Absolute total and partial electron impact ionization cross sections of hexamethyldisiloxane

Abstract

We studied the electron impact ionization of hexamethyldisiloxane (HMDSO), Si 2O(CH 3) 6, which is widely used in plasma-enhanced polymerization applications. Appearance energies and absolute partial cross sections for the formation of fragment ions with relative intensities >1% of the most abundant ion, the Si 2O(CH 3) 5 + fragment ion, were measured in a high resolution double focusing sector field mass spectrometer with a modified ion extraction stage for electron energies from threshold to 100 eV. Dissociative ionization was found to be the dominant process. The main ionization channel removes a complete methyl group to produce the fragment ion Si 2O(CH 3) 5 + with a cross section of 1.7 × 10 −15 cm 2 at 70 eV. The stoichiometric and isotope composition of the various fragment ions was determined by using the high resolution ( m/Δ m = 40,000) of the mass spectrometer used in these studies. The methyl ion is formed with considerable excess kinetic energy, whereas all other fragment ions are formed with essentially no excess energy. The experimental total single ionization cross section of HMDSO (2.2 × 10 −15 cm 2 at 70 eV impact energy) is in good agreement with the result of a semiempirical calculation (2.1 × 10 −15 cm 2 at the same energy).