Electron momentum spectroscopy study of 1,3-dimethyladamantane: Methyl substitution effects on the outer valence orbitals

This paper investigates the effects of methyl substitution on the valence orbital momentum distributions of adamantane. Kinematically complete electron-impact ionization experiments on 1,3-dimethyladamantane are performed at an incident electron energy of 1.2 keV, and the resulting electron momentum profiles are compared with those of pristine adamantane. Moreover, the influences of molecular vibration and distorted-wave effects on the electron momentum profiles are examined. The highest occupied molecular orbital (HOMO) of adamantane, 7t₂, is split into the 10b₁, 18a₁, and 12b₂ orbitals in 1,3-dimethyladamantane. It is revealed that the momentum profile for the sum of the 10b₁, 18a₁, and 12b₂ orbitals exhibits a markedly higher intensity at low momentum than that of the 7t₂ parent orbital. This strongly suggests that the HOMO is spatially more extended when surface hydrogens are replaced with methyl groups, which causes an increase in the absorption cross section of the 3 s Rydberg excitation.