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P. POTZINGER AND F. W. LAMPE
An Electron Impact Study of Ionization and Dissociation of Monosilane and Disilanel by P. Potzinger and F. W. Larnpe Department of Chemistry, The Pennsvlvania State University, University Park, Pennsylvania 16803 (Received May 7, 1969)
Appearance potentials of the principal positive ions from monosilane and disilane have been determined and upper limits to the heats of formation of the various ions have been derived, as well as a value of AHro(SiHs)= 51 kcal/mol. From the energetic onsets of some of the resonance-capture processes and pair-production processes, thermochemical data for some of the negative ions from monosilane are derived. This is not possible in the case of disilane because it appears that all the negative ions are formed in different dissociation channels of common parent SizHe-* ions and SizHa* molecules.
Introduction Ionization and dissociation of the simple paraffinic hydrocarbons by electron impact have been the subject of numerous studies and there exists general agreement concerning the appearance potentials and the standard heats of formation of the principal ions and free radicals formed.2 The structurally similar silanes, on the other hand, have received only scant attention and some of the available electron impact data are not in good agreement. a As part of a general program of study concerned with the effects of ionizing radiation on silanes and simple organosilanes, we have had occasion to study the major ionization and dissociation processes that occur from the impact of electrons on monosilane and disilane. In the hope that our results may contribute to a clearer understanding of the energetics and thermochemistry involved, we report them herewith.
-'
Experimental Section A11 the measurements reported herein were carried out in a Nuclide Associates 12-90G mass spectrometer (12-in. radius of curvature, 90" sector field). All studies of positive ions were made using the retardingpotential difference (rpd) methods of obtaining ionization efficiency curves that originate from the impact of electrons which are monoenergetic within about k O . 1 eV. A1 least three replicate determinations were carried out for each ion with the experimental errors reported being the average deviation from the average value. I n these rpd experiments, the electron beam was pulsed at 100 kHz, obtaining an average electron current of about 5 PA. A 22-Hz square wave, having a peakto-peak amplitude of 0.1 V and clamped about 1-V negative with respect to the filament, was applied to the retarding grid in a three-grid electron gun. The output of the electron-multiplier detector, consisting of a 22-Hz square-wave signal superimposed on an The Journal of Physical Chemistry
essentially dc level, (rms multiplier current caused by ions arriving a t 100 kHz) was fed to a Princeton Applied Research lock-in amplifier (par), tuned to 22 Hz and proper phase. The par amplifies the entire signal, extracts the 22-Hz rpd component as a dc signal which is displayed on the y axis of a htoseley 135-AM X-Y recorder. Since the electron energy, scanned by means of an appropriate motor drive, is displayed on the x axis of the recorder, direct continuous rpd ionization-efficiency curves are obtained. Typical curves for SiH3+ and SiH2+ from SiH, are shown in Figure 1. Argon ( I s = 15.77 eV) was used to calibrate the electron energy scale and all measurements were done in a t least triplicate. The rpd technique was not used in the study of negative ions, since the ion intensities were considerably lower than obtained during the study of positive ions. The electron energy scale for negative ion measurements was calibrated using the onset (9.2 eV) for the resonant capture formation of 0 - from COa9 Monosilane was obtained from Air Products, Inc. It was purified by fractionation on a high-vacuum line immediately before use until satisfactory purity by mass spectral standards was obtained. Disilane was prepared by the reduction of hexachlorodisilane (Peninsular Chemical Co.) with lithium aluminum hy(1) AEC Document No. NYO-3570-9. (2) F. H. Field and J. L. Franklin, "Electron Impact Phenomena," Academic Press, New York, N. Y . ,1957. (3) H. Neuert and H. Clasen, Z . Naturforsch., 78,410 (1952). (4) W. C. Steele, L. D. Nichols, and F. G. A. Stone, J. Amer. Chem. Soe., 84,4441 (1962). (5) W. C. Steele andF. G. A. Stone, ibid., 84,3599 (1962). (6) F. E. Saalfeld and H. J. Svec, Inorg. Chem., 2, 46 (1963); (b) F. E. Saalfeld and H. J. Svec, ibid., 2,50 (1963). (7) G. G. IIess, F. W. Lampe, and A. L. Yergey, Ann. N . Y . Acad' Sci., 136, 106 (1966). (8) R. E. Fox, W. NI. Hiokam, D. J. Grove, and T. Kjeldaas, Rev. Sci. Instrum., 26, 1101 (1955). (9) G. J. Schula, PhZ/s. Rev., 128, 178 (1962).
AN ELECTRON IMPACT STUDY OF IONIZATION AND DISSOCIATION
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Table I : Energetics of Positive Ion Formation in Monosilane" AP, eV
Prooess assigned
12.30 f 0.03 11.90 f 0.02 15.3 f 0 . 3 13.56 f 0.08
SiH4 -t e + SiHa+ H f 2e SiH4 e + SiHz+ Hz 2e SiH4 e SiH+ Hz H -I- 2e SiH, e Si+ 4-2Hz -I- 2e
Ion
SiHa + SiHz+ SiH +
Si + a
-+
+ + + + +
239 282 308 318
-+
AHfo(SiH4)was taken to be 7.3 kcal/mol after S. R. Gunn and L. G. Green, J . Phyls. Chem., 65,779 (1961).
I-
z
W LIT
ar 3
0
z 0
II
+ + +
AHfo (ion), koal/mol
12
I
I
I
I
J
13
I4
I5
16
I7
ELECTRON ENERGY
Figure 1. Ionization-efficiencycurves of SiHs+ and SiHz+ from SiH4.
dride using bis [2-(2-methoxyethoxy)ethyl] ether as solvent. When the gases formed in this reaction were separated by fractionation on the vacuum line, it was found that the Siz& fraction contained SiH3C1 as an impurity. In order to remove this impurity, the gases evolved from the reaction mixture were collected and treated again with lithium aluminum hydride. Fractionation of the gases subsequent to the second treatment with lithium aluminum hydride yielded a sample of disilane free of SiH3C1and sufficiently pure for our purposes.
Results and Discussion 1. Positive Ions from SiH4. The 70-eV mass spectrum of SiH4 is in excellent agreement with that reported by previous investigator^^^^" and therefore is not reproduced here. The lowest threshold potentials of the principal positive ions are tabulated in Table I. I n addition, Table I presents the formation processes assigned and the standard heats of formation of the ions, as calculated from the assigned processes and the assumption that the minimum appearance potentials are equal to the enthalpy changes of the assigned reactions. SiH3+ (m/e 31). Our value for the threshold potential of this ion is in excellent agreement with the value reported by Steele, Nichols, and Stone4 and within the combined experimental errors agrees with the value reported by Neuert and C l a ~ e n . ~It is in serious disagreement with the value reported by Saalfeld and Svec.6a Since there is no doubt, in the case of this ion, about the formative process, and since Saalfeld
and SvecBafound no evidence of kinetic energy effects in the silane fragment ions, we conclude that AHfO(SiH3+) = 239 f 2 kcal/mol. Two discontinuities in the ionization-efficiency curve were found a t 13.1 f 0.2 and 14.2 f 0.3 eV in all experiments. These breaks are indicative of the onset of other formation processes of SiH3+ and, although the nature of the processes cannot be ascertained, they must represent the onsets of different states of excitation of the SiH3+ ion. Isotopic interference may be an influence in the determination of the appearance potential of SiH3+, since the appearance potential of SiHz+ is lower by in the ionizaabout 0.5 eV. The presence of SiZQHz+ tion-efficiency curve of m/e 31 would be indicated by a slight tailing near onset with a slope that is 5% of the initial slope of m/e 30 (SiHz+). Inspection of Figure 1 shows that a t our level of detection sensitivity such tailing is not significant. SiHz+ (m/e SO). The threshold potential for the appearance of this ion is likewise in excellent agreement with the value reported by Steele, Nichols, and Stone.4 Within the combined experimental errors it can be considered to agree with the work of Neuert and Clasen3 (after correction of an obvious erroneous mass assignment by these authors) and with that of Saalfeld and Svec,6a Thus, one may conclude with confidence that AHt(SiH2+) = 282 f 1 kcal/mol. A reproducible break was found a t 13.0 f 0.2 eV. Since a similar break was found in SiDz+ by Hess and Lampe'O and since there is no such low-lying we believe that this break correelectronic state of Hz, sponds to formation of SiHz* in a different state of excitation. Saalfeld and SvecGnhave reported an even higher appearance potential of SiHz+a t 16.5 f 0.3 eV which corresponds to the process SiH4
+e
SiHz+
+ 2H + 2e
(1) I n only one of our continuous rpd curves have we been able to detect a break beyond that at 13.0 eV. This particular break exhibited only a very slight change of slope and although its occurrence was a t 16.4 f 0.2 eV, its absence in the other curves prevents us from reporting a definite confirmation of (1). --j
(10) G. G. Hess and xi'. W. Lampe, J . Chem. Phus., 44,2257 (1966). Volume 73, Number 11 November 1989
P. POTZINGER AND F. W. LAMPE
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SiH+ (m/e 29). The ionization-efficiency curve for this ion showed a long tail and is indicative of a formation process involving considerable excess energy, although in this case the tail may be partly due to contribution from Siz9+. We did not attempt to correct this tail for the contribution for Si29+, since the very small slope of the Si28+curve over a large energy range suggested that the contribution was small. Extrapolation of the tail to onset yielded a value of 15.3 f 0.3 eV for the minimum appearance potential. This value lies between those reported by Neuert and Clasen3 and Saalfeld and SveclGaalthough with the rather large combined experimental error, could be considered in agreement with the former authors. In agreement, with Saalfeld and SvecGawe found a second appearance potential a t 20.0 f 0.2 for this ion which most likely corresponds to SiH4
+ e +SiH+ + 3H + 2e
(2)
Saalfeld and SvecGaemployed a linear extrapolation method to obtain their appearance potentials, and since it is known that this method gives results too high for curves exhibiting long tails, we take our value to be more nearly correct and conclude AHfO(SiH+) = 308 f 7 kcal/mol. Si+ (m/e 68). The threshold for the appearance of Si+ was found to be quite reproducible a t 13.56 f. 0.08 eV. This value is not in good agreement Tyith the value of 11.7 eV reported by Saalfeld and SvecBa nor with the value of 14.5 reported by Keuert and Clasena3 The standard heat of formation of gaseous Si is calculated from vapor pressure measurements to be 105 kcal/mol’l and this value combined with the ionization potential of Si (8.15 eV12) yields a heat of formation of Si+ of 293 kcal/mol. The value of 11.7 eV5 is simply too low to be compatible with this figure. If we take our minimum-energy process to be SiH4
+ e +Si+ + 2H2 + 2e
(3)
we calculate AHf”(Si+)to be 318 kcal. For a process where so many bonds are broken such a large excess energy is not unexpected and hence, as is often true in electron impact fragmentation, the appearance potential is an upper limit to the heat of reaction. 0.2, Reproducible breaks were observed at 14.8 18.4 f 0.2, 20.2 f 0.2, and 22.9 k 0.2 eV, with the breaks at 18.4 and 22.9 eV prabably signifying the onsets of the two processes
+ e Si+ + HZ+ 2H + 2e SiH4 + e -+ Si+ + 4H + 2e
SiH4
(4) (5)
The other discontinuities at 14.9 and 20.2 eV do not correspond to known excited states of Si+. d. Negative Ions from S’L”4. Xegative ion mass spectra of SiH, a t three different electron energies are shown in Table 11. The spectra shown have been The Journal of Physical Chemistry
~~
Table I1 : Negative Ion Mass Spectra of SiHe
----Ion
SiHaSiI-12SiH -
Si-
Relative intensity
at----y
20 eV
50 eV
70 eV
100 33 39 4.8
100 38 76 52
100 33 64 54
corrected for isotopic contributions from Si29and Sia0 and so represent the monoisotopic spectra of Si28. The negative ion ionization-efficiency curves were recorded from 0 to 30 eV nominal electron energy, However, below 4 eV the trap current decreased so rapidly that quantitative ion intensities could not be obtained in this region. I n addition to measuring onset potentials of the negative ions, the cross sections of the highest resonance peak of all negative ions were measured relative to those for the resonance peak of 0 - from CO (u = 1.610-19 ~1119.~ These data are shown in Table 111. SiH3- (m/e 31). The ionization-efficiency curve of SiH3- shows a resonance peak having an onsct a t 2.5 f 0.3 eV and a second resonance peak at 6.7 eV. A third onset a t approximately 11 eV is probably also due to a resonance process while a t 15.5 f 0.5 eV a pair-production process sets in with yet another resonance peak superimposed on it. From the appearance potential of the lowest-energy resonance peak we calculate a maximum heat of forma0.5 eV. Comtion of SiHa-, namely AHfo(SiH3-) bination of this result with the heat of formation of the SiH3 radical leads to a minimum electron affinity of 41.5 kcal/mol for SiH3. The resonance processes at 6.7 eV and 11 eV may result in an electronically excited %Ha- ion or the energy may go in kinetic and/or vibrational energy of the fragments formed, but we are unable to say more about these processes. The onset of the pair-production process gives the value AHfo(SiH3-) % 0.0 f 0.5 eV, which is somewhat lower than the upper limit obtained above from the lowest-energy resonance peak. SiH2- (m/e 30). This ion also shows a resonance peak at 2.5 eV. I n addition, there are two further resonance peaks, one at 7.7 eV and a very broad one with an onset at around 13 eV. A sharp pair-production onset appears a t 20 f 0.5 eV. The first resonance peak exhibits the same threshold potential as the corresponding resonance peak for SiHa-, suggesting that this peak is characteristic of the vertical transition of the SiH4 molecule to SiHd- and is not
