Electron impact spectroscopy of methane and methane–d4

Electron impact spectra of methane and methane-d4 have been obtained employing electrons of 200 eV incident energy at scattering angles of 2° to 15° over an energy-loss range that includes the second ionization potential. In the bound state spectrum evidence is presented for the existence of at least one additional state in the region of the 3s transition. A new vibrational progression has been found on the side of a conjectured 4s band lending support to that assignment. One diffuse structure and three new optically forbidden vibrational progressions have been found in a 4 eV range below the 2A1 ion threshold. Evidence indicates that the first three transitions belong to the symmetry forbidden Rydberg series (2a1, ns)←(2a1)2 where n=3, 4, and 5. It is suggested that the fourth transition is the first member of a series converging to an excited ion state.