Studies of molecular orbital momentum distributions by ( e , 2 e ) spectroscopy

Binary electron impact ionization or ( e , 2 e ) spectroscopy has recently been employed to measure molecular orbital momentum distributions in a variety of molecules. Interpretation of the momentum distributions ρ( q ) has been facilitated by the use of the wave‐function autocorrelation function B ( r ). In some cases differences in the autocorrelation function of two orbitals may be semiquantitatively interpreted in terms of contour plots of the average and difference of their position‐space orbital amplitudes. The σ g and σ u orbitals of a homonuclear diatomic provide a simple illustration of this point. Calculated and experimental spherically averaged ρ( q ) for the predominantly N 2p lone‐pair HOMOs of NH 3 and CH 3 NH 2 are shown using a Δ B ( r ) analysis to indicate significant participation of the CH 3 hydrogens in the lone‐pair orbital of CH 3 NH 2 . Although the difference in ρ( q ) between the lone pairs of NH 3 and CH 3 NH 2 is adequately reproduced by split valence (SV) level ab initio SCFMO calculations the experimental ρ( q ) have their maxima at considerably smaller absolute values of q then that obtained from either SV or near Hartree–Fock SCFMO calculations. Experimental and calculated momentum distributions are also presented for the 2t 2g HOMO of Cr(CO) 6 . A Δ B ( r ) analysis of ρ( q ) for this orbital confirms that it has mixed Cr 3d and CO 2π character.