.
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Strong-field manipulation of autoionizing states is a crucial aspect of electronic quantum control. Recent measurements of the attosecond transient absorption spectrum of helium dressed by a few-cycle visible pulse [Ott et al., arXiv:1205.0519[physics.atom-ph] ] provide evidence of novel ultrafast resonant phenomena, namely, two-photon Rabi oscillations between doubly-excited states and the inversion of Fano profiles. Here we present the results of accurate \emph{ab-initio} calculations that agree with these observations and in addition predict that (i) inversion of Fano profiles is actually periodic in the coupling laser intensity and (ii) the supposedly dark 2p22p^22p2 {1^11S} state also appears in the spectrum. Closer inspection of the experimental data confirms the latter prediction.
In the present paper, we investigate the time-resolved transient absorption spectroscopy of doubly excited states of helium atoms by solving the time-dependent two-electron Schr\"odinger equation numerically based on a one-dimensional model. The helium atoms are subjected to an extreme ultraviolet (XUV) attosecond pulse and a time-delayed infrared (IR) few-cycle laser pulse. A superposition of doubly excited states populated by the XUV pulse is identified, which interferes with the direct ionization pathway leading to Fano resonance profiles in the photoabsorption spectrum. In the presence of an IR laser, however, the Fano line profiles are strongly modified: A shifting, splitting, and broadening of the original absorption lines is observed when the XUV attosecond pulse and infrared few-cycle laser pulse overlap in time, which is in good agreement with recent experimental results. At certain time delays, we observe symmetric Lorentz, inverted Fano profiles, and even negative absorption cross sections indicating that the XUV light can be amplified during the interaction with atoms. We further prove that the above pictures are general for different doubly excited states by suitably varying the frequency of the IR field to coherently couple the corresponding states.
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.