Studies of the single and double photoionization of two-electron systems at the Advanced Light Source

Abstract

The Advanced Light Source (ALS) at Lawrence Berkeley Laboratory is a "third generation" synchrotron radiation machine that incorporates wigglers and undulators to enhance the spectral brightness of the emitted radiation. Commissioned in October 1993, its performance to date has been nothing short of spectacular. Its demonstrated characteristics make it an ideal tool for carrying out high resolution measurements of photoionization studies of two-electron atomic and ionic systems. ALS Beamline 9.0.1, a facility dedicated to the study of photoprocesses in atoms, molecules, and ions, utilizes an undulator with an 8 cm period and a monochromator with spherical gratings to produce light at energies between 20 and 300 eV. At a photon energy of, 50 eV, the beamline is designed to deliver about 1.5 × 1013 photons/s into a spot measuring approximately 50 × 800 μm with a resolving power of 10 000. Measurements of low-lying autoionizing resonances in He, carried out shortly after the ALS was commissioned, provided the first confirmation of the essential beamline characteristics. The 1.0 meV line width, more recently observed for the 2p3d double-excitation state of He at 64.12 eV, demonstrates that the actual resolving power of the beamline far exceeds the design value. The properties of the ALS, combined with improvements in the reliability and operating characteristics of ion sources, make it possible to extend high-resolution synchrotron experiments beyond the study of neutral systems. Investigations of low-Z ions are important, because they provide an easy way to vary the relative contribution of the electron–electron interaction. An international collaboration has recently been formed to perform such studies. Its list of priorities includes the doubly excited spectra and the threshold region for two-electron photoionization in Li+ and H−. Metastable He*, formed by resonant charge transfer from He+, will provide access totriplet states.