Abstract
Photoelectron emission from excited states of laser-dressed atomic helium is analyzed with respect to laser intensity-dependent excitation energy shifts and angular distributions. In the two-color XUV (exteme ultra\-violet) -- IR (infrared) measurement, the XUV photon energy is scanned between \SI{20.4}{\electronvolt} and the ionization threshold at \SI{24.6}{\electronvolt}, revealing electric dipole-forbidden transitions for a temporally overlapping IR pulse ($\sim\!\SI{e12}{\watt\per \centi\meter\squared}$). The interpretation of the experimental results is supported by numerically solving the time-dependent Schr\"odinger equation in a single-active-electron approximation.
Highlights
Photoelectron spectroscopy is a powerful technique to obtain compositional and structural information about matter and to investigate light-matter interactions in general
We report the use of XUV radiation with tunable wavelength provided by the free-electron laser in Hamburg (FLASH) in combination with a synchronized infrared (IR) laser to obtain a detailed picture of excited states in laser-dressed atomic helium
We have measured and analyzed photoelectrons stemming from laser-dressed atomic helium
Summary
Photoelectron spectroscopy is a powerful technique to obtain compositional and structural information about matter and to investigate light-matter interactions in general. One step further in the investigation of light-matter interactions is the implementation of two-color ionization and excitation schemes, which reveal laser-induced continuum structures [8] and light-induced structures (LIS) [9] In the former case, the dressing laser field couples bound states to the continuum, giving rise to a resonant structure [10,11,12,13]. The interpretation of the experimental results is supported by numerical calculations based on the time-dependent Schrödinger equation (TDSE) within the single-active electron (SAE) approximation This two-color scheme has the clear advantage over, e.g., single-color REMPI setups, where the dominant contribution to the excitation energy is delivered by only one XUV photon, and the laser intensity can be kept low. The presented measurement focuses on multiphoton excitation, enabled by the combined interaction of XUV and IR photons
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