Investigation of the two-photon resonance ionization of helium
Using a new experimental method, physicists from the Max Planck Institute for Nuclear Physics in Heidelberg have investigated the ionization of two resonant photons of helium with improved spectral resolution and angular resolution. For this purpose, they used an interaction microscope with a high-resolution ultraviolet (EUV) photon spectrometer developed at the institute.
The measurements were made at the Free Electron Laser in Hamburg (FLASH), a remarkable radiation source, providing intense EUV laser flashes. This allows events from each individual laser flash to be analyzed in terms of photon energy, resulting in high-resolution spectral datasets.
Helium, as the simplest and most accessible multi-electron system, is ideally suited for basic theoretical and experimental studies. Here, the mutual electrical repulsion between the two electrons plays a fundamental role – it accounts for one third of the total binding energy. Of particular interest and fundamental is the interaction with photons (the amount of light).
Researchers from the groups around Christian Ott and Robert Moshammer at the Thomas Pfeiffer Department at the Max Planck Institute for Nuclear Physics in Heidelberg have investigated the ionization of two resonant photons of helium in detail in DESY’s FLASH free-electron laser in Hamburg.
In this nonlinear process, both electrons simultaneously absorb two photons of extreme ultraviolet radiation and form a double excitation state, illustratively, both electrons are in a large orbit around a helium nucleus. The pairwise dance of the electrons is unstable and their mutual repulsion causes one to leave the atom while the other regresses to the ground state of the helium ion – a process called self-ionization (see Figure 1). It occurs when the energy of the photons corresponds to the energy of the discrete excitation, that is, when the so-called resonance condition is achieved.
To get a detailed measurement, the researchers used reaction microscopy (REMI), which allows full motion detection of both photoelectrons and helium ions. However, a fundamental difficulty remains to be overcome: although the free electron laser presents very intense ultraviolet radiation, the energy of the photons has a fairly wide frequency range, and the higher energy intensity range also varies from laser flash to laser flash.
However, it is this very property that has now been exploited: “We used a spectrophotometer to measure the energy distribution of the photons in each individual snapshot and then sort them according to the photon energy of higher intensity (peak position),” he explains. First author Michael Straub. “In conjunction with REMI signals, we thus obtain spectrally high-resolution datasets, digitally tunable over the entire bandwidth.” (Picture 2).
The resonance was solved with this trick and measured the angular distribution of the photoelectrons in the resonance. In direct comparison with theoretical calculations from Chris Green’s group (Purdue University), there was good agreement, but there are also deviations on closer examination. One explanation is the small contributions from non-resonant ionization with single photons of twice the energy (red curve in Fig. 1), which represents about 1% of the flash photon flux.
“These results and newly developed experimental methodology open a promising avenue for exploring the fundamental interactions of a few photons with a few electrons,” says group leader Christian Ott, summarizing the scope of the work, now published in physical review messages.
Michael Straub et al, Differential measurement of electron ejection after two-photon two-electron excitation of helium, physical review messages (2022). DOI: 10.1103/ PhysRevLett.129.183204
Submitted by Max-Planck-Institut für Kernphysik
the quote: Retrieved November 9, 2022 from https://phys.org/news/2022-11-resonant-two-photon-ionization-helium.html Investigation of the Ionization of Two Resonant Photons of Helium (2022, November 9)
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