Imaging of the HOMO electron density in Cr(CO)(6), Mo(CO)(6) and W(CO)(6) by electron momentum spectroscopy: A comparison with Hartree-Fock and DFT calculations

Studies of the electron density distributions in the t(2g) HOMO orbitals (essentially metal valence d character) of the transition metal carbonyls Cr(CO)(6), Mo(CO)(6) and W(CO)(6) have been made using electron momentum spectroscopy (EMS). The EMS measurements provide a test of basis set effects and the quality of theoretical methods in the description of these large systems. The experimental momentum profiles are compared with theoretical spherically averaged momentum profiles calculated using the plane wave impulse approximation (PWIA) at the level of the target Hartee-Fock approximation (THFA) with a range of basis sets. Electron density maps for the HOMO orbitals in both momentum and position space have also been calculated for the oriented molecules at the Hartree-Fock level. Further comparisons of the measured momentum profiles to PWIA calculations are made in the target Kohn-Sham approximation (TKSA) using density functional theory (DFT) with the local density approximation and also with gradient corrected exchange correlation potentials. In addition to momentum profiles, other electronic properties have also been calculated by the various theoretical methods and compared to experimental values. The experimental momentum profiles for the HOMO orbitals of all three hexacarbonyls show significant cross-section at momenta below similar to 0.5 au including a large contribution near p = 0 which is not expected from symmetry considerations or predicted by either the Target Hartree-Fock or the DFT target Kohn-Sham PWIA calculations. The discrepancies may be due to the significant vibrational excitation of the initial state metal carbonyl molecules expected at room temperature. However, theoretical studies of the Cr3d and Mo4d atomic systems show that significant low momentum components are introduced into the EMS cross-section when the PWIA treatment is replaced with the distorted wave impulse approximation (DWIA). While DWIA calculations are not possible for molecules the results for atomic Cr3d and Mo4d strongly suggest that the unusual effects observed experimentally in the M(CO)(6) HOMO (essentially Cr3d, Mo4d and W5d in character) cross-sections are due to distortion of the electron waves in the collision process.