Abstract
The laser-field-modified dipole response of the first ionization threshold of helium is studied by means of attosecond transient absorption spectroscopy. We resolve light-induced time-dependent structures in the photoabsorption spectrum both below and above the ionization threshold. By comparing the measured results to a quantum-dynamical model, we isolate the contributions of the unbound electron to these structures. They originate from light-induced couplings of near-threshold bound and continuum states and light-induced energy shifts of the free electron. The ponderomotive energy, at low laser intensities, is identified as a good approximation for the perturbed continuum response.
Highlights
With the advent of high-order harmonic generation, it became possible to generate extreme ultraviolet (XUV) pulses on the attosecond time scale, which paved the way for direct timeresolved measurements on the natural time scale of electronic excitations
We investigate the laser-field-modified dipole response of the continuum threshold of helium by means of XUV transient absorption spectroscopy in the presence of a near infrared (NIR) laser pulse; see figure 1
A recently developed in situ method for the recording of the XUV reference spectrum in transient absorption facilitates the observation of weak structures in the absorption spectrum at energies closely above the ionization threshold [34]. Comparing these structures with a model simulation we demonstrate direct sensitivity to laser-induced energy shifts of the ionized electron in the XUV photoabsorption spectrum
Summary
To cite this article: Paul Birk et al 2020 J. 53 124002 View the article online for updates and enhancements. This content was downloaded from IP address 132.234.229.125 on 18/09/2020 at 07:10. Paul Birk1,3 , Veit Stooß, Maximilian Hartmann , Gergana D Borisova , Alexander Blättermann, Tobias Heldt , Klaus Bartschat , Christian Ott and Thomas Pfeifer. Received 6 December 2019, revised 5 February 2020 Accepted for publication 3 March 2020 Published 27 May 2020
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