Valence orbital electron momentum distributions for oxygen: comparison of EMS measurements with theory

The valence shell binding energy spectra and orbital electron momentum profiles of O-2 have been measured by energy dispersive multichannel electron momentum spectroscopy at an impact energy of 1200 eV + binding energy. The effects of electron correlation on the valence binding energy spectrum are investigated using multi-reference singles and doubles configuration interaction calculations. The presently reported experimental momentum profiles of O-2 display considerably improved statistics compared with previously published EMS results. The measured momentum profiles are compared with cross sections calculated using both unrestricted and restricted open shell Hartree-Fock methods with basis sets ranging from minimal to near Hartree-Fock limit in quality. In addition, the effects of correlation and relaxation on the calculated momentum profiles are investigated using multi-reference singles and doubles configuration interaction calculations of the full ion-neutral overlap distributions. Electron correlation effects in the ground state are further examined using several density functional approaches for the momentum profiles. The present EMS measurements and MRSD-CI calculations clearly show that the binding energy peak at similar to 27.3 eV has significant contributions from both (4) Sigma(u)(-) and (2) Sigma(u)(-) processes in contrast to earlier assignments which have attributed this peak to the C-2 Sigma(u)(-) State alone. Similarly, the binding energy peak at 33 eV is shown to be due to (2) Sigma(u)(-) rather than earlier assignments of (2) Pi(u) character. (C) 1998 Elsevier Science B.V. All rights reserved.