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

Although breakdowns in the dipole approximation in the soft-X-ray photon-en ergy range (hv less than or equal to 5 keV) were first observed 30 years ag o and have been studied theoretically for many years, their significance at low energies has remained generally unappreciated within the broader photo emission community. Advances in gas-phase photoemission experiments using s ynchrotron radiation have recently highlighted nondipole effects at relativ ely low energies while probing the limits of the dipole approximation. Brea kdowns in this approximation are manifested primarily as deviations from di polar angular distributions of photoelectrons. Detailed new results demonst rate nondipolar angular-distribution effects are easily observable in atomi c gases at energies well below 1 keV, and, in molecules, a previously unexp ected phenomenon greatly enhances the breakdown of the dipole approximation just above core-level ionization thresholds. A progress report on this new ly burgeoning area from an experimental perspective is presented here, incl uding a brief history, a description of recent advances, graphical represen tations 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) 199 9 Elsevier Science B.V. All rights reserved.