Fragmentation of tetravinylsilane and dimethylphenylsilane by electron impact O. Struzinsky 1 , H. Sahankova 1 , F. Krcma 1 , J. L. D. Cayao 2 , J. Kocisek 2 , S. Matejcik 2 1 Faculty of Chemistry, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech Republic 2 Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius University Bratislava, Mlynska dolina F2, 842 48 Bratislava, Slovakia Abstract: Fragmentation of tetravinylsilane and dimethylphenylsilane by electron impact was investigated by mass spectroscopy. Mass spectra was measured at 70eV and compared with NIST databse. Some of the most abundant peaks were discussed and reaction mechanisms of ion decomposition were predicted. Furthermore, there are pre- sented tables which are containing treshold energies of most abundant ions. Was discussed influence of molecule structure on ionisation properties. Measured were positive ions only. Keywords: Ionisation, tetravinylsilane, dimethylphenylsilane, mass spectroscopy, treshold energy. 1. Introduction The plasma deposited organosilicone thin films are nowadays used in a number of applications, e.g. microe- lectronics, protecting layers or intermediate phase in composites [1]. The most widely technologically used monomers are, hexamethyldisiloxane (HMDSO), tetra- ethoxysilane (TEOS), tetravinylsilane (TVS) and dimeth- ylphenylsilane (DMPS) which are an object of our inves- tigation. These monomers are used for creation thin plasma polymer film. The film properties differ by their composi- tion mainly reflecting the original monomer structure and plasma deposition conditions [2]. Very important feature of TVS thin films is that they have nanoporous structure (semiconducting industry exploited) [3] . The monomer fragmentation degree and the fragmentation process itself are the main factors influencing the structure of the de- posited film. The film properties significantly depend also on the oxygen ammount in the film. Both TVS and DMPS monomers contain no oxygen and they are isomers. The very recent calculations of TVS fragmentation compared with the experimental data partialy described here showed that (during the TVS frangmentation) some phenyl struc- tures can be generated [4]. This contribution compares the fragmentation of both these monomers by the elektron impact. The cross section for electron impact ionization reaction was described by more theories [5]. 2. Experimental Experiment set up allows measuring of two dif- ferent electron-molecular interactions. The first one is electon impact ionisation (EII, positive ion), the second one is dissociative electron capture (negative ion). The aim of this article is an experimental study of electron impact ionisation of TVS and DMPS by electron-crossed beam. EII process can be described by a following reaction: M + e - → M + + 2 e - , where M + is molecular ion. It can be stable or it can be decomposed under the reaction M + → M 1 + + M n , where M 1 + is molecular ion again and M n is neutral frag- ment. EII process can not create fragments under lower energy than molecular ion is. Due to this fact, treshold energy of molecular ion is so called a ionisation energy. The experiments were performed at the Department of experimental physics Comenius University in Brati- slava. The aparature scheme can be seen in (Fig. 1). The electron molecular reaction was performed in a high vac- uum reaction chamber (RC). Laminary molecular beam (created by using the effusion capillary) collides with electron beam (emitted by heated tungsten wire and formed by the trochoidal electron monochromator TEM). An ionic signal is amplified by an electron multiplier. The rising ions are selected by ion optics (IO) into a quadrupole mass spectrometer (KHA). The obtained data are computer recorded (PC). There is a Faraday cup (FC) situated behind the reaction chamber for the accurate measurement of the electron beam energy. The software is able to adjust energy of electrons and KHA selectivity by DA converter.