Electron impact spectroscopy of silane and germane

Electron impact spectra of silane and germane have been recorded using electrons of 200 eV incidence at scattering angles of approx.20--90. The range of energy losses covers the entire region of single electron excitation from the valence shell. In the bound state spectrum evidence is presented supporting the existence of two valence transitions in addition to anticipated s, p, and d Rydberg transitions. The 90 spectra reveal much structural detail in the approx.4 eV energy range preceding the 2A1 ion state. The most prominent band consists of a superposition of an optically allowed valence excitation and a symmetry forbidden Rydberg transition. The remaining peaks may be members of vibrational progressions belonging to either [d(t2), a1], [p, a1] reverse arrow (a1)2 dipole allowed or [d(e), a1], [s,a1] reverse arrow (a1)2 dipole forbidden channels. Of these two possibilities the evidence presented here favors the latter choice