Dramatic Nondipole Effects in Low-Energy Photoionization: Experimental and Theoretical Study of Xe5s

Abstract

The Xe 5s nondipole photoelectron parameter is obtained experimentally and theoretically from threshold to 200 eV photon energy. Significant nondipole effects are seen even in the threshold region of this valence shell photoionization. In addition, contrary to previous understanding, clear evidence of interchannel coupling among quadrupole photoionization channels is found. DOI: 10.1103/PhysRevLett.91.053002 PACS numbers: 31.25.Eb, 32.80.Fb Until recently, conventional wisdom had assumed nondipole effects in photoionization were negligible at relatively low photon energies, perhaps for energies up to a few keV, but certainly for photon energies below a few hundred eV [1][2][3]. Indeed, despite indications to the contrary [4 -7], the usual practice in the field of photoionization, particularly for experiment, had been to ignore effects beyond the dipole approximation for photon energies as high as several keV. While this may be a reasonable assumption for integrated cross sections, recent work has shown it is certainly wrong for differential cross sections (i.e., photoelectron angular distributions). Experiments have shown the importance of nondipole effects in the 1-3 keV photon-energy region [8,9], in the hundreds-of-eV range [10], and, in one case, at 13 eV [11]. Concurrently, theory has predicted significant nondipole contributions to electron angular distributions for atomic valence shells down to threshold at a few tens of eV photon energy [12 -14].In addition, for dipole photoionization, interchannel coupling, which is simply configuration interaction in the continuum, has been shown to be important for most subshells of most atoms at most energies [15,16]; this work was contrary to the previous conventional wisdom that the independent-particle approximation (IPA) was generally valid away from thresholds. It had been suggested, however, that such interchannel coupling was not important in quadrupole photoionization channels [17], but recent theory has suggested that interchannel coupling can indeed be significant in quadrupole photoionization channels as well [14].To test these two ideas, significant nondipole contributions to the photoionization of a valence shell in the threshold region, and the existence of interchannel coupling effects in quadrupole photoionization channels, we have performed a benchmark experiment on the differential photoionization cross section of Xe 5s from 26 eV (close to threshold) to 200 eV to obtain the nondipole contribution to the photoelectron angular distribution which arises from interference between dipole (E1) and quadrupole (E2) channels. The differential cross section is given by [6,[18][19][20] where is the angle-integrated cross section, is the dipole anisotropy parameter, P 2 cos 3cos 2 ÿ 1=2, and and are nondipole asymmetry parameters. The coordinate axes have the positive x axis along the direction of the photon propagation vector, the z axis along the photon polarization vector, and and are the polar and azimuthal angles of the photoelectron momentum vector. ...

Non-dipole angular anisotropy parameters of photoelectrons from semi-filled shell atoms

“…[10]). Note, that in spite of the fact that the non-dipole anisotropy parameter is usually for low enough photon energies much smaller than the dipole one (equal to 2 for s-subshells), it is quite measurable experimentally almost from the threshold even for He [19].…”

Dipole and nondipole photoionization of molecular hydrogen

“…Technical improvements were made since our previous experiments [23,27,31,37] to further reduce spurious effects. The vacuum chamber is shielded with two layers of high permeability Mu-metal that reduce the magnetic field to <1 mG.…”

“…The accurate methods that were developed for calculations of atomic photoionization within the dipole approximation [29,30] have been extended to calculations of nondipole asymmetries, including boundcontinuum and continuum-continuum interactions [25,26]. Nondipole asymmetries yield insight into photoionization dynamics such as Cooper minima and interchannel coupling [27,31,32].…”

Nondipole Effects in Photo-Double-Ionization of He by a vuv Photon

“…Although the first nondipole SPI measurements for Ar were in the x-ray region [1], very recently the forward-backward asymmetry of the photoelectron angular distributions (i.e., with respect to the photon wave vector k, originating from the interference of dipole and quadrupole SPI amplitudes) has been found to be significant also in the vuv region (see, e.g., results for Xe at E & 200 eV [2] and for He at E & 160 eV [3]). Besides the asymmetry in the angular distributions, nondipole effects lead also to new features in SPI for spin-resolved photoelectron measurements [4] and for the case of polarized atoms [5].…”