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another can tend to cause the oxides to be deposited simultaneously. If that interaction did not occur, preferential deposition of one of the oxides would occur as the gas stream traverses the hot zone. (This would be especially pronounced in the case of POC13 and SiC14 mixtures.) We have also shown that the reactions will go to completion under normal glass fiber fabrication conditions. This information serves to define the problem of increasing deposition efficiency as one involving the collection of reaction products, rather than one involving incomplete reaction.
Achnowledgment. We thank D. Edelson, P. K. Gallagher, W. R. Sinclair, and D. L. Wood for helpful discussions. One of us (W.G.F) expresses his gratitude to Professor J. E. Willard for his guidance and support from 1965 to 1969. References and Notes (1) W. G. French, J. B. MacChesney, and A. D. Pearson, Annu. Rev. Mater. Sci., 5, 373 (1975). (2) D. L. Wood, J. B. MacChesney, and J. P. Luongo, J. Mater. Sci., in press. (3) V. Geiss and E. Froschle, J. Electrochem. Soc., 123,133 (1976). (4) D. Powers, J . Am. Ceram. SOC., in press.
Gas Phase Photolysis of Ethyl Bromide at 253.7 nm Arthur J. Frank and Robert J. Hanrahan" Department of Chem/stry, University of Florida, Gainesvi//e,Florida 326 1 1 (Received January 12, 1978) Publication costs assisted by the US. Department of Energy
The photolysis of ethyl bromide was studied at 100 Torr and 23 "C using 253.7-nm radiation. In the pure system between 60 and 90 s at an absorbed light intensity of 8.3 X 1013quanta/cm3 s the major products and their respective quantum yields axe as follows: hydrogen bromide, 0.26; ethane, 0.40; ethylene, 0.028; 1,l-dibromoethane, 0.102; l,Bdibromoethane, 0.0092; vinyl bromide, 0.009; 1,1,2-tribromoethane,0.0027; methane, 0.00052; and methyl bromide, 0.00091. When 5 mol % oxygen is added, the quantum yields in this time period become the following: hydrogen bromide, 0.47; ethane, 0.000 32; ethylene, 0.0081; 1,l-dibromoethane, 0.0040; 1,2-dibromoethane, 0.022; vinyl bromide, 0; 1,1,2-tribromoethane,0; methane, 0.0001; and methyl bromide, 0.091, Bromine is also formed with a quantum yield of 0.22. At long photolysis times the HBr and CzH4go into a stationary state, and the main photolysis products are C2H6and 1,1-C2H4Br2with quantum yields of 0.24 and 0.17, respectively. The behavior of the system is interpreted based on a model involving C-Br rupture as the main primary process (4 = 0.8) with a substantial contribution of HBr elimination (4 = 0.2). Primary C2H6. and Br. fragments abstract hydrogen from the substrate to produce mainly the l-C2H4Br. radical. Net product M), formation involves reaction of radical species with Br, (present at low, steady-state concentration Br. atoms, or HBr. The dynamic behavior of the system was investigatedby computer simulation and compared with experimental results.
I. Introduction The present study is an outgrowth of a parallel investigation of the mechanism of the y radiolysis of ethyl bromide vapor a t room temperature.l We undertook an investigation of the photolysis of this compound since all previous gas-phase photolysis studies were carried out at high temperatures and/or in the presence of various additives. Roof and Daniels2 investigated the 313-nm photolysis of ethyl bromide-acetaldehyde mixtures a t 310 "C. Results were interpreted in terms of the formation of CzH6-and Br.; the former was assumed to catalyze the decomposition of the acetaldehyde. Friedman, Bernstein, and Gunning3studied the photolysis of ethyl bromide in the presence of mercury and excess cyclopentane, over a temperature range of 30-250 "C. Ethane was formed with a quantum yield of nearly unity, but only a small amount of ethylene was detected; results were interpreted on the basis of C-Br bond scission as the major primary photolytic event in the wavelength range 210-260 nm. Barker and Maccol14 photolyzed gaseous ethyl bromide at 253.7 nm over the temperature range 150-300 "C. At these rather high temperatures, they reported a radical chain reaction forming CzH4 and HBr. Gurman, Dubinski, and Kovalev6 studied the photolysis of liquid ethyl bromide at 253.7 nm and room temperature. Major products were ethane, dibromoethane, and bromine, 0022-3654/78/2082-2194501 .OO/O
as well as a small amount of ethylene. They made no mention of the presence of hydrogen bromide. The present investigation is concerned with the 253.7-nm photolysis of ethyl bromide vapor at 100 Torr pressure and 23 "C. Quantum yields are reported for several products both in the absence and in the presence of oxygen, added as a free radical scavenger. Only the major features of the photolytic decomposition of the compound are reported here and these are discussed in light of a computer simulation of the reaction mechanism. Further details of the reaction mechanism are described elsewhere.6 11. Experimental Section Baker reagent grade ethyl bromide was dried with Drierite and fractioned through a 4-ft glass-helix packed Todd still. The middle cut boiling at 38.3 f 0.1 "C was degassed on a mercury-free vacuum line, vacuum distilled through a 25-cm column of barium oxide, and stored in the dark at -196 "C. This material showed no observable impurities by flame ionization gas chromatography. Matheson hydrogen bromide used in actinometry experiments was passed through columns of P205and copper filings; research grade oxygen was dried with silica gel. Gas chromatography standards and other miscellaneous reagents were used as received in the best available grades. 0 1978 American Chemical Society
Gas Phase Photolysis of Ethyl Bromide
Photolysis experiments were carried out at room temperature under mercury-free conditions using radiation emitted by a 15-W General Electric germicidal lamp, Type G15E8. Except for the measurement of Br2, all experiments utilized 91-cm3cylindrical vessels constructed of 18 mm i.d. GE Type 204 clear fused quartz. For analysis of bromine, a 34-cm3vessel consisting of a cylindrical 10-cm light path quartz spectrophotometer cell was used. Photolysis vessels were positioned longitudinally parallel to the photolysis lamp at a perpendicular distance of about 2 cm; a standard desk lamp fixture was used, modified slightly to allow blowing air for cooling purposes. The emission spectrum of the germicidal lamp was examined with a McPherson Model 218 vacuum ultraviolet monochrometer and associated recording electronics. In the spectral range 170-300 nm the emission lines and their respective relative intensities (log scale) were 253.7 nm, 1.0 units and 296.7 nm, 0.05 units. The near-UV absorption spectrum of the hydrogen bromide actinometer gas resembles closely that of ethyl bromide. The HBr absorption band is rather broad, with a maximum a t about 185 nm and a long wavelength tail extending to 299 nm.7 Similarly, the absorption band of ethyl bromide reaches a broad maximum a t 203 nm and tails to approximately 300 nma8Photolytic decomposition of both ethyl bromide and HBr was due essentially to 253.7-nm radiation. Production of H2 from the HBr actinometer gas was assumed to have a quantum yield of unity.9 Conditions were arranged to give essentially identical light absorption with HBr and ethyl bromide respectively, in order to cancel geometric factors. Absorbed intensity was 8.5 f 0.1 X 1013 quanta/cm3 s for the 91-cm3photolysis vessel and 8.0 f 0.01 X 1013quanta/cm3 s for the 34-cm3cell. (To facilitate comparisons, the ordinate of the Br2 time-yield plot in Figure 4 was adjusted to compensate for the lower absorbed intensity in the 34-cm3vessel.) Products volatile at liquid nitrogen temperature were collected by a Toepler pump and measured on a McLeod gauge and/or delivered to a gas loop for gas chromatographic analysis. Higher boiling products were identified by combined gas chromatography-mass spectrometry and measured using flame ionization gas chromotography. Columns used were 3-m silica gel (for methane, ethane, ethylene, and acetylene) and 4.2-m 30% OV-101 on 60-80 mesh acid-washed Chromosorb P. Hydrogen bromide was determined as aqueous Br- by titration with 0.02 or 0.05 M AgNO,. A 50X gas-tight (Teflon tipped plunger) Hamilton microsyringe, equipped with a platinum needle, was used in lieu of a buret. The titration was monitored electrochemically, using an Orion Model 94-35A bromide ion specific electrode and a double junction calomel electrode; the output was read on a Hickok digital volt-ohm meter. When both HBr and Brz were produced (experiments with added oxygen) a small correction was made for Br2 disproportionation to Br- and HOBr, using a simple equilibrium calculation. Bromine was measured spectrophotometrically in the gas phase at room temperature, using a photolysis cell with attached cuvet. Absorbance measurements were made a t 416 nm on a Beckman DU spectrophotometer, using an extinction coefficientlo of 170 M-l cm-l. 111. Experimental Results We were able to observe 21 products from the gas-phase photolysis of ethyl bromide; 15 of these were definitely identified, and another 4 were tentatively identified. A number of these species had very small quantum yields, of the order of 0.0005 or less, and are discussed elsewhere.6
The Journal of Physical Chemistty, Vol. 82,No. 20, 1978 2195
0.0
2.0
4.0
~nor04.i~
6.0 (minutes)
8.0
10.0
Figure 1. Production of hydrogen bromide (pure, 0; 5 % Opr 0 )as a function of photolysis time. Broken line shows computer sirnulation of HBr yield.
4.0 6.0 Phatolpi. T h e (minutes)
2.0
0.0
10.0
8.0
Flgure 2. Production of ethane (pure, 0; 5 % 02, 0 )as a function of photolysis time. Broken line shows computer simulation of ethane yield.
I
0.08
1
I
s
:1,20
f 1
n.in
u.no 0.0
2.0
L.0
6.0
8.Q
Pbotalysli Tine Iminutcs)
Flgure 3. Production of ethylene (pure, 0; 5 % 02, e) as a function of photolysis time. Broken line shows computer simulation of C&I4 yield; ordinate compressed 5 times.
Table I presents the yields of the major products including hydrogen bromide, ethane, ethylene, 1,l-dibromoethane, and 1,Zdibromoethane. Yields of several minor products, including vinyl bromide, 1,1,2-tribromoethane, methane, and methyl bromide, are also given, since formation of these species is mechanistically interesting. As Table I indicates, molecular bromine was found in the presence of oxygen scavenger, but not in its absence. Since essentially none of the quantum yields are constant with photolysis time, Table I gives the quantum yields at several stages of the experiments. The rather complex behavior of the system can be seen more clearly in Figures 1-7,
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TABLE I: Quantum Yields for Major and Minor Photolysis Products from Ethyl Bromide Vapor at 100 Torr as a Function of Photolysis Time pure systemb yields at
0, scavenged systemb yields at 3.5-4.0 6.0-7.0 0-0.5 1.0-1.5 3.5-4.0 6.0-7.0 Convex Upward HBr 0.42 0.26 0.13 -0 0.47 0.29 0.29 0.55 0 0 0 0.22 0.22 0.19 0.12 Br, 0 0.40 0.24 0.24 0.00032 0.00032 0.00032 0.00032 CZH, 0.40 0.028 0.017 -0 0.0081 0.0033 0.0032 0.0081 CZH, 0.028 C,H,Br 0.015 0.009 0.0008 -0.004 0 0 0 0 1,1,2-C,H3Br, c 0.0027 0.0016 0.0016 0 0 0 0 Concave Upward l,l-C,H4Br2 0.102 0.102 0.169 0.169 0.0040 0.0040 0.0091 0.0091 1,2-C,H,Brz 0.0012 0.0092 0.014 0.016 0.0073 0.022 0.041 C 0.00052 0.00052 0.0031 0.0031 0.00010 0.0001 0.00018 0.00018 CH, CH,Br C 0.00091 0.0024 0.0031 c 0.091 0.21 0.21 The time-yield plots for the listed products are found in Figures 1-7. Data for very minor photolysis products with quantum yields
