Electron impact ionization of SiCl_x and TiCl_x (x = 1-4): Contributions from indirect ionization channels

We measured absolute partial cross sections for the formation of various singly charged positive ions produced by electron impact on SiCl_x (x = 1-4) using two different experimental techniques, a time-of-flight mass spectrometer (TOF-MS) and a fast-neutral-beam apparatus. The energy range covered was from the threshold to 900 eV in the TOF-MS and to 200 eV in the fast neutral beam apparatus. In the case of SiCl₄, the absolute cross sections obtained by the two different experimental techniques were found to agree very well. The ionization of the SiCl_x (x = 1-3) free radicals can only be studied using the fast-beam technique. Some of the partial cross section curves were found to exhibit an unusual energy dependence with a pronounced low-energy maximum at an energy around 30eV, which is in some cases followed by a shallow minimum at about 40eV and a broad second maximum at around 100eV. This pronounced low-energy maximum is indicative of the presence of an indirect ionization channel and is similar to what has been observed earlier for TiCl₄ and the TiCl_x (x = 1-3) radicals. Similarities in the contribution of the indirect ionization channel to the formation of the various SiCl_x⁺ and TiCl_x⁺ (x = 1-4) ions will be highlighted in this paper.