Investigation of the two-photon resonant ionization of helium

Using a new experimental method, physicists from the Max Planck Institute for Nuclear Physics in Heidelberg investigated the two-photon resonant ionization of helium with improved spectral and angular resolution. To this end, they used a reaction microscope combined with a high-resolution extreme ultraviolet (EUV) photon spectrometer developed at the Institute.

The measurements have been carried out at the Free Electron Laser in Hamburg (FLASH), a brilliant radiation source that provides intense EUV laser flashes. This allows the events from each individual laser flash to be analyzed in terms of photon energy, yielding high-resolution spectral data sets.

Helium, as the simplest and most accessible multi-electron system, is ideal for fundamental theoretical and experimental studies. Here, the mutual electrical repulsion of the two electrons plays an essential role – accounting for a good third of the total binding energy. Of particular and fundamental interest is the interaction with photons (the quanta of light).

Researchers from the groups around Christian Ott and Robert Moshammer in Thomas Pfeifer’s department at the Max Planck Institute for Nuclear Physics in Heidelberg investigated in detail the two-photon resonant ionization of helium at the free-electron laser FLASH of DESY in Hamburg.

In this nonlinear process, both electrons simultaneously absorb two extreme UV photons and form a doubly excited state in which, indicatively, both electrons are in a long orbit around the helium nucleus. The correlated electron pair dance is unstable, and their mutual repulsion causes one to leave the atom while the other falls back to the ground state of the helium ion—a process called autoionization (see Fig. 1). It occurs when the sum of the energy of the photons simply corresponds to the discrete excitation energy, i.e. when the so-called resonance condition is met.

For a detailed measurement, the researchers used a reaction microscope (REMI), which allows a kinematically complete detection of both photoelectrons and helium ions. However, a fundamental difficulty had to be overcome: Although the free-electron laser emits sufficiently intense UV radiation, the energy of the photons has a fairly large bandwidth, and the energy range of the highest intensity also varies from laser flash to laser flash.

However, this very property has now been exploited: “We used a spectrometer to measure the energy distribution of the photons in each individual shot and then sorted them according to the photon energy with the highest intensity (peak position),” he explains. first author Michael Straub. “Synchronized with the REMI signals, we thus obtain high-resolution spectral data sets, digitally tuned across the entire bandwidth.” (Fig. 2).

The resonance was resolved by this trick and the angular distribution of the photoelectrons in the resonance was measured. In direct comparison with theoretical calculations by the group of Chris Greene (Purdue University), there was good agreement, but also discrepancies upon closer examination. One explanation is small contributions from non-resonant ionization by single photons of twice the energy (red curve in Fig. 1), which account for about 1% of the FLASH photon flux.

“These results and the newly developed experimental methodology open a promising avenue for investigating fundamental interactions of a few photons with a few electrons,” says team leader Christian Ott, summarizing the scope of the work, now published in Physical Review Letters.