J . Pkys. Chem. 1987, 91, 5777-5781
Energetics and Reaction Mechanisms of SiH' Collision- Induced Dissociation of SiD,'
+ D, and SiD'
5777
-t H, and
B. H.Boot and'P. B. Armentrout** Department of Chemistry, University of California, Berkeley, California 94720 (Received: April 14, 1987)
The title reactions are investigated by guided ion beam mass spectrometry. Absolute cross sections are determined as a function of the relative collision energy. In the reaction of SiH+ with D2, the lowest energy process is the near thermoneutral H/D exchange reaction. At energies above about 2 eV, several minor processes are observed: deuterium atom transfer to form SiHD', HID2 exchange to yield SiD2+,and collision-induced dissociation into Si+ and H. The reaction of SiD+ + H2 is observed to have analogous product channels with nearly identical energy behaviors. The translational energy dependence of the collision-induced dissociation of SiD3+is shown to correlate well with the reaction of SiH+ (SiD') + D2 (HJ. This supports the intermediacy of SiHDz+ (SiH,D+) for the H/D isotope exchange reaction. The thermochemistry of all these reactions is in good agreement with previous studies indicating that there are no activation barriers in excess of the endothermicities.
Introduction In recent studies, we have examined the detailed interactions of Si+(2P) with dihydrogen' and silane2 using guided ion beam mass spectrometry. A major impetus for this work is the elucidation of the properties of reactive silicon hydrides. These species are of interest in the chemical vapor deposition (CVD) of silicon, and the plasma deposition and etching of silicon surfaces. Our studies evaluated the mechanisms of the reactions and made a systematic measurement of the thermochemistry for comparison with previous data.+1° In the present study, this work is continued by studying the title reactions. These reactions may be of particular interest since they lend insight into H-H activation by silicon species. For neutral reactions, H2 activation by silylene, SiH2, is of considerable interest and has been studied by time-resolved laser spectroscopy" and ab initio calculations.12 The experimental and theoretical studies indicate that the SiHz + D2 reaction proceeds with a = l kcal/mol bamer, a value consistent with theoretical calculations for thermal decomposition of silane. Since ground-state SiH+('Z+) and SiH2('Al) are both singlets, their reactions with molecular hydrogen may have some similarities. In both cases, the reaction may occur via insertion to form the closed-shell intermediates SiH3+('AI) and SiH4(IAI),respectively. In the present study, this possibility is tested in part by examining the collision-induced dissociation (CID) of SiD3+. Thus, the decomposition of SiD3+formed via reaction of SiD+ + D2 (or isotopic variants) is directly compared with that of collisionally activated SiD3+. In essence, this permits us to enter the pertinent potential energy surface from two independent points. This helps provide information on how efficiently translational energy is converted into the internal energy necessary to break bonds. Experimental Section The guided ion beam apparatus has been described at length in the 1 i t e r a t ~ r e . l Silicon ~ hydride (deuteride) cations of SiH+, SiD', and SiD3+are produced as described below. The ions are extracted, accelerated, and focussed into the 60' magnetic momentum analyzer. Here, the desired ions are mass selected, then decelerated to a particular translational energy, and focused into an octopole ion beam guide which passes through the reaction chamber filled with the neutral reactant. The octopole beam guide utilizes radio-frequency electric fields to create a radial potential well which traps ions over the mass range examined. The pressure of the reactant gas is maintained sufficiently low that it is unlikely that ions undergo more than a single collision. The product and unreacted ions drift from the gas cell to the end of the octopole ?Present address: Department of Chemistry, Chungnam National University, Dae Jeon 300-31, Korea. *NSFPresidential Young Investigator 1984-1989; Alfred P. Sloan Fellow.
where they are extracted and injected into a quadrupole mass filter for mass analysis. Finally, ions are detected by a secondary electron scintillation ion counter using pulse counting techniques. Ion intensities are converted to absolute cross sections as described previo~s1y.l~Absolute cross sections are estimated to have an uncertainty of f20%. Ground-state silicon hydride ions, SiH+ (SiD+ and SiD3+),are produced by electron impact (EI) ionization of silane (or SiD4) at -100-eV electron energy followed by the passage through a drift cell (DC).14 The first excited state of SiH+(311)is calculated to be 2.23 eV higher than the ground state.l0 Thus, the highenergy electron-impact ionization can produce excited SiH+. These ions are cooled in the DC via =lo3 collisions with Ar gas 2 As discussed further below, these studies find no at ~ 0 . Torr. indication for formation of vibrationally or electronically excited SiH+. However, this source also produces atomic silicon ions, 29Si+,at the same mass as the *%H+ ion beam. Optimal conditions can be found such that this isotopic contribution to the total intensity of the beam is significantly smaller than 2%. Nevertheless, even a small amount of 29Si+contributes to the observed product signal at mass 31 since the reactivity of SiH+ toward molecular hydrogen is low compared to that of Si+. However, the energy dependence of the cross section for reaction of atomic silicon ions with molecular hydrogen (H, and D2) is independently known.'J5 Therefore, we can subtract the contribution of the 29SiD+product ion arising from the reaction of 29Si+ Dz from the mass 3 1 signal to leave only the contribution due to reaction of 28SiH+ D1. Likewise, impurities of 30Si+in the 28SiD+ beam can be handled in the same way. Similar problems are not encountered for the SiD3+ beam since much higher intensities of this ion are observed. Therefore, the 30SiD2+
+
+
(1) Elkind, J. L.; Armentrout, P. B. J . Phys. Chem. 1984, 88, 5454. (2) Boo, B. H.; Armentroqt, P. B. J . Am. Chem. SOC.1987, 109, 3549. (3) (a) Carlson, T. A.; Copley, J.; Duric, N.; Elander, N.; Erman, P.; Larsson, M.; Lyyra, M. Astron. Asrrophys. 1980.83, 238. (b) Douglas, A. E.; Lutz, B. L. Can. J. Phys. 1970, 48, 247. (4) Ding, A.; Cassidy, R. A.; Cordis, L. S.; Lampe, F. W. J. Chem. Phys. 1985,83, 3426. (5) Bijrlin, K.; Heinis, T.; Jungen, M. Chem. Phys. 1986, 103, 93. (6) Shin, S. K.; Beauchamp, J. L. J . Phys. Chem. 1986, 90, 1507. (7) Berkowitz, J.; Greene, J. P.; Cho, H.; Ruscic, B. J . Chem. Phys. 1987, 86, 674. (8) Frey, H. M.; Walsh, R.; Watts, I. M. J. Chem. SOC.,Chem. Commun. 1986, 1189. (9) (a) Ho, P.; Coltrin, M. E.; Binkley, J. S.; Melius, C. F. J. Phys. Chem. 1985, 89, 4647. (b) Ho, P.; Coltrin, M. E.; Binkley, J. S.; Melius, C. F. J . Phys. Chem. 1986, 90, 3399. (10) Bruna, P. S.; Hirsh, G.; Buenker, R. J.; Peyerimhoff, S . D. Molecular Ions, Berkowitz, J., Groeneveld, K., Ed.; Plenum: New York, 1983; p 309. (11) Jasinski, J. M. J. Phys. Chem. 1986, 90, 555. (12) Gordon, D. R.; Binkley, S.; Frisch, M. J. J. Am. Chem. SOC.1986, 108, 2191. (13) Ervin, K. M.; Armentrout, P. B. J. Chem. Phys. 1985, 83, 166. (14) Ervin, K. M.; Armentrout, P. B. J. Chem. Phys. 1986, 84, 6738. (15) Elkind, J. L.; Boo, B. H.; Armentrout, P. B., unpublished results.
0022-3654/87/209~-5777%01.50/0 0 1987 American Chemical Society
Boo and Armentrout
5778 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 ENERGY feV, Lob)
t
c ENERGY (eV. CM
ENERGY (eV. CM)
Figure 1. Variation of product cross sections for the reaction of SiH' with D2 as a function of kinetic energy in the laboratory frame (upper scale) and the center of mass frame (lower scale). The line shows the total cross section for all products. The dashed line shows the collision
Figure 2. Variation of product cross sections for the reaction of SiDt with H2 as a function of kinetic energy in the laboratory frame (upper scale) and the center-of-massframe (Iower scale). The line shows the total cross section for all products. The dashed line shows the collision cross section, uLos. Arrows indicate the thermodynamicthresholds for reactions 6 and 10.
impurity ion is present in negligible amounts,
