Absolute electron-impact ionization cross sections of free radicals from threshold to 30 eV: CHx=2,3, Si(CH3)x=1,2,3, and H2Si(CH3) radicals

We report absolute electron-impact ionization cross sections from threshold to 30 eV for the formation of the parent ions from free radicals. A microwave discharge is used as a source to produce a large amount of these free radicals, and measurements are mainly performed by means of mass spectrometry. We detail the method used to measure electron-impact ionization cross sections of the main radicals produced. First, this method is tested on well known radicals such as CH3 and CH2, produced in a discharge sustained in an Ar-CH4 gas mixture; then it is applied to the free radicals Si(CH3)(3,2,1) and H2Si(CH3), which are produced in a discharge sustained in Ar-Si(CH3)(4). The experimental results are compared with the calculations performed using the binary-encounter-Bethe model and using a simplified form of this theory, the total ionization cross section model. Absolute electron-impact ionization cross sections have been measured at 30 eV for Si(CH3) and CH3 by means of the selective energy transfer reaction process between Ar(P-3(2)) metastable species and the tetramethylsilane molecule [Si(CH3)(4)]. The value measured for Si(CH3) is used as the reference to calculate absolute values from relative ionization cross sections measured for the other radicals, Si(CH3)(3,2,1) and H2Si(CH3), from threshold to 30 eV. For example, we find at 30 eV the absolute cross section values, 13.9x10(-20)+/- 1.8x10(-20), 2.0x10(-20)+/- 0.3x10(-20), 1.5x10(-20)+/- 0.1x10(-20), and 12.8x10(-20)+/- 1.7x10(-20) m(2) for m/z=73, 58, 43, and 45, respectively.