W. Lindinger. Institut für Ionenphysik der Universität Innsbruck, Technikerstr. 25, Innsbruck, Austria

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1 FORMATION OF SiH 3 IONS IN REACTIONS OF SMALL HYDROCARBON IONS WITH SiH 4 G. Bano, A. Luca, J. Glosík,P.Zakouřil Charles University Prague, Mathematics and Physics Faculty, Department of Electronics and Vacuum Physics, V Holešovičkách 2, Prague 8, Czech Republic W. Lindinger Institut für Ionenphysik der Universität Innsbruck, Technikerstr. 25, Innsbruck, Austria Received 6 April 2000 Selected ion flow drift tube (SIFDT) was used to study the kinetics of the formation of SiH 3 ions in the reactions of silane, SiH4, withseveral small hydrocarbon ions at nearthermal collision energies. The reaction rate coefficients and the product distributions were measured for the reactions of ions CH 3,C2H 3,C2H 5. The reactions were studied for the collision energies from 0.04 up to 1.5 ev and for several pressures of the buffer gas. The only product ion in the reactions of CH 3 and C2H 5 ions withsih4 is SiH 3.Inthe reaction of H 3 there are three product ions, SiH 3,SiH 5 and SiC2H 5.Atallcovered collision energies, all three reactions are very fast with the reaction rate coefficients close to the collisional rate coefficients. 1 Introduction Investigations of ion-molecule reactions of different ions with silane are mostly motivated by their importance in plasmas for technological applications. In recent years, our research has been directed towards understanding of ion- molecule reactions where Si-C bonds are formed (see, e.g., Refs. [1,2,3,4]). The reactions of silicon containing molecules, neutral and ionized, in the ion-molecule reactions and ions formed in these reactions have recently been the subject of a number of theoretical (see, for example, the discussions on the structure SiH 5 in Refs. [5,6]) and experimental ([7,8]) investigations. See also the reviews by Bohme [9], Armentrout [8], the compilation of Anicich [10], and references therein. The reactions of silicon ions and silicon containing molecules are important in the interstellar and circumstellar chemistry [11,12], and they could be of interest to theoretical chemists because of the fundamental aspects of the chemical bonding of atoms to silicon. One of our reasons to study the reactions of hydrogen containing molecular ions with SiH 4 was the attempt to produce SiH 5 ions in sufficient quantities to study their collisional dissociation and structure [5,6]. 2 Experiment The measurements have been carried out using the Innsbruck selected ion flow drift tube (SIFDT) apparatus of a conventional design (see description in Refs. Czechoslovak Journal of Physics, Vol. 50 (2000), Suppl. S3 C 1

2 G. Bano et al. [13,14]). The used SIFDT is shown schematically in Fig. 1. The hydrocarbon reactant ions were generated from CH 4 in an electron impact ion source. The ions RP Mo Re Bu ID Q 1 V Q 2 IS Th TP DP Fig. 1. The Innsbruck Selected Ion Flow Drift Tube (SIFDT). Qu - quadrupole mass filter, IS - ion source, Th- thermalization region, ID - ion detector, RP - roots pump, TP - turbo pump, DP - diffusion pump. were subsequently mass selected in a quadrupole mass filter and injected into the flow tube via a Venturi-type inlet. In the first thermalization part of the drift tube, the collision energy of the ions with the buffer gas atoms was kept below 0.1 ev in order to quench the internal excitation of the injected ions. The reactant and the product ions were monitored down-stream with the second quadrupole mass spectrometer as a function of the flow rate of the added neutral reactant (silane). The reactant ion/reactant neutral center-of-mass kinetic energy,,wasderived by means of the Wannier expression (for details see Refs. [15,16]). Helium at pressures Torr was used as the buffer gas. Mobilities of the reactant ions in He were measured in the present experiment. The mobility of CH 3 is taken from our previous studies and is in good agreement with a former study of Peska et al. [17]. The accuracy of the obtained reaction rate coefficients is ± 30% as it is usual for swarm experiments of the SIFDT type. 3Results 3.1 The reaction of CH 3 with SiH4 In the covered energy range, the reaction of CH 3 with SiH 4 is proceeding with a rate coefficient close to the collisional rate coefficient (approximated by Langevin rate coefficient, k L ). In Fig. 2, the measured reaction rate coefficient is plotted versus the average kinetic energy in center-of-mass frame,, for two different pressures of the buffer gas. SiH 3 is the dominant ionic product. Neutral products cannot be detected, nevertheless, the only energetically possible product is CH 4 ( H=-52.8 kcal/mol, energetics of the reactions is calculated on the base of the data given in the Refs. [18,19]). The reaction scheme is: 2 C Czech. J. Phys. 50/S3 (2000)

3 Formation of SiH 3 ions in reactions of small hydrocarbon ions... k L k[cm 3 s -1 ] 10-9 CH 3 SiH Torr 0.19 Torr Fig. 2. The reaction rate coefficient of the reaction of CH 3 withsilane. Different buffer gas pressures are indicated by different symbols. k L indicates the collisional rate coefficient calculated according to simple Langevin theory. CH 3 SiH 4 SiH 3 CH 4 (1) The formation of SiH 5 by the proton transfer and the consequent dissociation to SiH 3 and H 2 is endoergic and can be excluded at low. Traces of SiCH 3 and SiCH 5 product ions, ( 1%) were also observed in agreement with the previous tandem mass spectrometer study (see compilation by Anicich et al. [10]). At near-thermal collision energies, the obtained rate coefficient is equal to the collisional reaction rate coefficient as calculated from Langevin theory. Note the small increase at higher ; such increase is very often observed in drift tubes at elevated energies, where simple polarization theories are no longer applicable (see the discussion of the phenomena in Refs. [3,20,21]). 3.2 The reaction of H 3 with SiH4 The reaction has three product channels as indicated in the reaction scheme (2). Indicated are the energetically possible neutral products, there is not enough energy in the reaction for further dissociation of neutral molecules. H 3 SiH 4 SiH 3 H 4 (a) SiH 5 H 2 (b) Si H 5 H 2 (c) The reaction channel (2b) is thermoneutral and is the only one in the present Czech. J. Phys. 50/S3 (2000) C 3 (2)

4 G. Bano et al. study where production of SiH 5 was observed ( 10% at low ). The measured overall reaction rate coefficient, k, isplottedinfig.3.the overallreaction rate coefficient (k) is nearly constant (note that k k L /2). The product distribution is plotted in Fig. 4. At low collision energies, the production of Si H k L k[cm 3 s -1 ] Torr 0.25 Torr 0.33 Torr 0.1 H 3 SiH Fig. 3. The reaction rate coefficient of the reaction of H 3 withsilane. Different buffer gas pressures are indicated by different symbols. k L indicates the collisional rate coefficient calculated according to the simple Langevin theory. 1 H 3 SiH 4 Product distribution Torr SiH 3 Si H 5 SiH Fig. 4. Product distribution of the reaction of H 3 withsih4. 4 C Czech. J. Phys. 50/S3 (2000)

5 Formation of SiH 3 ions in reactions of small hydrocarbon ions... ion is dominant with a small enhancement with increasing pressure of He. That can indicate that at low, the reaction is proceeding via formation of longlived complex (Si H 7 ) which decomposes to Si H 5 and H 2.Notethatwith increasing, the production of SiH 3 is increasing at the expense of the other two channels. For > 0.2 ev, SiH 3 is the dominant product ion. The corresponding partial reaction rate coefficients are plotted in Fig. 5. Actually, there are 10-9 k β [cm 3 s -1 ] k β SiH 3 H 5 Si SiH Fig. 5. Overall and partial reaction rate coefficients corresponding to production of ions SiH 3,SiH 5 and SiC2H 5. Data are corrected by a small factor β considering the small increase of the collision reaction rate coefficient. The data correspond to the data given in Figs. 3 and 4. Because of the minimal pressure dependence of the data for one product ion, just one symbol is used independently on the actual pressure of the buffer gas. plotted the partial reaction rate coefficients, k β, obtained from the partial reaction rate coefficient after a small correction considering the small increase of the collision rate coefficient at superthermal energies [20]. The production of SiH 3 cannot be explained by the formation of SiH 5 or SiH 5 and their consequent dissociation, because in such a case, H 2 and H 2 are formed and the reaction channel is endoergic ( H = 17.5 kcal/mol). In the study of Mayer and Lampe, only the production of SiH 3 and SiH 5 was reported with reaction rate coefficient cm 3 s 1, this is corresponding to the sum of the partial reaction rate coefficients of channel (2a) and (2b) at near-thermal energies. 3.3 The reaction of H 5 with SiH4 The reaction rate coefficient of the reaction of H 5 ion with SiH 4 versus is plottedinfig.6.onlysih 3 was observed as a product ion. The reaction is exoergic ( H = -10 kcal/mol) if the neutral product is H 6. If two neutral molecules Czech. J. Phys. 50/S3 (2000) C 5

6 G. Bano et al. k L H 5 SiH k m [cm 3 s -1 ] 0.16Torr 0.18Torr 0.24Torr 0.26Torr Fig. 6. The reaction rate coefficient of the reaction of H 5 withsilane. Different buffer gas pressures are indicated by different symbols. k L indicates the collisional rate coefficient calculated according to the simple Langevin theory. (H 2 and H 4 ) are produced, the reaction is endoergic ( H = 22 kcal/mol). This endothermicity rules out the formation of SiH 5 and its consequent dissociation. The reaction scheme is: H 5 SiH 4 SiH 3 H 6 (3) 4 Concluding Remarks The aim of the presented experiments was the study of the production of SiH 3 and SiH 5 ions. The siliconium ion SiH 5, an analog of the carbonium ion CH 5,isof considerable interest in understanding the nature of nonclassical bonding [6]. The dissociation energy of SiH 5 is approximately 14 kcal/mol (see calculation in Ref. [5]). In order to avoid the dissociation of eventually formed SiH 5, only the reaction of ions parent molecules with the proton affinity close to the proton affinity of SiH 4 (155 kcal/mol, see Ref. [23]) were studied. We expected a higher production of SiH 5 by soft proton transfer from hydrocarbon. SiH 5 ions were produced only in the reaction of ions H 3 with SiH 4 at low, channel (2b), with small branching ratio ( 10 %) and the corresponding reaction rate coefficient < cm 3 s 1.The measured reaction rate coefficients and product distributions were independent on pressure. P.Z., A.L. and G.B. are thankful to Österreichischer Akademischer Austauschdienst for Scholarships, which allowed their stay in the Institute für Ionenphysik, Universität 6 C Czech. J. Phys. 50/S3 (2000)

7 Formation of SiH 3 ions in reactions of small hydrocarbon ions... Innsbruck. J.G., P.Z., A.L. and G.B. wish to express their thankfulness for the hospitality during visits at the Institute für Ionenphysik. This work was supported in part by the Grant Agency of the Czech Republic under projects No. 203/98/0508, No. 202/99/D061 and by researchproject VZ / References [1] J. Glosík, P. Zakouřil, and W. Lindinger: Int. J. Mass Spec. Ion Proc. 149/150 (1995) 499. [2] P. Zakouřil, J. Glosík, V. Skalský, and W. Lindinger: J. Phys. Chem. 99 (1995) [3] J. Glosík, P. Zakouřil, and W. Lindinger: J. Chem. Phys. 103 (1995) [4] J. Glosík, P. Zakouřil, and W. Lindinger: Int. J. Mass Spec. Ion Proc. 145 (1995) 155. [5] Ching-Han Hu, Mingzuo Shea, and H.F. Schaefer III: Chem. Phys. Letters 190 (1992) 543. [6] Doo Wan Boo and Yuan T. Lee: J. Chem. Phys. 103 (1995) 514. [7] B.L. Kickel, E.R. Fisher, and P.B. Armentrout: J. Phys. Chem. 96 (1992) [8] P.B. Armentrout: Adv. Gas Phase Ion Chem. 1 (1992) 83. [9] D.K. Bohme: Adv. Gas Phase Ion Chem. 1 (1992) 225. [10] V.G. Anicich: J. Phys. Chem. Ref. Data22 (1993) [11] E. Herbst, T. J. Miller, S. Wlodek and D. K. Bohme: Astron. Astrophys. 222 (1989) 205. [12] T.J. Millar: in Chemistry and Spectroscopy of Interstellar Molecules, (Eds. D.K. Bohme, E. Herbst, N. Kaifu, and S. Saito), University Of Tokyo Press, Tokyo, [13] W. Federer, H. Ramler, H. Villinger, and W. Lindinger: Phys. Rev. Lett. 54 (1985) 540. [14] J. Glosík, W. Freysinger, A. Hansel, P. Španěl, and W. Lindinger: J. Chem. Phys. 98 (1992) [15] G.H. Wannier: Bell Syst. Tech. J. 32 (1953) 170. [16] J. Glosík, V. Skalský, and W. Lindinger: Int. J. Mass Spec. Ion Proc. 134 (1995) 67. [17] K. Peska, H. Stori, F. Hegger, G. Sejkora, H. Ramler, M. Kriegel, and W. Lindinger: J. Chem. Phys. 70 (1982) [18] B.H. Boo and P.B. Armentrout: J. Am. Chem. Soc. 113 (1991) [19] S. Wlodek, A. Fox, andd.k. Bohme: J. Am. Chem. Soc. 113 (1991) [20] J. Glosík,D.Smith,P.Španěl,W.Freysinger,andW.Lindinger:Int.J.MassSpec. Ion Proc. 129 (1993) 131. [21] S.C. Smith, M.J. McEvan, K. Gilet, D. Smith, and N.G. Adams: Int. J. Mass Spec. Ion Proc. 96 (1990) 77. [22] T.M. Mayer and F.W. Lampe: J. Phys. Chem. 78 (1974) [23] S.G. Lias, J.F. Liebman, R.D. Levin: J. Phys. Chem. Ref. Data 13 (1984) 695. Czech. J. Phys. 50/S3 (2000) C 7