771
Photolysis of Hydrogen Selenide
Photolysis of Hydrogen Selenide D. C. Dobson, F. C. James, 1. Safarik, H. E. Gunning, and 0. P. Strausz* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada (Received May 20, 1974; Revised Manuscript Received December 2, 1974) Publication costs assisted by the National Research Council of Canada
The photochemical decomposition of hydrogen selenide has been studied by flash photolysis-kinetic spectroscopy, ESR spectroscopy, and continuous photolysis techniques. From the combined results including those obtained in scavenging experiments with added ethylene, it was concluded that the only primary step operative in the X >200.0 nm photolysis is the free-radical mode of decomposition, H2Se hu H SeH, with a quantum efficiency of unity. The intermediacy of H and SeH in the reaction, and the presence of Se and Sez originating from the secondary reactions of the SeH radical under flash conditions, has been demonstrated by spectroscopic means. The SeH radicals decay largely by the disproportionation reaction 2SeH HzSe Se(3P). The rate constant for the abstraction reaction, H HzSe H Z SeH has been determined in competition with the H CzH4 C2H5 reaction to have the value of (7.1 f 2.0) X 10l2cm3 mol-’ sec-l.
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The photolysis of hydrogen selenide has received little attention compared with studies on the photolysis of water and hydrogen sulfide. An early study1 using a mercury arc lamp, and tungsten-steel and aluminum spark lamps identified the primary photolytic step as decomposition into molecular hydrogen and selenium atoms H,Se + kv
-+
H,
+
Se
This contrasts with the anticipated primary decomposition H,Se + hv
+
SeH (2n)+ H(2S)
(1)
which is analogous to the well-established free-radical mode of decomposition of both water2J and hydrogen sulfide:4s5 H20
H,S
+ hv + hv
-.-c 4
OH (‘n)+ H(‘S) SH(’n)
A
H(2S)
The presence of OH6 and SH4,7radicals in the H20 and HzS systems has been detected by flash spectroscopy. Porter4 looked for the SeH radical in flash photolyzed hydrogen selenide and attributed the failure to detect this species to either the predissociative nature of the 0,O transition of the A28 - X2II system, or to the chemical reactivity of the radical. Later Radford,8 and more recently Carrington,g succeeded in obtaining the ESR spectrum of SeH. As the experimental part of this work was near completion,’O Lindgren’l reported the observation of the SeH radical as a weak diffuse spectrum in the region 300.0-325.0 nm following the flash photolysis of H2Se. More recently Donovan et a1.12 have reported the observation of strong Rydberg bands of SeH in the vacuum uv. The present study was undertaken to elucidate the nature of the primary step in the photodecomposition of the hydrogen selenide molecule, to obtain spectral information on the SeH radical, and to gain an insight into the chemistry of SeH radicals. The present article describes the primary processes while the rates of recombination of the transient SeH, Se, and Sez species and the spectrum of the SeH radical will be reported in forthcoming communications.
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Experimental Section Flash Photolysis, The uv flash photolysis apparatus used was similar in design to that described in earlier articles frqm this 1ab0ratory.l~ Continuous time-absorption records for specific wavelengths were obtained by replacing the spectroscopic lamp with a 450-W xenon arc lamp and the photographic plate with a photomultiplier (EMI-6265B) slit combination. The 18,O band of the B3Z,- - X3Z,- system of Sez a t 334.0 nm14 was monitored in these experiments. The far-uv spectra were obtained in a flash apparatus incorporating a 0.5-m Jarell Ash (Type 78-650) Seya Namioka vacuum monochromator in which spectra were recorded photographically. The pressures of H2Se and inert gases used in the flash experiments were in the ranges 0.05-1 and 50-350 Torr, respectively. Electron S p i n Resonance Experiments. ESR spectra were obtained in a Varian V4502 spectrometer. Photolyses were conducted in situ a t 77 K in the cavity using an Osram high-pressure mercry arc and Vycor 7910 and Pyrex filters. Continuous Photolysis Experiments. A medium-pressure mercury lamp (Hanovia) and a cadmium resonance lamp (Osram) were used to irradiate a quartz cell 10 cm long and 5 cm in diameter. Pyrex and Vycor 7910 filters could be interposed between the lamp and the air-cooled face of the cell. An interference filter was used to isolate the 228.8-nm resonance line of cadmium used for quantum yield measurements. Carbonyl sulfide was used as an actinometric gas (4co from COS = 1.8 a t X 228.8 nm and X 253.7 nrn),l5 and the pressures of hydrogen selenide used were such that total absorption occurred within the cell (above ca. 100 Torr). Yields of noncondensable products of photolysis were measured with a gas burette and analyzed by gas chromatography using a 10-ft. molecular sieve 13X column. Materials. H2Se and D2Se (Matheson 98.0 and 99.8%, respectively), SF6 (Matheson 98%), and CO2 (Matheson 99.8% “bone dry”) were purified by distillation in vacuo and degassed before use. C2H4 (Phillips 99.99%) was passed through traps a t 77 K. COS (Matheson 97.5%) was purified by a method previously described.’j Tank N2 (Union CarThe Journal of Phywcai Chemistry, Voi. 79, No. 8 , 7975
772
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Dobson, James, Safarik, Gunning, and Strausz
TABLE I: Hydrogen Yield in the Flash Photolysis of Hydrogen Selenide Pressure, Torr H,Se
Total
0.53
53 162 176
0.54 0.55 0.46 0.98 0.81 1.29 0.98 0.94
2'76 98 244 129 98 94
Diluent
Yield of H,, pmol/flash
F6
SF, SF, SF, CQZ
COZ C,H4 COZ
SF,
35
3.01 3.70 3.54 3.34 5.91 6 .OO 0.28 5.91 6.90
300
600
TOTAL P R E S S U R E ,
900
Torr
Figure 1. Plot of the quantum yield of hydrogen production as a function of H2Se pressure in the photolysis of pure H2Se (open circles) and with added SF6 (filled circles).
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bide Canada Ltd. 99.993%) and He (Canadian Liquid Air Co. 99.999%) were used without further purification. Results The absorption spectroscopic experiments showed the presence of the following intermediates in flash photolyzed mixtures: Sez, Se, and SeH. The most prominent feature of the uv spectrum was the Se2(B32,,-) - (X32&-) band system.14 Bands at X 300.0, 310.0, and 322.0 nm, attributed t o the A%+ - X 2 n system of SeH,I1 were seen only a t short delay times. Absorption by HzSe, and after photolysis, reduction of the overall transmission by solid deposits, prevented studies a t wavelengths below 250 rim in the uv experiments. The resonance lines of S e ( 3 P ~were ) observed in the vacuum uv at 206.3, 204.0, and 196.1 nm, corresponding to J = 0, 1, and 2, respectively.16 A careful search for lD2 and state selenium atoms which absorb a t 188.5 nm and a t 199.5 nm, respectively,16 with transition probabilities (43P2),17indicated the comparable to that of Se(j3S1) absence of these species. SeH and Sez were also observed in the vacuum uv region, the SeH a t 179.0 nml* corresponding to the C1 X3n3/2 transition and S e ~ ( ~ x , -as ) the C X and D X band systems.18 The concentration of Se2 rapidly rose to a maximum and subsequently decayed by second-order kinetics in the presence of He, Nz,or CO2 as inert diluents. With SF6 as diluent the decay was complex; nevertheless the amounts of Hz per flash formed were independent of the nature of diluent gas used, Table I. Ethylene when used as diluent exhibited a strong suppressing effect on both Se2(X32,-) and the hydrogen product. The most important feature of the S e ( 3 P ~spectrum ) was that it appeared weakly a t zero delay time and its rate of growth was intermediate between that of SeII and Sez. In the presence of added ethylene the spectrum of ethylene episelenide could be seen.lg This molecule is known to form from the addition of selenium atoms to ethylene.20 When H2Se was photolyzed in an ice matrix at 77 K the two-line proton spectrum with a splitting of 505 G appeared.21 In similar experiments with D2Se the three-line deuterium spectrum with a splitting of 77 G was observed. The ESR spectrum with a g value of 0.808 attributed by Radford8 to the gaseous SeH radical could not, of course. be detected under these conditions. The kinetics of Hz formation was studied quantitatively in continuous photolysis experiments. A plot of the quantum yield of hydrogen formation as a function of hydrogen selenide pressure is shown in Figure 1. The value of a(H2) is 1.0 f 0.05 in the pressure range, 200-800 Torr, studied. +
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The Journa! of Physical Chemistry, Vo!. 79, No 8, 1975
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L
L L
t
(L
@ 8 io0
30
900
630
PRESSURE, fori
Figure 2. Variation of the rate of hydrogen production in the photolysis of H2Se with the pressure of added ethylene and sulfur hexafluoride: 10 Torr of H2Se C2H4 no filter (open circles); as above but with a Vycor 7910 filter (filled circles); 10 Torr of H2Se -t SFc with a Vycor 79 10 filter (squares). Photolysis by medium-pressure mercury lamp (Hanovia).
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110
100
90
t
::v
IC
AGC
,
,
63C
723
,
>G
0
0
100
230
30C
4CC
53C
ETHYLENE PRESSURE,
8C3
903
tori
Figure 3. Plot of [ @(H2)/R(H2)] - 1 vs. ethylene pressure (eq I).
Ethylene was used to scavenge atomic hydrogen and any radical species present. The suppressing effect of ethylene on the yield of hydrogen is shown in Figure 2. This suppression is evidently due to the competing reaction of ethylene with hydrogen selenide for H atoms: H + H,Se
-+
H t- CZH,
-
H,
-C
SeH
C,Hs
(2)
(3)
Steady-state treatment of the two reactions leads to the rate expression (1) [Ro(H,)/R(Hz)] - 1 = (K,/k,)([C,H,I/[H,SeJ)
where RO(H2) and R(H2) are the rates of hydrogen production in the absence and presence of ethylene, respectively.
Photolysis of Hydrogen Selenide
773
Plotting the left-hand side of (I) against ethylene pressure (at a fixed pressure of 10 Torr of H2Se) a straight line relation is obtained, Figure 3, as predicted. From the slope, k 3 / k z equals 0.105 f 0 01. The value of 123 is uncertain although a recent review article22 includes most high-pressure values in the range (7.5 i 1.0) X 10l1 cm3 mol-1 sec-l. Using this average value of k3 we obtain k z = (7.1 f 2.0) X 10l2 cm3 mol-l sec-l. Thus, it appears that all the hydrogen produced in the reaction is scavengeable with ethylene, but because of the high value of k z a more substantial excess of ethylene than was used here would be required for the complete suppression of hydrogen. The k 3 / k z ratio does not seem to be affected by the wavelength of the photolysis. Figure 2 also illustrates that the diluent gas SFs has no effect on the quantum yield of hydrogen, indicating that the hydrogen producing sequence does not involve an intermediate species that can be collisonally deactivated.
Discussion The electronic absorption spectrum of H2Se consists of a continuum from 340.0 nm to below 190.0 nm (emax 1.5 X lo3 M - l cm-l a t 215.0 nm),l with strong discrete bands starting in the vacuum uv a t 169.0 nm.23 Weak vibrational structure in the 250.0-nm absorption region of H2S indicates that the lowest excited state of H2S is nominally a Rydberg state. Although no fine structure has been detected in the 340-190-nm region of HZSe, it has been suggested that this absorption also corresponds to excitation to a Rydberg state.23 The only primary step which is consistent with the pressure independent unit efficiency of hydrogen production, the suppressing effect of added ethylene, the appearance of the intense spectrum of SeH, even at short delay times, and the ESR spectrum of H, is the free radical mode of decomposition H,Se
Izu
-.
H('S) + SeH('n)
b = 1.0
(1)
Using a value of 76 kcal mol-l for the HSe-H bond dissociation energy,25the threshold for reaction 1 is a t -380.0 nm. Of the two alternative primary steps H,Se
+ lzu H, + + hu -+ 2H + 4
Se
(41
H,Se Se (5) neither can be important; the former on account of the suppressing effect of added ethylene on the yield of hydrogen, and the latter on energetic grounds. Step 5 is endothermic by 146 kcal mol-' and would become feasible only a t wavelengths below 197.0 nm. This photobehavior of HzSe may be compared to that of H20 and H2S both of which undergo primary decompositions analogous to step 1 in their first absorption region,28J9although molecular hydrogen production has been observed on photolysis of hydrogen sulfide at X
