Near-Threshold Photoionization of Hydrogenlike Uranium Studied in Ion-Atom Collisions via the Time-Reversed Process

Abstract

Radiative electron capture, the time-reversed photoionization process occurring in ion-atom collisions, provides presently the only access to photoionization studies for very highly charged ions. By applying the deceleration mode of the ESR storage ring, we studied this process in low-energy collisions of bare uranium ions with low- Z target atoms. This technique allows us to extend the current information about photoionization to much lower energies than those accessible for neutral heavy elements in the direct reaction channel. The results prove that for high- Z systems, higher-order multipole contributions and magnetic corrections persist even at energies close to the threshold.

Observation of Coherence in the Time-Reversed Relativistic Photoelectric Effect

“…The total solid angle coverage was 1%. This is an order of magnitude increase as compared to a typical x-ray detection setup used at the gas-jet target in earlier experiments [7,8]. Correspondingly, the x-ray coincidence efficiency increases by 2 orders of magnitude.…”

“…These limitations were removed by studying the photoelectric effect via its time-reversed process of radiative recombination (RR) of initially free or weakly bound electrons into heavy highly charged ions (HCIs). One experimental approach exploits the properties of the emitted RR x rays, such as their angular distribution with respect to the propagation direction of the incoming electron [7,8] and their linear polarization [9]. For example, the latter experiment studied, in time reverse, the angular distribution of the electrons photoionized by linearly polarized x rays.…”

“…Details about beam preparation, the experimental environment at the ESR, as well as the gas-jet target, can be found in Refs. [7,8].…”

“…The measurement was conducted at the experimental storage ring (ESR) of the GSI accelerator complex in Darmstadt [7,8,29]. Typically, 10 7 fully ionized uranium ions of the spinless isotope 238 U 92þ were injected into the ring with a kinetic energy of 230 MeV=u.…”

Radiative-electron-capture-to-continuum cusp inU88++N2collisions and the high-energy endpoint of electron-nucleus bremsstrahlung

“…This process, closely related to the fundamental process of electron-nucleus bremsstrahlung, is termed radiative electron capture to continuum (RECC) or radiative ionization [26][27][28]. RECC in turn is closely related to radiative electron capture to bound states (REC) [29,30], analog to the described relationship between bremsstrahlung and radiative recombination. Measurements of triple-differential cross sections of RECC thus provide the most stringent tests for the theory of electronnucleus bremsstrahlung and the wave functions of low-energy continuum states in the vicinity of the nucleus [3][4][5][6][7][8][9][10].…”