Production of vacuum ultraviolet radiation by electron impact on silicon-organic molecules

The vacuum ultraviolet (VUV) emissions of the hydrogen Lyman series following the dissociative excitation of the Si-organic compounds tetraethoxysilane (TEOS), tetramethylsilane (TMS), and hexamethyldisiloxane (HMDSO) by controlled electron impact were analyzed. Absolute photoemission cross sections from threshold to 200 eV and appearance potentials were determined for the Lyman-α and Lyman-β emissions for each of the three targets. The measured apparent emission cross sections at 100 eV impact energy for the Lyman-α lines are 6.0 × 10^-19 cm² for TMS, 6.5 × 10^-19 cm² for HMDSO and 1.1 × 10^-18 cm² for TEOS. The apparent Lyman-α cross sections for all three targets contained significant cascade contributions from the H Balmer series. Detailed studies of the near threshold regions indicated that several break-up mechanisms of the parent molecules contribute to the emissions. The lowest onsets for HMDSO, and TEOS were attributed to the removal of a single excited atomic hydrogen atom in the n = 2 state. Additional onsets at higher energies could not be uniquely correlated in all cases with a particular break-up channel and (or) cascading, since the other fragments in these processes were not identified. The Lyman-β apparent emission cross sections were also measured and we found values of 1.3 × 10^-19 cm² for TMS, 1.6 × 10^-19 cm² for HMDSO, and 2.0 × 10^-19 cm² for TEOS at an impact energy of 100 eV.