Electron impact spectra of methane, ethane, and neopentane

Electron impact spectra of methane, ethane, and neopentane have been obtained at scattering angles of 0° and 90° and at impact energies from ?30 to 250 eV. The data are consistent with the lowest excitation in all of these systems to involve promotion of an electron to a 3s Rydberg-like orbital. Differences between 0° and 90° onsets are attributed to large-angle intensity enhancements of transitions to the 3s Rydberg triplets. At 90° all of the spectra exhibit very similar intensity redistributions with strong enhancement of transitions in the 12 and 15 eV region as compared to lower-lying transitions. Assignments of the spectra and possible origins of the angular dependence are discussed.