Differential Measurement of Electron Ejection after Two-Photon Two-Electron Excitation of Helium

Open Access

Differential Measurement of Electron Ejection after Two-Photon Two-Electron Excitation of Helium

Michael Straub, Thomas Ding, Marc Rebholz, Gergana D. Borisova, Alexander Magunia, Hannes Lindenblatt, Severin Meister, Florian Trost, Yimeng Wang, Steffen Palutke, Markus Braune, Stefan Düsterer, Rolf Treusch, Chris H. Greene, Robert Moshammer, Thomas Pfeifer, and Christian Ott

Phys. Rev. Lett. 129, 183204 – Published 28 October 2022

Abstract

We report the measurement of the photoelectron angular distribution of two-photon single-ionization near the $2 p^{2}$ ${{}^{1}D}^{e}$ double-excitation resonance in helium, benchmarking the fundamental nonlinear interaction of two photons with two correlated electrons. This observation is enabled by the unique combination of intense extreme ultraviolet pulses, delivered at the high-repetition-rate free-electron laser in Hamburg (FLASH), ionizing a jet of cryogenically cooled helium atoms in a reaction microscope. The spectral structure of the intense self-amplified spontaneous emission free-electron laser pulses has been resolved on a single-shot level to allow for post selection of pulses, leading to an enhanced spectral resolution, and introducing a new experimental method. The measured angular distribution is directly compared to state-of-the-art theory based on multichannel quantum defect theory and the streamlined $R$ -matrix method. These results and experimental methodology open a promising route for exploring fundamental interactions of few photons with few electrons in general.

Received 3 June 2022
Accepted 4 October 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.183204

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Single- and few-photon ionization & excitation Strong field ionization & excitation Ultrafast phenomena

Free-electron lasers

Atomic, Molecular & Optical

Authors & Affiliations

Michael Straub ¹, Thomas Ding¹, Marc Rebholz ¹, Gergana D. Borisova ¹, Alexander Magunia¹, Hannes Lindenblatt ¹, Severin Meister¹, Florian Trost ¹, Yimeng Wang ², Steffen Palutke ³, Markus Braune ³, Stefan Düsterer³, Rolf Treusch ³, Chris H. Greene ², Robert Moshammer^1,‡, Thomas Pfeifer ^1,†, and Christian Ott ^1,*

¹Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany
²Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, USA
³Deutsches Elektronen Synchroton DESY, Notkestrasse 85, D-22607 Hamburg, Germany

^*christian.ott@mpi-hd.mpg.de
^†thomas.pfeifer@mpi-hd.mpg.de
^‡r.moshammer@mpi-hd.mpg.de

Article Text

Click to Expand

Supplemental Material

Click to Expand

References

Click to Expand

Issue

Vol. 129, Iss. 18 — 28 October 2022

Reuse & Permissions

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) The experimental setup: Linearly polarized FEL pulses arrive from the left, propagating in the $x$ direction, and are focused into the center of the reaction microscope, crossing a supersonic gas jet of cryogenically cooled atomic helium. The created ions are then accelerated toward a position- and time-sensitive detector. The transmitted FEL beam is refocused into a grating spectrometer, measuring the XUV photon spectrum on a shot-to-shot basis with the GOTTHARD detector [20], compatible with the burst-mode high-repetition rate pulse pattern at FLASH, similar to Ref. [17]. (b) The measured recoil-ion momentum distribution in the $x − y$ plane with a slice in the $z$ direction ( $| p_{z} | \leq 0.1 a . u .$ ). The FEL is polarized in the $y$ direction. Two-photon ionization events are selected by their momentum $1.5 a . u . \leq | {\vec{p}}_{e} | \leq 1.7 a . u .$ as marked by the red circles. Because of a locally confined inhomogeneity in the detector efficiency at very negative $p_{y}$ momenta, only events in the upper hemisphere ( $p_{y} \geq 0$ ) are considered for the analysis.
Reuse & Permissions
Figure 2
(a) Energy level scheme of the observed ionization channel. The doubly excited state $2 p^{2}$ ( ${{}^{1}D}^{e}$ ) at 59.9 eV can be reached with two FEL photons (vertical purple arrows) tuned to half the transition energy. This state primarily decays through autoionization, emitting an electron (thin black arrow) into the ${He}^{+}$ ionization continuum. (b) Calculated total two-photon cross section in blue [5]. The blue inset shows the PAD at the peak of the cross section which features a dominant D symmetry. Additionally, the calculated one-photon cross section is shown (red, dashed) at twice the photon energy [22, 23]. The red inset shows the corresponding one-photon $P$ lobe.
Reuse & Permissions
Figure 3
Lower panel: mean FEL spectra sorted by the photon-energy position of the highest peak in each SASE spectrum, where the pulse energy is restricted to values between 4.2 and $4.7 μ J$ . Upper panel: the blue data depict the relative counts of a fast recoil ion ( $1.5 a . u . \leq | {\vec{p}}_{e} | \leq 1.7 a . u .$ ), normalized to the number of pulses in each bin. The gray data show the relative counts (scaled by $10^{- 2}$ ) of direct one-photon ionization events ( $| {\vec{p}}_{e} | \approx 0.6 a . u .$ ) for each bin. The green curve is obtained by convoluting the experimental FEL spectra with the theoretical cross section [cf. Fig. 2], assuming a 1% photon flux of the second harmonic compared to the fundamental. The red data are obtained by selecting fast recoil ions ( $1.5 a . u . \leq | {\vec{p}}_{e} | \leq 1.7 a . u .$ ) with the additional condition of a small momentum in the polarization direction ( $| p_{y} | \leq 0.3 a . u .$ ).
Reuse & Permissions
Figure 4
(a) Accumulated FEL spectra, measured with the XUV photon spectrometer, for high (orange) and low (blue) relative photon flux on resonance, i.e., between 29.9 to 30.0 eV. (b) PAD of fast recoil ions ( $1.5 a . u . \leq | {\vec{p}}_{e} | \leq 1.7 a . u .$ ), measured with the REMI, and accumulated for the condition of FEL spectra with resonant (orange) and off-resonant (blue) pulses, resulting from the corresponding FEL spectra which are plotted in (a). The counts of the off-resonant case have been renormalized by the ratio of the total number of incoming photons for both cases. (c) Subtracting the measured background (renormalized off-resonant counts) from the signal (resonant counts) from (b) isolates the PAD of the two-photon double-excitation resonance shown in (c). The dashed black line depicts the least-squares fit to the experimental data, together with the best fit parameters and statistical error bars given by the standard deviation. The solid green line denotes the theory prediction at the peak of the resonance with given $β_{2}$ and $β_{4}$ parameters.
Reuse & Permissions

Physical Review Letters