3315
IONIC REACTIONS IN GASEOUS MIXTURES OF MONOSILANE AND ACETYLENE
its existence. Since &Ha+is a primary ion from monosilane and since it is produced in a hydride-ion transfer reaction3 to SiH2+, additional increases of an already intense ion via reaction 27 would be difficult to detect unless (27) was extremely fast. The concordance of the rate constant for total reaction of C2H2+ with the sum of the rate constants obtained from the products of C2H2+ reactions suggests that if (27) occursit does so with a rate constant that is a t most ‘/4 that of reaction 2, namely, the H2- transfer r e a ~ t i o n . ~ l - ~ ~
Table V : Upper Limits“ of the Heat of Formation for Secondary Ions Observed in C2H2-SiHa Mixtures Secondary Ion
m/e
SiCHa + SiGH SiCaHz SiCZH8 + SiCzH4+ SiC2HB +
43 53 54 55 56 57
+
+
AHf I
196 273 325 273 325 273
a The heats of formation of the reactants and neutral products used to calculate these limits were obtained from Steele et al.,al Rossini et aLla2Field and Franklin,%and G u n and Green.aa
for the loss of these reactant ions by cross reaction shows agreement within experimental error, attesting to the validity of our treatment. Although the H- transfer reaction
+ SiH4 +C2H3 + SiH3+
C2H2+
(27)
is energetically feasible, we have not been able to verify
Acknowledgments. The work was supported by Contract No. AT (30-1) - 3570 with the United States Atomic Energy Commission. We also wish to thank the National Science Foundation for providing funds to assist in the original purchase of the mass spectrometers. (31) W. C. Steele, L. D. Nichols, and F. G. A. Stone, J. Amer. Chem. SOC.,84,4441 (1962). (32) F. D. Rossini et al., “Selected Values of Chemical Thermodynamic Properties,” National Bureau of Standards Circular 500,U. S. Government Printing Office, Washington, D. C., 1952. (33) 8. R. Gunn and L. G. Green, J. Phys. Chem., 65,779 (1961).
Ionic Reactions in Gaseous Mixtures of Monosilane and Ethylene1
by D. P. Beggs2 and F. W. Lampe Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania (Received April 3, 1969)
16803
I n ionized mixtures of monosilane and ethylene the only major ions that are observed to undergo collision reactions with the opposite molecule are CzH2+, CZH4+, and SiHz+. The principal reactions of both CzH2+a n d CzH4+ are those involving H2- (or H and H- in one step) abstraction from SiH4. The principal reaction of SiHz+ with C2H4 is to form SiCH3+ and CH3. Rate constants for all the reactions of C2Hz+ and CzHa+ with SiH4 and of SiHz+ with CzH4 are reported. Isotope labeling experiments indicate reactive intermediates in which complete mixing of hydrogen atoms takes place. The very fast reactions of C&+ with SiH4 and SiHz+ with CzHa provide a rationale for the effects of CzH4 011 the radiolysis of monosilane.
Introduction Previously in this laboratory we have studied the ion-molecule reactions in pure monosilane3 and in monosilane-acetylene mixture^;^ we have also examined the radiolysis of pure monosilane and ethylene-monosilane mixtures.6 Certain aspects of the radiolytic behavior of the silane and the ethylenesilane mixtures point to chain reactions propagated by species that are not free radicals and which we have suggested to be ionic. For a more complete understanding of the radiation chemistry of simple silane systems, we have extended our study of ion-molecule reactions to ethylenemonosilane mixtures and we report the results herewith.
Several studies have been made of the ionic reactions in pure gaseous ethyle~ie,~-’~ and among the various (1) AEC Document NYO-3570-8. (2) Based in part on a thesis by D. P. Beggs submitted to the Pennsylvania State University in partial fulfillment of the requirements for the Ph.D. degree. (3) G. G. Hess and F. W. Lampe, J . Chem. Phys., 44,2267 (1966). (4) D. P.Beggs and F. W. Lampe, J . Phys. Chem., 73,3307 (1969). (5) F. W. Lampe and J. Schmidt, ibid., 73,2706 (1969). (6) F. H.Field, J. L. Franklin, and F. W. Lampe, J . Amer. Chem. SOC.,79,2419 (1957). (7) C.E.Melton and P. S. Rudolph, J. Chem. Phys., 32, 1128 (1960). (8) F. H.Field, J . Amer. Chem. SOC.,83,1523 (1961). (9) A.G.Harrison, Can. J.Chem., 39,1523 (196;). Volume 73, Number 10 October 1969
D. P. BEGGS AND F. W. LAMPE
3316
IO L
m4.55
0
.-
I1
*m/
12 I3 E.E., eV.
14
15
= 56
e
E.E.,
m?e
Figure 1.
e.V.
= 57
Ionisation-efficiency curves of secondary ions.
authors there seems to be general agreement as to the identification of the bimolecular reactions occurring and the specific reaction rates. This investigation is concerned with identifying the cross reactions (the reactions between ions of one compound and the molecules of the other compound) in monosilane-ethylene mixtures, determining the rate constants for these reactions, and exploring the mechanistic consequences of these reactions on the radiation chemistry of silane and silane-ethylene mixtures.
Experimental Section A detailed discussion of the apparatus and experimental procedures is presented in the preceding paper4 and need not be repeated here. The only difference in experimental procedure between this paper and the preceding is that in this case ionization-efficiency curves were obtained using the retarding-potential-diff erence (RPD) method20 with the Bendix time-of-flight mass spe~trometer.~~~~ The R P D ionization efficiency curves were obtained with potential configurations that have been described previously.22 A variable-speed motor drove the electron-beam energy helipot so that an ionization-efficiency curve could be continuously plotted on an X-Y recorder. After one ionization-efficiency curve was so plotted (-15 sec), the potential on the retarding grid in the electron gun was reduced by 0.3 V and another ionization-efficiency curve was plotted. The difference between the two curves, a t a given electron energy, was The Journal of Physical Chemistry
then plotted vs. the ionization energy of the electron beam as shown in Figure 1. Krypton was used to calibrate the electron-energy scale, the concentration of the krypton being such that the ion current of the m/e 84 isotope was approximately equal to the ion currents of the secondary products a t 50 eV. Ethylene (C.P. grade) was purchased from the Matheson Company while the silane was obtained from Air Products and Chemicals Inc. Ethylene-& (Minimum isotopic purity of 99%) was acquired from Merck Sharp and Dohme. Silane-d4 was prepared by the action of LiAlD4 on SiCld, that had been purchased from the Peninsular Chemical Company. All gases were (10) S.Wexler and R. Marshall, J. Amer. Chem. Soc.,86,781 (1964). (11) P. Kebarle and A. M. Hogg, J. Chem. Phys.,42,668 (1966). (12) P. Kebarle, R. M. Haynes, and S. Searles, Advances in Chemistry Series, No. 58, R. F. Gould, Ed., American Chemical Society, Washington, D. C., 1966,p 210. (13) G. G. Meisels, ref 12,p 243. (14) I.Szabo, Arkiv Fysik, 33,67 (1966). (15) P. Kebarle and R. M. Haynes, J . Chem. Phga., 47, 1676 (1967). (16) R.Gordon, Jr., and P. Ausloos, ibid., 1799 (1967). (17) J. J. Myher and A. G. Harrison, Can. J . Chem., 46, 101 (1968). (18) T. 0.Tiernan and J. H. Futrell, J. Phys. Chem., 7 2 , 3080 (1968). (19) M.T.Bowers, D. D. Elleman, and J. L. Beauchamp, ibid., 72, 3699 (1968). (20) R. E. Fox, W. M. Hickam, D. J. Grove, and T. Kueldans, Rev. Sci.Instrum., 26,1101 (1956). (21) W. C. Wiley and I. H. McLaren, ibid., 26, 1160 (1966). (22) G. G. Hess, F. W. Lampe, and L. H. Sommer, J . Amer. Chem. SOC., 87,6327 (1966).
IONIC REACTIONS IN GASEOUS MIXTURES OF MONOSILANE AND ACETYLENE fractiona.ted on the vacuum line and checked mass spectrometrioally for satisfactory purity before use.
3317
I
Results A . Reaction Identi$cation. A typical mass spectrum of a (1 :1) silane-ethylene mixture (uncorrected for isotopic contributions of 29Si, 3oSi,and I3C) is shown in Table I. The variation of some of these ion fractions Table I : Spectrum of SiH4-C8H4 System (1.0 psec Delay Time, 5.0 X 10-8 Torr) Ii/ZI
