J. Phys. Chem. 1996, 100, 10215-10222
10215
Vacuum-Ultraviolet Photodecomposition of Stannane D. J. Aaserud and F. W. Lampe* 152 DaVey Laboratory, Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: March 11, 1996; In Final Form: April 10, 1996X
The 147-nm photolysis of SnH4 results in the formation of H2, Sn(s), and a very small amount of Sn2H6. The quantum yields decrease with increasing partial pressure of SnH4, at 0.2 Torr being Φ(-SnH4) ) 5.6 ( 0.3, Φ(H2) ) 11.4 ( 0.6, and Φ(Sn2H6) ) 0.059 ( 0.02. A mechanism is proposed that is shown to be internally consistent and in agreement with the experimental facts.
1. Introduction It has been shown that the 147-nm photodecompositions of silane1-3 and germane4-6 occur via the formation of silylene, germylene, and, to a lesser extent, silyl and germyl radicals in the respective primary photodissociation steps. Much less information is available on the decomposition processes in the structurally similar stannane, a fact that is, perhaps, not surprising when one considers the even greater instability of the stannane molecule as compared to silane and germane. A few kinetic studies of the thermal decomposition of stannane were reported some 30-40 years ago;7-9 a much more recent investigation of the 193-nm photodecomposition10 suggests that stannylene is the principal intermediate in the reaction, which is analogous to the cases of silane and germane. In a continuation of our studies of the radiation-induced decomposition of group IV hydrides,2,6,11-19 we have had occasion to investigate the 147-nm photodecomposition of stannane. This paper is a report of our results. 2. Experimental Section All photolyses were carried out in a cylindrical stainless steel cell that was modified to be slightly different from those previously used2,6 in our laboratory. The cell is fitted, at one end of the cylinder, with a CaF2 window for passage of the vacuum-ultraviolet light and with a quartz window at the opposite end. A glass, pinhole leak, leading directly into the ionization chamber of a Nuclide 12-90G magnetic sector mass spectrometer, was positioned in the side of the cylinder perpendicular to the light path and at a distance of 4.6 cm from the CaF2 window. This T-shaped configuration was chosen to keep the sampling orifice closer to the entrance window than in our previous work,2,6 thus minimizing diffusion effects, since most of the photolysis occurs near the window. The total length of the cell was 9.4 cm and the diameter was 3.5 cm. For experiments in which the temperature was varied, the quartz window was replaced by a stainless steel flange holding a nickel-chromium vs nickel-aluminum (chromel-alumel) thermocouple, and in this case the cell length was 7.8 cm. To limit reflection, the inside surface of this flange was coated with a black paint that is resistant to high temperature and vacuum. The photolysis cell was connected via Nupro valves and 1/4-in. diameter Teflon tubing to a high-vacuum line. Gas pressures in the photolysis were measured with a 0-50-Torr Wallace and Tiernan gauge. Although the leakage of the gases from the photolysis cell into the mass spectrometer is in the transition region between X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(96)00740-X CCC: $12.00
molecular flow and viscous flow, the rate of decrease in concentration of any gas due to this flow can be expressed by a first-order, leak-rate constant, λ, that is dependent on the total pressure in the cell. For a typical pinhole leak, at 15-Torr total pressure, the value of λ was 3.62 × 10-4 s-1; a 3-fold reduction of the total pressure results in an 18% decrease in λ. Since all experiments were carried out with gas mixtures containing about 90% of helium as diluent and conversions of stannane were kept at about 10%, the maximum pressure changes (increases) observed were no greater than about 2%. Hence, over any of our runs λ may be taken to be a constant that is determined at 15 Torr of total pressure for each pinhole. The leak-rate constant depends only slightly on mass, decreasing by about 8% for an increase in mass from 120 to 220 amu. During a photolysis, the dependence of reactant and product concentrations on time was determined by continuous mass spectrometric monitoring of the ion currents as follows: SnH4 (117SnH3+ + 118SnH2+ + 119SnH+ + 120Sn+, m/z 120); H2 (H2+, m/z 2); H2O (H2O+, m/z 18); 15NO (15NO+, m/z 31); N2O (N2O+, m/z 46). The concentration of Sn2H6 was monitored at m/z 240, an ion current that is due to the sum of 26 isotopic variants of Sn2Hx (x ) 0-6), namely 6
∑ x)0
m
SnnSnHx
m+n)240-x
where the isotopic mass numbers m and n range over the stable isotopes of tin. The electron impact mass spectra of natural SnH4 and Sn2H6 at 70 eV are shown in Figure 1. The relationships between ion currents and photolysis cell concentration were determined, when possible, using mixtures of pure samples of the reactants and various products in helium at partial pressures comparable to those obtaining in the photolysis. This procedure could not be used for Sn2H6 because of the lack of thermal stability of the compound. The procedure used in this case was based on our observation that for a given homologous series, MnX2n+2 (M ) C, Si; X ) H, F), the ionization cross section at 70 eV, σ, can be represented by the formula
σ ) γ + RMZ
(1)
where Z is the number of electrons in the molecule and γ and RM are constants, RM depending on the nature of M. In this calibration for Sn2H6 we determined [Sn2H6] relative to [SnH4] by the well-known mass spectrometric relationship © 1996 American Chemical Society
10216 J. Phys. Chem., Vol. 100, No. 24, 1996
Aaserud and Lampe
Figure 2. Electron impact ionization cross sections at 70 eV for alkanes (0), silanes (]), germanes (O), stannanes (4), and perfluoroalkanes (open cross) as a function of the number of molecular electrons.
Figure 1. Mass spectra of SnH4 and Sn2H6 at 70 eV of ionizing energy.
[Sn2H6] [SnH4]
)
( )(
)(
)
i240 σi(SnH4) f120(SnH4) i120 σi(Sn2H6) f240(Sn2H6)
(2)
which assumes equal instrumental transmission for m/z 120 and 240, and in which i120 and i240 are the ion currents at the denoted masses, σi(SnH4) and σi(Sn2H6) are the ionization cross sections of the respective compounds, f120(SnH4) is the fraction of the ions in the mass spectrum of SnH4 of m/z 120, and f240(Sn2H6) is the fraction of ions in the mass spectrum of Sn2H6 of m/z 240. However, to use eq 2 we require the ionization cross sections of SnH4 and Sn2H6 at 70 eV of ionizing energy. σi(SnH4) was measured to be 8.04 ( 0.40 Å2 from the total ion currents observed in a mixture of SnH4 and Xe, taking σi(Xe) to be 7.51 Å2.20-23 We have estimated σi(Sn2H6) from an observed linear correlation (cf. eq 1) between the ionization cross section at 70 eV and the total number of electrons within a group of similar molecules. The solid lines in Figure 2 are plots of the ionization cross sections versus the number of electrons for alkanes,20-23 perfluoroalkanes,23 and silanes.24,25 Assuming the same relationship to hold for stannanes (SnnH2n+2), with the mean intercept for the three solid lines of 0.058 ( 0.485, we used our measured value of σi(SnH4) to derive the expression where σ is given in Å2. From eq 3 we calculate the value σi-
σ(SnnH2n+2) ) 0.058 ((0.485) + 0.148 ((0.007)Z (3) (Sn2H6) ) 15.7 ( 1.3 Å2 and have depicted the relationship for σi(Sn2H2n+2) (n ) 1-3) as a dotted line in Figure 2. In a similar way, we measured σi(GeH4) and have depicted the cross sections so determined for the series GenH2n+2 (n ) 1-4) as a dotted line in Figure 3. Knowledge of the relevant ionization cross sections (Figure 2) and the mass spectra (Figure 1) permits calculation of Φ(Sn2H6) from rate data such as shown in Figure 3. The source of radiation was a xenon resonance lamp26 powered by a 2450-Mhz microwave generator. This lamp produces resonance radiation at 147 nm (98%) and 130 nm
Figure 3. Reactant and product ion currents as a function of photolysis time.
(2%). The intensity of the lamp on the reaction mixture was determined before each photolysis by observation of the decomposition rate of N2O, which has an absorption cross section27 of 5.0 × 10-18 cm2 and a quantum yield for decomposition of 1.728-31 at 147 nm. The average photon flux on the reaction mixture through a freshly cleaned window was 2.6 × 1014 photons/cm2‚s. The absorption cross section of SnH4 for 147-nm radiation has been reported32 to be 2.2 × 10-17cm2. This value is such that, for the partial pressures of SnH4 obtaining in our experiments, 75% to essentially complete absorption of the incident radiation occurred. This means that most of the light absorption occurs near the entrance window of the cell and leads to the question of whether diffusion effects will influence the measurement of the rates of reactant depletion and product formation and, thus, the quantum yields. Although one may estimate the average time required for a product molecule formed near the entrance window to diffuse to the sampling orifice to be 33 s,33 a value that suggests a significant influence of diffusion effects, experimental evidence indicates these are negligible. First, reactant depletion and product formation are observed to occur simultaneously with the onset of photolysis over the entire range
Vacuum-UV Photodecomposition of Stannane
J. Phys. Chem., Vol. 100, No. 24, 1996 10217
of partial pressures studied. Second, a 3-fold variation of the incident light intensity had no effect on the observed quantum yields. We believe that during photolysis effective mixing of the cell contents by convection currents caused by thermal gradients is the explanation for the observed absence of diffusion effects. The major overall reaction occurring in the cell is
SnH4(g) f Sn(s) + 2H2(g)
(4)
which is exothermic by 38.9 kcal/mol; this heat release combined with the energy input from the radiation leads to an overall power input to the system of 4.1 × 10-5 W/cm3. Given the heat capacity of the gas (13.5 Torr He and 1.5 Torr SnH4) to be 1.1 × 10-5J/cm3‚K; we obtain a heating rate of 3.7 K/s-1 averaged over the entire cell. The heating rate and hence the temperature rise will be greatest at the window and will decrease toward the exit aperture; in addition, there will be cooling at the walls. We believe these temperature gradients and accompanying convection currents provide efficient mixing of the cell contents. Similar effects were noted6 in GeH4 photolysis. Stannane was synthesized by the reduction of tin(IV) chloride (SnCl4) with lithium aluminum hydride (LiAlH4) in diethyl ether.34,35 In this procedure, a slurry of SnCl4 in (C2H5)2O is slowly added to the reaction vessel, containing a solution of LiAlH4 in (C2H5)2O, which is kept at -60 to -70 °C. The gases evolved are passed through a series of cold traps to collect the product, and the stannane is purified by repeated distillation through a trap at -116 °C (ethanol slush). Because of its instability, stannane was stored in a refrigerator at 4 °C in freshly cleaned Pyrex bulbs, either with 1.2 Torr of oxygen present to inhibit decomposition36 or as a dilute mixture (10-15%) in helium. Hydrogen, nitrous oxide, and xenon were purchased from Matheson. Helium and nitrogen were obtained from MG Industries and [15N]nitric oxide was purchased from Cambridge Isotope Laboratories. All gases condensable at liquid nitrogen temperature were subjected to at least one freeze-pump-thaw cycle prior to use. All gases were checked mass spectrometrically for impurities prior to use. The lithium aluminum hydride, tin(IV) chloride, and diethyl ether were purchased from Aldrich. 3. Results and Discussion (a) General Nature of the Photodecomposition. The products observed in the 147-nm photodecomposition of SnH4 are H2, Sn2H6, and Sn, the latter of which appears as a solid film. While it is possible that higher hydrides such as Sn3H8 and Sn4H10 were formed at concentrations below our detection limit, it seems highly unlikely as they should be extremely unstable; indeed, to our knowledge they have not yet been synthesized and isolated. Typical recorder tracings of ion currents as a function of time, showing the depletion of SnH4 (m/z 120 amu) and the formation of H2 (m/z 2 amu) and Sn2H6 (m/z 240), are shown in Figure 3. It may be seen here that the increases in H2 and Sn2H6 concentrations occur simultaneously with the decrease in SnH4 concentration, indicating H2 and Sn2H6 to be primary products. In Figure 3, t ) 0 corresponds to the beginning of photolysis and, as discussed in the Experimental Section, the absence of any delay in the onset of observed reaction attests to the absence of significant diffusion effects. The addition of small amounts of 15NO has very little effect on the observed rate of depletion of SnH4 and the rate of formation of H2 but causes a significant increase in the formation rate of Sn2H6. This was a somewhat surprising result
TABLE 1: Quantum Yields in 147-nm Photodecomposition of SnH4 P(SnH4),a Φ(H2), Φ(Sn2H6), Φ(-SnH4), molecules/photon molecules/photon molecules/photon Torr 5.6 ( 0.3 5.1 ( 1.1 4.9 ( 0.08 4.4 ( 0.2 4.7 ( 0.4
0.20 0.58 1.06 1.51 1.99
11.4 ( 0.6 9.8 ( 0.4 9.0 ( 0.5 8.6 ( 0.9 8.3 ( 0.8
0.059 ( 0.017 0.058 ( 0.015 0.049 ( 0.013 0.053 ( 0.012 0.048 ( 0.015
a Total pressure ) 15 Torr of the indicated partial pressure of SnH 4 in He.
because, in the analogous photodecompositions of SiH42 and GeH46, the presence of NO results in a significant decrease in the formation rates of Si2H6 and Ge2H6, respectively. Additional products formed when NO is present were N2O and H2O. Although we searched for SnH3OSnH3 and higher stannoxanes, we could find no evidence for their presence, a finding also contrary to expectations based on the observed formation of SiH3OSiH3 and GeH3OGeH3 in the photodecompositions of SiH4 and GeH4, respectively. (b) Quantum Yields. The quantum yields for the depletion of SnH4 were determined from replicate measurements of the initial rate of disappearance of SnH4 in photolyses carried out immediately after measurement of the initial rate of disappearance of N2O in a photolysis of pure N2O. The initial rates of depletion were determined from the initial rates of decrease of the ion currents of 120 amu (SnH4) and 44 amu (N2O) in the respective photolyses, as shown in
-
(
)
[SnH4]0 di120 d[SnH4] )) Φ(-SnH4)qa(SnH4) dt 0 dt i1200
-
(
)
(5)
[N2O]0 di44 d[N2O] )) Φ(-N2O)qa(N2O) (6) dt 0 i 0 dt 44
where i1200 and i440 are the currents at the denoted masses before photolysis is begun, [SnH4]0 and [N2O]0 are initial concentrations, Φ(-SnH4) and Φ(-N2O) are the quantum yields at 147 nm for depletion of SnH4 and N2O, respectively, and qa(SnH4) and qa(N2O) are the respective rates of absorption of the 147nm radiation. The rate of absorption of radiation by a gas j in our photolysis cell is given by
qa(j) ) (I0/L)(1 - e-σj[j]L)
(7)
where I0 is the photon flux, L is the cell length, σj is the absorption cross section of gas j, and [j] is the concentration of j molecules. We may combine eq 7 with j ) N2O with eq 6 to compute I0 and, hence, Φ (-SnH4) from eqs 7 and 5 with j ) SnH4. The quantum yields for the products, namely Φ(H2) and Φ(Sn2H6), were similarly determined from the initial slopes of rate curves for these products such as shown in Figure 3. It was necessary to correct the current-time curve for H2 (H2+ at m/z 2) for the contribution to H2+ from SnH4; the yields of Sn2H6 were so small that contribution to H2+ from it was negligible. The quantum yields for the depletion of SnH4 and for the formation of H2 and Sn2H6 over a 10-fold partial pressure range of SnH4 are shown in Table 1. Each quantum yield is the mean of between three and five replicate experiments, and the uncertainties shown are the average deviations from these means. As may be seen in Table 1, and as is shown graphically in Figure 4, all quantum yields decrease with increasing partial pressure of SnH4, showing zero-pressure limits of
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+ Figure 5. Ion current ratio i+ 116/i123 as a function of photolysis time.
Figure 4. Quantum yields of SnH4 depletion and H2 formation as a function of partial pressures of SnH4.
lim Φ(-SnH4) ) 5.41 ( 0.39 molecules/photon f0
PSnH4
lim Φ(H2) ) 11.3 ( 0.4 molecules/photon f0
PSnH4
lim Φ(Sn2H6) ) 0.060 ( 0.003 molecules/photon f0
PSnH4
The inverse dependence of the quantum yields on SnH4 partial pressure contrasts sharply with the behavior seen in the 147nm photodecomposition of SiH42 and GeH4.6 In the former case the quantum yields were pressure independent, while in the GeH4 case the quantum yields increased with pressure. Material balances calculated from the quantum yields in Table 1 indicate that the gas phase products H2 and Sn2H6 account for 2.2 ( 0.2% of the Sn and 96 ( 5% of the H in the decomposed SnH4. Moreover, within experimental error, these balances are independent of the partial pressure of SnH4 over the 10-fold range investigated. Since virtually all of the H in the decomposed SnH4 is found in the gas phase, and since no compounds containing Sn other than Sn2H6 were found in the gas phase, we conclude that Sn(s) is the material deposited on the walls and window of the photolysis cell and that Sn(s) and H2(g) are the predominant products of the photocomposition. On the basis of material balances
Φ(Sn(s)) ) 5.35 ( 0.39 molecules/photon at the low-pressure limit. Of course, at all of the partial pressures of SnH4 studied, Φ(Sn(s)) is just slightly less than Φ(-SnH4). We attempted mass spectrometrically to observe the formation of the tin produced in the reaction by examining the ratio of
Figure 6. Effect of temperature on the quantum yields for SnH4 depletion and Sn2H6 formation.
the ion current at m/z 116 amu to that at m/z 123 amu as a function of photolysis time. The ion current at m/z 116 from SnH4 is comprised of 96% 116Sn+, >[HNO], a not surprising conclusion. The same kinetic treatment yields the expression for Φ(Sn2H6), with NO present, shown in
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