J. Phys. Chem. 1985, 89, 5344-5347
ARTICLES Formation of Silylgermane by a Silylene Insertion Reaction in the Infrared Photochemistry of Monosilane-Monogermane Mixtures' Pei-ran Zhu, M. Piserchio, and F. W. Lampe* Davey Laboratory, Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: August 14, 1984; In Final Form: July 22, 1985) The infrared photodecomposition at 944.2 cm-' of silane-germane mixtures has been studied over a pressure range of 24-46 torr and over a temperature range of 295-356 K. The products observed are hydrogen, disilane, silylgermane, trace amounts of trisilane, and solid polymeric material. Digermane formation was not observed. The primary photodecompositionof SiH4 is to SiH, + H2; this is followed by insertion of SiH2 into an Si-H bond of SiH4 or a Ge-H bond of GeH, to form Si2H6 or SiH,GeH3, respectively. On the basis of our studies of the competitive rates of SiH3GeH3and Si2H6formation as a function of temperature, and those in the literature relative to the absolute rate of insertion of SiH2 into SiH,, we derive the value k2 = 10'0.4*0.s expi-10.7 f 5.3 X lo3 J/RT} L mol-' s-' for the specific reaction rate of insertion of SiH, into a Ge-H bond of germane.
Introduction Silylgermane was first reported by Spanier and MacDiarmid2 as a product of the reaction induced in SiH4-GeH4 mixtures by an ozonizer type of electric discharge. A mechanism for the formation of the compound in this complex reaction was not proposed, although from the nature of electric discharge chemistry one may assume that reactive intermediates such as H, SiH,, SiH3, GeH2, GeH3, as well as ions, were all present. Timms, Simpson, and Phillips3 reported the formation of silylgermane in a pyrolysis of mixed silicon and germanium hydrides. They proposed the formation of GeHz as an intermediate and implied that silylgermane arose from the addition of this species to silane, this proposal being consistent with earlier r e p o r t ~ ~of9the ~ homogeneous thermal decomposition of GeH4 to GeH2 and H2. Silylgermane is also formed as a product of the Hg(3Pl) photosensitized decomposition of SiH4-GeH4 mixtures along with disilane, digermane, hydrogen, and a solid deposit.6 The mechanism of this photosensitized process was proposed to be the various recombination reactions of SiH, and GeH3 radicals generated by the reactions of Hg()P,) and H atoms with the substrate molecules.6 Gaspar and c o - w ~ r k e r presented s~~~ strong evidence that recoiling 75Geatoms, from the 76Ge(n,2n)7SGenuclear reaction, interact with germane and silane to form 75GeH2.The radioactive GeH2 molecules then react by bond insertions into the substrate germane and silane to form digermane and silylgermane, respectively. NO was shown to have a moderate effect in suppressing product yields. The pyrolysis of silylgermane in the presence of methylsilane has been shown9 to lead primarily to the products CH3SiH2GeH3 and SiH3GeH2GeH3.This has been interpreted by the authors' to indicate a predominant decomposition of the parent silylgermane (1) 35. (2) (3) 1467. (4)
U S . Department of Energy Document No. DE-AS02-76ER03416Spanier, E. J.; MacDiarmid, A. G. Znorg. Chem. 1963, 2, 2 15. Timms, P. L.; Simpson, C. C.; Phillips, C. S.G. J . Chem. Soc. 1964,
Fensham, P. J.; Tamaru, K.; Boudart, M.; Taylor, H. S. J . Phys. Chem. 1955, 59, 801. ( 5 ) Tamaru, K.; Boudart, M.; Taylor, H. S. J . Phys. Chem. 1955,59, 806. ( 6 ) Gibbon, G . A.; Rousseau, Y.; Van Dyke, C. H.; Mains, G . J. Inorg. Chem. 1966, 5, 114. (I) Gaspar, P. P.; Levy, C. A,; Frost, J. J.: Bock, S.A. J . Am. Chem. SOC. 1969, 91, 1573. ( 8 ) Gaspar, P. P.; Frost, J. J. J . Am. Chem. Soc. 1973, 95, 6567. (9) Elliot, L. E.; Estacio, P.; Ring, M. A. Inorg. Chem. 1973. 12, 2 1 9 3 .
0022-3654/85/2089-5344$01.50/0
to GeH, and SiH,, followed by insertion of GeHz into an Si-H bond of methylsilane and a Ge-H bond of silylgermane; the data suggest that insertion of GeH, into Ge-H bonds is much more rapid than insertion into Si-H bonds. Evidence thus exists in the literature that silylgermane is formed by the insertion of GeH2 into the Si-H bonds of ~ i l a n e and ~~~-~ by the recombination of SiH3 and GeH3 radicals6 To our knowledge no evidence for the insertion of SiH, into Ge-H bonds had been reported. Previous work in our laboratorylWl2indicates that photocomposition of silane by irradiation with an infrared beam of 944.2 cm-' from a C 0 2 TEA laser is an excellent source of SiH2. Moreover, the infrared spectrum of germane13-15is such that no absorption of laser energy should occur and, therefore, any reaction induced should arise solely from the interaction of SiH2 with the germane and silane substrate molecules. Experimental Section The infrared laser photodecompositions were carried out in a cylindrical stainless steel cell having a diameter of 3.45 cm and a length of 15.5 cm. A pinhole leak, located in the wall of the photolysis cell, led directly into the ionization source of a timeof-flight mass spectrometer. The ends of the photolysis cell were fitted with NaCl windows so that the laser beam was perpendicular to the axis of the time-of-flight mass spectrometer. Molecules leaving the cell, along the axis of the flight tube of the mass spectrometer, reach the ionization source after a transit time of, at most, 2-3 ms. The source of infrared radiation was a CO, TEA laser (Lumonics Research Ltd., Model 103-2) pulsed at a frequency of 0.5 Hz. All irradiations were carried out with an unfocussed beam and with the laser tuned to the P(20) line of the 10.6-pm band, i.e. at 944.19 cm-I; this corresponds to a photon energy of 0.1 1706 eV. The average cross section of the beam was 6.6 cm2 and the incident energy as measured by a GenTec joulemeter and an evacuated photolysis cell, was 3.5 J/pulse, resulting in an incident fluence of 0.53 J/cm2. The laser beam illuminated approximately 70% of the photolysis cell volume. Longeway, P. A.; Lampe, F. W. J . Am. Chem. SOC.1981,103,6813. Blazejoswki, J.; Lampe, F. W. J . Photochem. 1982, 20, 9. Moore, C. B.; Biedrzycki, J.; Lampe, F. W. J . Am. Chem. SOC.1984, 106, 7161. (13) Straley, J. W.;Tindal, C. H.; Nielsen, H. H . Phys. Reu. 1942,62, 161. (14) McKean, D. C.; Chalmers, A. A. Spectrochim. Acta, Port A 1967, 23A, 111. ( 1 5 ) Levin, I. W. J . Chem. Phys. 1965, 42, 1244.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5345
IR Photochemistry of Monosilane-Monogermane Mixtures During a photolysis the dependencies of the reactant and product concentrations on the reaction time were determined mass spectrometrically with continuous monitoring of ion currents as follows: H2 (H2+,m / z 2 amu); SiH4 (SiH3+,m / z 31 amu); GeH, (74GeH2+4- 73GeH3+,m / z 76 amu); Si2&(%2H6+, m / z 62 amu); Si3H8(Si3H+,m / z 85 amu); and SiH3GeH3(Si7'GeH6+ (9%) Si72GeH4+(41%) Si73GeH3+(4%) Si7,GeH2+ (42%) Si76Ge+(4%), m / z 104 amu). Mass spectrometric calibrations to establish the relationships between ion currents and partial pressure were carried out on pure samples of H2, SiH,, Si2H6,GeH,, and Ge2H6. SiH3GeH3was synthesized by a silent electric discharge in an SiH4-GeH4mixture, according to the method of Spanier and MacDiarmid.2 The resulting gaseous product also contained Si2H6,Ge2H6,and a trace of Si3H8. This gas mixture was used to calibrate for SiH3GeH3 by correcting for contributions from Si2H6, Ge2H6,and Si3H8. The extent of decomposition of SiH4 was about 1% per pulse. As may be seen from Figure 2, the ion-current traces of m / z 62 (Si2H6)and m / z 104 (SiH3GeH3)vs. time (or number of pulses) are reasonably linear up to about 10 pulses or about 10% conversion of SiH,. This indicates that at the conversions utilized there is no significant loss of products via direct photolysis in the laser beam. The decomposition of GeH4was always less than that of SiH,. Yields of Si2H6were typically a few tenths of a percent per pulse with those of SiH3GeH3varying from about 0.1 to 0.6 of the yields of Si2H6, depending on the reactant ratio WH4I / [SiH41. SiH, and GeH, were obtained from the Matheson Co. and subjected to several freeze-pump-thaw cycles on a high-vacuum line prior to use; H2, also obtained from Matheson, was used as received. Ge2H6was prepared by the silent electric discharge of GeH, and Si2H6was synthesized by methods described previously.16 All gas mixtures were prepared on a Saunders-Taylor apparatus.17
+
+
+ +
Results and Discussion 1 . Absorption of Energy. As shown by Deutsch,Is the frequency of the P(20) line of the 10.6 wm band of the C 0 2 laser (944.19 cm-I) is very close to that of an R-branch transition in the v4 mode of SiH, (944.21 cm-I). On the other hand, infrared radiation of this frequency is not absorbed by GeH,. Therefore, this laser line is a useful one with which to carry out the photodecomposition of SiH, in the presence of GeH,. The absorption of laser energy of 944.19 cm-I by pure SiH, and SiH4-GeH, mixtures is shown in Figure 1 as the average number of photons absorbed per SiH, molecule present in the path of the beam. As is usual in the irradiation of complex molecules with an intense laser beam, the absorption does not follow the Beer-Lambert law, which would predict that D would decrease monotonically as the pressure is increased. The numbers shown parenthetically indicate the percent of the laser energy absorbed by the gas. As may be seen in the figure, in pure SiH, the average number of photons absorbed per SiH, molecule in the beam increases with increasing pressure to a maximum, which occurs when about 60% of the beam energy is absorbed. As the pressure is increased further, the fraction of the beam energy absorbed is increased but the average number of photons absorbed per molecule of SiH, decreases. The addition of GeH, to 20 torr of SiH4 also enhances the fraction of laser energy absorbed; since the number of SiH, molecules in the beam path remains constant, the average number of photons absorbed per SiH4molecule also increases. When pure GeH, is irradiated the energy absorption is negligible at all temperatures investigated and no decomposition of GeH, is observed. These effects of pressure on the absorption of laser energy by SiH, have been noted previously10Jl~18 and are attributed to the col(16) Perkins, G. G. A.; Lampe, F. W. J . Am. Chem. SOC.1980,102, 3764. (17) Saunders, K. N.; Taylor, H. A. J. Chem. Phys. 1941, 9, 616. (18) Deutsch, T. F. J . Chem. Phys. 1979, 70, 1187.
1.5 -
V
1-
0.5 -
10
20
30
40
50
Total Pressure (Torr) Figure 1. The average number of laser photons absorbed per SiH, molecule in beam path as a function of total pressure: 0 , pure SiH.,; 0, 20 torr of SiH, and variable partial pressures of GeH,.
lisional redistribution of vibrational energy that repopulates the low-lying v4 states of SiH,. GeH4 also participates in this collisional redistribution although not as effectively as SiH,. 2. Nature of the Reaction. Mass-spectrometric a n a l y ~ i s ~ ~ - ~ ~ of the contents of the reaction cell as a function of irradiation time or, equivalently, of the number of laser pulses leads to the conclusion that both SiH4 and GeH4 are consumed and the principal products are H2, Si2H6,SiH3GeH3,trace amounts of Si3H8,and a brown-black solid polymeric material that on the basis of mass balance must contain silicon, germanium, and hydrogen. Although we expected to find Ge2H6as a product, and searched for it, it is not formed in amounts that could be detected (Le. greater than 10% of the SiH3GeH3yield). As may be seen in Figure 2, which shows the strip-chart recorder tracings of the ion currents of m / z 2 (H2+), m / z 62 (28Si2H6+),and m / z 104 (28Si70GeH6++ 28Si72GeH4C 28Si73GeH3+ 28Si74GeH 2 2sSi76Ge+)as a function of irradiation time, the products Ha, Si2H6,and SiH3GeH3are formed simultaneously and are, therefore, all primary products of the photodecomposition of SiH4 in the presence of GeH,. Measurement of the initial slopes of these tracings enables us to calculate easily the initial rates of formation of the reaction products. Surface effects do not seem to affect the course of the primary reactions. In the course of our work on the infrared photochemistry of SiH4I0and of SiH4-PH3" and SiH4-HC112 mixtures, we have had occasion to use two vessels that differ in the respective surface to volume ratios by a factor of 1.8. No differences in overall reaction rates have been observed. Moreover, repetitive experiments in all cases including the present work in the same reaction vessel but with varying amounts of added quartz wool showed good reproducibility indicating that only a minor role, if any, is played by surface reactions. 3. Kinetics and Mechanism. The irradiation of pure GeH, with the P(20) line from the C 0 2 laser causes no decomposition, a fact that is not surprising when one examines the infrared spectrum of the c o m p ~ u n d ' ~ -and '~% confirms ~~ that radiation of
+
+
+
+
(19) Potzinger, P.; Lampe, F. W. J . Phys. Chem. 1969, 73, 3912. (20) Spanier, E. J. Ph.D. Thesis, University of Pennsylvania, Philadelphia PA, 1964. (21) Gibbon, G. A.; Rousseau, Y . ;Van Dyke, C. H.; Mains, G. J. Inorg. Chem. 1966.5, 114. (22) Saalfeld, F. E.; Svec, H . J. J . Phys. Chem. 1966, 70, 1753.
5346
r
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
VI
c_ C 2
n
Zhu et al.
I
295K
m= 2 e 0.4
O'I
0.2 a
/
/ 'O
I t
P '
324K /
p
/o /O
I
/Q 0'
,
I
0.5
10
/"
-
I
/ L
L
m
f=62
c
c ai L
3 U
0 c
05
1.0
[GeHkl/ [ S i H L l
-
/r
J
Figure 3. Dependence of the initial-rate ratio, R(SiH3GeH3)/R(Si,H6), on the initial concentration ratio, [GeH,]/[SiH4], at various temperatures.
!=I04
(x2)
0 10
Irradiation time (SI Figure 2. Dependence of ion currents of reaction products on irradiation time.
this frequency is not absorbed to any significant extent by GeH4. In previous studies of the infrared-laser-induced decomposition of SiH410J1.24 we have shown that the only reactive entity produced in chemically significant amounts is SiH,. It is thus strongly suggested that the mechanism of the major product formation is described by SiH, SiH,
n221
+ nhu
+ GeH4
SiH,
F=
+ H,
(1)
SiH3GeH3*
( 2 , -2)
SiH,
+ SiH4+ Si2H6*
+ -
SiH3GeH3*+ M Si2H6* M SiH3GeH3*
SiH3GeH3*
-
Si2H6*
(3, -3)
+
SiH3GeH3 M Si2H6
+M
+ GeH, SiGeH, + H, SiH,
Si2H4+ H 2
(4) (5) (6)
+
(7)
(8)
A standard treatment of this proposed mechanism leads to the following expression for the ratio of the initial rate of SiH3GeH3 formation to that of Si2H6formation: R(SiH3GeH3) = -k-,- [GeH,] P4 R(Si2H6) k3 [SiH,] P5
similarity of the two molecules. At the pressures involved in this study it has been shownl2Sz6that the predominant fate of Si2H6* formed in (3) is collisional deactivation. Further, comparison of our finding of no significant formation of Ge2H6with the report of Elliot, Estacio, and Ring9 that the principal thermal decomposition channel of SiH3GeH3is (6) leads to the conclusion that collisional deactivation is also the predominant fate of SiH,GeH,*. Hence, we may assume that k , [ M ] >> k-3 k s and k , [ M ] >> k-, + k6 + k 7 , assumptions that result in P4/P5 N 1. On the basis of these assumptions, (9) predicts that the initial rate ratio R(SiH3GeH3)/R(Si2H6)should be a linear function of the concentration ratio [GeH4]/[SiH4]with the slope being the rate constant ratio k 2 / k 3 . A confirmation of this prediction is given in Figure 3, whefe the initial rate ratios are plotted vs. the concentration ratios for several temperatures in the range of 295-357 K. The fact that the straight lines of Figure 3 do not go through the origin is attributed to a systematic error in preparing the SiH4-GeH4 mixtures on the Saunders-Taylor apparatus." This same type of error was observed previously in the preparation of SiH4-HC1 mixtures.I2 If an Arrhenius form is assumed for the rate constants k 2 and k 3 , the slopes of the lines in Figure 3 are then related to the temperature by the relationship
(9)
In (9), P4 and P5 represent the respective probabilities of collisional deactivation of SiH3GeH3*and Si2H6*. This probability ratio, which in terms of the rate constants in the mechanism is given by P4 k4 k,[M] + k-3 + ks -=(10) k5 k , [ M ] + k-2 + k6 + k7 P5 would be expected to be very close to unity, in view of the close (23) Wilkinson, G. R.; Wilson, M. K. J . Chem. Phys. 1965, 4 4 , 3867. (24) O'Keefe, J. F.; Lampe, F. W. Appl. Phys. Lerr. 1983, 42, 219.
where A, and A, are the Arrhenius preexponential factors for (2) and (3), respectively, and E, and E , are the respective activation energies. A plot of the left-hand side of (1 1) vs. 1/ T is shown in Figure 4 and it may be seen then that a satisfactory linear relationship is obtained. The Arrhenius parameters derived from Figure 4 are (A2/A3) = 5.2 f 1.4; E , - E, = 5330 f 670 J/mol Combining the results of a study of the pyrolysis of Si2H,27with those of a subsequent study of the copyrolysis of Si2H,-SiH4 mixtures, John and Purne1126derived the following Arrhenius parameters for the insertion of SiH, into SiH4:
=
109.7f0.4
L
s-l.
E3 = 5.4
* 4.6 kJ/mol
( 2 5 ) Deleted in proof.
( 2 6 ) John, P.; Purnell, J . H.; J . Chem. SOC.,Faraday Trans. I 1973, 69, 1455. (27) Bowrey, M.; Purnell, J. H. Proc. R SOC.London, Ser. A 1971, 321A, 341.
J . Phys. Chem. 1985,89, 5347-5353
5347
of insertion of SiH2 into the Si-H bonds of a series of silanes and, in addition, into the Ge-H bonds of CH3GeH3. Their results, extrapolated to a comparison of SiH4 and GeH4, yield the value k2/k3 = 0.5 f 0.2. The agreement is not particularly good although, considering the very different nature of the two experiments and the crude approximation involved in extrapolation of results for CH3SiH3to SiH4, this is, perhaps, not surprising. The presence of one to two quanta of vibrational energy from the laser in the average SiH4 molecule (cf. Figure 1) would tend to make (3) be faster and, hence, cannot be the explanation for the disagreement. (b) Decomposition of SiH3GeH3. Combination of our activation energy for (2) of 10.7 kJ/mol with the standard enthalpy change26~29~30 of the process
0.0
-0.1 Log k2 k3
- 0.2
SiH3GeH,
2.80
3.00
3.20
3.4 0
w
T Figure 4. Arrhenius plots of slopes of lines in Figure 3, i.e. log ( k 2 / k 3 ) vs. 1J T .
If these Arrhenius parameters are combined with the values of A 2 / A 3and E , - E3 determined in this work, we obtain the following kinetic quantities for the insertion of SiH, into GeH4: A , = 1010.4*0.5 L mol-' s-1. E2 = 10.7 f 5.3 kJ/mol
( a ) Relative Rates of Insertion. According to the temperature dependence of the rate constant ratio k2/k3, insertion of SiH, into SiH4is faster than insertion into GeH4 at temperatures below 389 K but above this temperature the latter reaction is faster. At 350 OC the value of the ratio kz/k3 is calculated from our results to be 1.8 f 0.8. Sefcik and Ring2*have studied the relative rates (28) Sefcik, M.
D.; Ring, M. A. J . A m . Chem. SOC.1973, 95, 5168.
-
SiH,
+ GeH4
(-2)
leads one to conclude that the activation energy of (-2) is 227 f 7 kJ/mol. This is most probably not the decomposition channel of SiH,GeH3 with the lowest energy barrier. In a study of the pyrolysis of SiH3GeH3, Elliot, Estacio, and Ring9 found the predominant thermal decomposition channel to be the process
-
SiH3GeH3
SiH4
+ GeH,
(12)
Simple bond energy considerations indicate that (12) is 63 kJ/mol less endothermic than (-2). It is not likely that the activation energies for the two decomposition paths differ by this much, however, and so it must be that the activation energy for the insertion (-12) is significantly greater than that for the analogous reaction 2.
Acknowledgment. This work was supported by Contract No. DE-AS02-76ER03416 with the US. Department of Energy. (29) Gunn, S. R.; Green, L. G. J . Phys. Chem. 1961, 65, 779. (30) Gunn, S. R.; Kindsvater, J. H. J. Phys. Chem. 1966, 70, 1750.
Electronic-to-Vibrational Energy-Transfer Studies of Singlet Molecular Oxygen. 1. O*(a'AJ J. P. Singh,+ J. Bachar,* D. W. Setser,* and S. Rosenwakd Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: March 15, 1985; In Final Form: August I , 1985)
A gas flow reactor was used to measure the total quenching rate constants of Oz(a'A,) with 26 small molecules at room temperature. With the exception of CF3N0, which probably quenches by an excitation-transfer mechanism, the electronic-to-vibrational (E-V) energy-transfer rate constants are rather small and fall in the 10-16-10-18cm3moleculed s-I range. Attempts were made to observe the infrared emission from products of the E-V transfer, but conclusive assignment was only possible for the quenching of 02(a1A,) by HF. Both HF(u=2) and HF(u=l) seem to be primary products from the H F + Oz(a) reaction.
Introduction The first two excited states of molecular oxygen, 02(a'A,) (0.977 eV) and O,(b'Zg+) (1.627 eV), with radiative lifetimes of 3900 and 12 s, respectively, are interesting candidates for electronic-to-vibrational energy-transfer studies because high concentrations can be conveniently generated by chemical means.' The iodine-oxygen chemical laser, is evidence for the possible 'Present address: MHD Energy Center, Mississippi State University, Starkville, MS 39762. 'Permanent address: Department of Physics, Ben Gurion University, Beer Sheva, Israel.
utility of singlet oxygen, if a good E-V transfer system could be discovered. Our initial interest in reactions of singlet 0, arose from the accidental observation of infrared emission from HF, HCl, and HBr during a study3a of the reaction of 0 atoms with (1) McDermott, W. E.; Pchelkin, N. R.; Benard, D. J.; Bousek, R. R. Appl. Phys. Lett. 1978, 32, 469; 1979, 34, 40. (2) (a) Hays, G. N.; Fisk, G. A. IEEE J . Quantun Electron. 1981, QE-17, 1823. (b) Busch, G. E. IEEE J . Quantum Electron. 1981, QE-17, 1128. (c) Bachar, J.; Rosenwaks, S. Appl. Phys. Lett. 1982, 41, 16. (3) (a) Agrawalla, B. S.; Manocha, A. S.; Setser, D. W. J . Phys. Chem. 1981, 85, 2873. (b) Wickramaaratchi, M. A,; Setser, D. W. J . Phys. Chem. 1983,87, 64. (c) Malins, R. J.; Setser, D. W. J . Chem. Phys. 1980, 73, 5666.
0022-3654/85/2089-5347$01.50/00 1985 American Chemical Society
