Angular Distribution Studies for the Time-Reversed Photoionization Process in Hydrogenlike Uranium: The Identification of Spin-Flip Transitions

Abstract

The K-shell photoelectric effect for a hydrogenlike U 911 ion is studied by its time-reversed process occurring in relativistic collisions between bare uranium ions and low-Z target atoms. In the timereversed situation, an electron is captured into the 1s ground state with the simultaneous emission of a photon. We present an angular differential study of these transitions for laboratory observation angles between nearly 0 ± and 150 ± . Our observation of photon emission close to 0 ± allowed us to identify spin-flip contributions to the photoionization process and to determine their relative strength.[S0031-9007 (99)08948-6] PACS numbers: 34.70. + e, 12.20.DsPhotoionization is one of the most important interaction processes between radiation and matter [1,2]. We have previously shown that for high-Z ions photoionization is studied best via its inverse reaction, radiative electron capture (REC) [3][4][5] in energetic ion-atom collisions [6]. For very high-Z systems, relativistic effects become important and require an exact theoretical treatment [1,7,8] adopting relativistic electron wave functions and including retardation, i.e., all multipole orders of the photon field [9]. For the quantitative assessment of relativistic effects in high-Z ions, angular distributions are a very sensitive probe [10]. This has already been shown for direct photoionization of neutral atoms at high energies (.500 keV), where retardation leads to a strong forward peaking of the photoelectron angular distribution and where spin-flip effects were observed close to 0 ± (see, e.g., [11]). The inverse reaction for a pure hydrogenlike system lends itself to a different critical test, namely, measuring the deviation from a sin 2 u distribution. For REC into the K shell of bare nonrelativistic ions, this reference distribution comes about by the cancellation between the effects of retardation, on the one hand, and the Lorentz transformation into the laboratory system, on the other hand [12,13]. Similar cancellations for various atomic processes are known to occur also in relativistic systems [14]. For high enough charges and energies, however, these cancellations are no longer complete. In particular, for Z 92 and for REC into the K shell, a significant cross section at forward angles has been predicted and shown to be a unique signature of spin-flip transitions [4,8,15].In this Letter, we report the first experimental study of a complete photon angular distribution for radiative electron capture into the K shell of a bare high-Z ion (uranium, Z 92). Our measurements encompass laboratory observation angles from near 0 ± to 150 ± , permitting the identification of spin-flip contributions that lead to photon emission at 0 ± . Photon emission at 0 ± and at 180 ± is forbidden by angular momentum conservation unless a magnetic spin-flip transition accounts for the spin of the emitted photon [15]. Furthermore, we deduce the electron angular distribution following photoionization of hydrogenlike uranium, i.e., for an explicitly point Coulombic high-Z...

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.…”

Discrepancies in quantum electro-dynamics

“…The origin of these discrepancies is not yet clear. Other tests in this regime have pursued lifetime or quench studies which may be based on laser-optical or X-ray transitions (Serpa et al, 1998), or on radiative recombination and radiative electron capture (Stolker et al, 1997(Stolker et al, , 1999, and related processes involving X-ray or visible emission from continuum states. The remainder of this review will concentrate on X-ray spectroscopic transitions between discrete levels, where narrow natural widths permit high-resolution measurements.…”