Breakdown of the dipole approximation in soft-X-ray photoemission

Although breakdowns in the dipole approximation in the soft-X-ray photon-energy range (hv less than or equal to 5 keV) were first observed 30 years ago and have been studied theoretically for many years, their significance at low energies has remained generally unappreciated within the broader photoemission community. Advances in gas-phase photoemission experiments using synchrotron radiation have recently highlighted nondipole effects at relatively low energies while probing the limits of the dipole approximation. Breakdowns in this approximation are manifested primarily as deviations from dipolar angular distributions of photoelectrons. Detailed new results demonstrate nondipolar angular-distribution effects are easily observable in atomic gases at energies well below 1 keV, and, in molecules, a previously unexpected phenomenon greatly enhances the breakdown of the dipole approximation just above core-level ionization thresholds. A progress report on this newly burgeoning area from an experimental perspective is presented here, including a brief history, a description of recent advances, graphical representations of nondipolar angular distributions, a re-evaluation of the classic first experiment in the soft-X-ray range and a look to the future. (C) 1999 Elsevier Science B.V. All rights reserved.