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Orbital electron density from electron momentum spectroscopy : comparison with AB initio calculations Rolke, James M.


Orbital electron momentum profiles measured by electron momentum spectroscopy (EMS) at a total energy of 1200 eV are presented in this work for the HOMOs of open-shell molecules (NO and N0₂), phosphorus compounds (PH₃, PF₃ and P(CH₃)₃) and transition metal hexacarbonyls (Cr(CO) ₆, Mo(CO) ₆ and W(CO) ₆). In addition, the complete valence shell binding energy spectra and all valence momentum profiles have been obtained for molecular oxygen (0₂) and methanol (CH₃OH) in the 9-59 eV and 6-47 eV binding energy ranges, respectively. A highly sensitive energy-dispersive multichannel EMS spectrometer has been used to obtain most of the measurements in the present work. The experimental momentum profiles for NO, 0₂ and CH₃OH are a significant improvement over previously published single-channel EMS results. The measured momentum profiles are compared with calculated cross-sections using a range of Hartree-Fock (HF) wavefunctions from minimal basis set to near-Hartree-Fock limit in quality. The effects of correlation and relaxation on the calculated momentum profiles of NO, N0₂, 0₂, PH₃ and CH₃OH are investigated using multi-reference singles and doubles configuration interaction (MRSD-CI) calculations of the full ion-neutral overlap distributions. The experimental measurements have also been compared with calculated density functional theory (DFT) momentum profiles using the target Kohn-Sham approximation. The measured valence experimental shell binding energy spectra for 0 2 and CH₃O H have been compared with spectra calculated using MRSD-CI and many-body Green's function methods. Generally good agreement has been obtained between either high-level HF, MRSD-CI or DFT calculated momentum profiles and the experimental momentum profiles for NO, N0₂, 0₂, PH₃, PF₃, P(CH₃) 3 and CH₃OH. However, there are still some small but significant discrepancies at low momentum for the outermost orbitals of NO and 0₂ as well as the 2a" (HOMO), 7a' and 5a' orbitals of CH3OH. Post-Hartree-Fock calculations have little effect on the shapes of the calculated momentum profiles for NO, N0₂ and PH₃. In contrast, the 7a', (6a'+la") and 5a' orbital experimental momentum profiles for methanol display shapes and intensities that can only be predicted by the MRSD-C I and/or DF T calculations and it is clear that in these cases electron correlation is important. The presently reported MRSD-C I calculated pole strengths and energies for 0₂ and the measured momentum profiles have been used to assign the significant satellite splitting observed in the 2au and 2rjg ionization strength. Experimental determinations of the pole strengths for the inner valence (4a')"1 and (3a')_1 satellite ionization processes of CH₃OH have also been achieved in the present work. The observed increase in s-type character for the outermost orbitals of PF₃ and P(CH₃)₃ relative to the HOM O of PH₃ is explained by molecular orbital calculations. This increased s-type character has been interpreted in terms of a derealization of the charge density in the HOM O orbitals when H is replaced by F or CH₃. The present experimental results for the HOMO of Cr(CO) ₆ verify earlier measurements in that they display large intensity at low momentum which is not predicted by either symmetry considerations or calculations within the plane wave impulse approximation (PWIA). High intensity at low momentum is also observed for the HOMOs of Mo(CO) ₆ and W(CO) ₆ and it is suggested that this is due either to ground state vibrational effects or distortion of the incoming and outgoing electrons waves. The latter suggestion is supported by distorted wave impulse approximation (DWIA) calculations for Cr 3d and Mo 4d atomic orbitals.

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