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Updated: Mar 8, 2026

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy
Published on: June 28, 2016
Circular Dichroism in Multiphoton Ionization of Resonantly Excited He^{+} Ions
M Ilchen1,2, N Douguet3, T Mazza1
1European XFEL GmbH, Holzkoppel 4, 22869 Schenefeld Hamburg, Germany.
Abstract:
Intense, circularly polarized extreme-ultraviolet and near-infrared (NIR) laser pulses are combined to double ionize atomic helium via the oriented intermediate He^{+}(3p) resonance state. Applying angle-resolved electron spectroscopy, we find a large photon helicity dependence of the spectrum and the angular distribution of the electrons ejected from the resonance by NIR multiphoton absorption. The measured circular dichroism is unexpectedly found to vary strongly as a function of the NIR intensity. The experimental data are well described by theoretical modeling and possible mechanisms are discussed.
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