Near-threshold photoionization of hydrogenlike uranium studied in ion-atom collisions via the time-reversed process

Observation of coherence in the time-reversed relativistic photoelectric effect

The photoelectric effect has been studied in the regime of hard x rays and strong Coulomb fields via its time-reversed process of radiative recombination (RR). In the experiment, the relativistic electrons recombined into the 2p_{3/2} excited state of hydrogenlike uranium ions, and both the RR x rays and the subsequently emitted characteristic x rays were detected in coincidence. This allowed us to observe the coherence between the magnetic substates in a highly charged ion and to identify the contribution of the spin-orbit interaction to the RR process.

Direct determination of the magnetic quadrupole contribution to the Lyman-α₁ transition in a hydrogenlike ion

We report the observation of an interference between the electric dipole (E1) and the magnetic quadrupole (M2) amplitudes for the linear polarization of the Ly-α1 (2p3/2→1s1/2) radiation of hydrogenlike uranium. This multipole mixing arises from the coupling of the ion to different multipole components of the radiation field. Our observation indicates a significant depolarization of the Ly-α1 radiation due to the E1-M2 amplitude mixing. It proves that a combined measurement of the linear polarization and of the angular distribution enables a very precise determination of the ratio of the E1 and the M2 transition amplitudes and the corresponding transition rates without any assumptions concerning the population mechanism for the 2p3/2 state.

Radiative double electron capture in collisions of O8+ ions with carbon

Multielectron capture processes observed in low energy collisions of bare ions with target atoms open insight into electron correlations in electromagnetic fields. Radiative double electron capture (RDEC) provides the simplest tool for investigation of such processes. Here, the experimental observation of the RDEC process in collisions of O8+ ions with thin carbon films is presented and the RDEC cross section value obtained is compared with recent theoretical calculations.

First measurement of the linear polarization of radiative electron capture transitions

For radiative electron capture into the K shell of bare uranium ions, a study of the polarization properties has been performed. For this purpose a position sensitive germanium detector has been used as an efficient Compton polarimeter. This enabled us to measure the degree of linear polarization by analyzing Compton scattering inside the detector and to determine the orientation of the polarization plane. Depending on the observation angle and the beam energy used, the radiation is found to be linearly polarized by up to 80%. In all cases studied, the plane of polarization coincides with the collision plane. The results will be discussed in the context of rigorous relativistic calculations, showing that relativistic effects tend to lead to a depolarization of the radiation emitted.

Lyman- alpha(1) decay in hydrogenlike ions: interference between the E1 and M2 transition amplitudes

For the Lyman- alpha(1) transition (2p(3/2)-->1s(1/2)) in hydrogenlike ions an interference between the leading E1 decay channel and the much weaker M2 multipole transition gives rise to a remarkable modified angular distribution of the emitted photons from aligned ions. This effect is most pronounced for the heaviest elements but results in a still sizable correction for medium- Z ions. For the particular case of hydrogenlike uranium where the angular distribution of the Lyman- alpha(1) x rays following radiative electron capture has been measured, the former variance with theoretical findings is removed when this E1-M2 interference is taken into account.

Electron acceleration by a tightly focused laser beam

State-of-the-art petawatt laser beams may be focused down to few-micron spot sizes and can produce violent electron acceleration as a result of the extremely intense and asymmetric fields. Classical fifth-order calculations in the diffraction angle show that electrons, injected sideways into the tightly focused laser beam, get captured and gain energy in the GeV regime. We point out the most favorable points of injection away from the focus, along with an efficient means of extracting the energetic electron with a static magnetic field.