Orbital imaging for the valence shell of sulphur dioxide: comparison of EMS measurements with near Hartree–Fock limit and density functional theory Original Research Article

The momentum distributions of the valence orbitals of sulphur dioxide have been studied by electron momentum spectroscopy (EMS), Hartree–Fock (HF) and density functional theory (DFT) calculations. The experiments were performed using an energy dispersive multichannel EMS spectrometer at an impact energy of 1200 eV plus the binding energy and in symmetric non-coplanar kinematics. The valence-shell binding energy spectra have been measured for all valence orbitals over the energy range 8–60 eV. Calculated synthetic binding energy spectra derived from many-body Green’s function and DFT calculations are compared with experiment. In the inner valence region, strong splitting of the 3b2 and 5a1 ionization is observed due to final state electron correlation effects. The measured momentum profiles of the valence orbitals are compared with HF calculations at the level of the target HF approximation, and with DFT calculations using B3LYP functionals at the level of the target Kohn–Sham approximation, with basis sets ranging from minimum (STO-3G) to large (AUG-cc-pV5Z(-h)). Generally, the shapes of the experimental momentum profiles are well reproduced by HF and DFT calculations using large diffuse basis sets. However, small discrepancies still exist between all theoretical treatments and experiment in the low momentum region for the (4b2+7a1+2b1) and 6a1 orbitals.