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.