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The photolysis of water vapor at 1470 Å. H2 production in the primary process
volume
CHE?&J CAL PHYSICSLETTERS
28, number 4
t5 October
1974
‘.
THE PHOTOLYSIS._OF WATER VAPOR AT 1470 A. H2 PRODWCTION INTHE PR%ARY PROCESS*
’
A-Y.-M,UNG Institute of Spgce nrx’dt?nospheric Studies, U~~iversity of Saskatchewan. Saskatoon, Saskatchewan S7N OJVR, Canada Received 24 June 1974 TIE primary pro&es
in the photolysis
Hz0 + izu(h = 1470 ii, --) HZ + O(‘D) ,
of water vapor at
1470 A ace.due to
Hz0 + It&‘, = 1470 A) + H + OH
with the Hz yield of the fust process accounting for 23% of the overall H2 prodxtion. ike qwntum yield of this process is estimated to be 0.08 by using 02 as a scavenger for H-atcns. Secondary re;lctioris involving the photo-
lytic pioducts and added 02 are discussed.
1. Introduction Watei vapor possesses two distinctive absorption features in the vacuum ultra~olet spectrum. In tile
spectral region 1430-1860 a, there is a continuum whose maximum lies at about 1650 A. The second continuum, 1250-1430 W, is cIearly associated with a series of broad diffuse bands. It is generally believed that the photodissociation of water vapor in the second continutim will produce a variety of primary processes, whereas the photo&is in the first continuum involves only one primary prpcess which yields H and OH. However, based on energetic considerations and the spin selection rules, the follow.’ ing primary processes are possible in the fust continuum.
H20f1AI) + hv(h G 2420 a) --f H(! 2S) + OH(X21-I) m @I
-Fe-questions of-wether.process (A) can occur in.‘the fiist coritinuum, and, if it does,.to @at extent‘it.may occu;, ,remain uncertain. The results of,Ung and Back ‘The ~xpe~~n~i’~~r~ &as tied of Chemistry,
out’& the Depakknt McGill University, Montreal, P.Q_, Canada.
.(, ., ‘.
..:..
-’
of tile first continuum.
2. Experimental The PhotoIysis was conducted in an all glassquartz-sapphire system similar. to that described previously [2]_ The Xe resonance lamp was,excited with a microwave discharge operating at 2450 MHz and was separated from the phoiol@s ceflby a sapphke double-window set-up to’reduce the effect oFlight divergence. This ~ran~eme~t provided esseptiairy a tionochromatic radiation tit 1470 A. The reaction vessel, t0.cm’i.n length and 34 mm in-diameter, was a quartz cylinder and was comected ~0 a circulatory system comprising a glass-enclosed paddle wheel pvrnp actuated by an external magnetic field to mix and. circulate reagents and products in the system. This @rev&ted the a~~~~~a~~on of reaction products ‘- neai.the entrance window and thus Feduced the “irmei-filter” effect and the &mplicatiotis of.secon.. .&try reactions iii the iytiem. .,.
,::
,.
:
. ‘.. ._
[l] suggest that the primary process in the photoIysis of water vapcr at 1849 A which ~es~ust~~vith~ the sharply rising edge of the first continuum, is entirely due to reaction(B). It is therefore of interest to examine the p&nary processes in the water photolysis at 1470 A which lies wifiin *Jle descending edge
:
,’ ,’
‘. -.._
‘,
., ‘, ‘.‘. ‘.,-...
.., ‘. .’
-603
,Volume 28,
.:
nlimb&
CHEMICALPIZYSICSLE’I-IERS
4
:
:
.
:.
15 October 1974
.
:
-D&Bed water tias used without special purificaOXfjo3 except for vigorous degas&g and several trap-toPap, distillations undervacuum. The experimental pro: _&dure was also similar to that described before [z] _ Conderisable gases were removed at -196°C. Non.condens,able gases were transferred to an analytical ,.system by the use of a Toepler pump, and the whole $ osvple was analyzed by an absolutely @librated gas chromatography containing a 7 ft. column of 54 molecular sieve. Blank experiments showed that Hz, 5 O2 and several other simple gases couId be measured E OJ-accurately to a cprantity as little as -lo-lo mole. Ali -. experiments were carried out in a Hg-free system and at room temperature.,.
.’
ofpizotolysis
prod&
of pure
4
6
Water
16
‘2
Vapor
Prissure
20
(torr)
Fig. 1. Hydrogen yield LLT a function of water pressure. hadiation time: 15 min; temperatuG: 25°C.
,3. Results and discussion 3.1. Reqtiorr
-,
water
1 sh0n.s the effect of water.vapor pressure on the HZ yield at 25°C. It may be observed that H, yield increases with increasing pressure initiali~ and ap-
vapor
In a large number of preliminary experiments under a wide variety of conditions of radiation intensity, reaction time (7.5 to 120 min) and pressure of water vapor (3.5 to 20 torr), Hz was found to be the -only non-condensable product. No 02 could be detected in any of these experiments, although the sensitivity for its detection was at least lo-lo mole khich is several orders of magnitude lower than the .H, yields obtained in these experiments.. Condensable product was not subject to analysis due to dif-
proaches saturation at high pressures. This indicates that complete absorption of the incident radiation is achiev:d at a pressnre,of about 18 torr. Using the published value of 18.1 cm-r atm-l for the absorption coefficient at 1470 8, 131, a simple calculation sug gests that the absorption is about 99% at 18 torr, which is in very good agreement with observations. 3.3. ff:? yield as a fimction
of irradiation
time
ficulties in analyzing smah quantities of H202. However, according to the stoichiometric requirement, 2H20 G Hz + H202, HzOz must be the condensable product, and the ratio of H,/H,O, equal to unity.
In the investigation of the photolysis of water vapor at 1849 A, Ung and Back [l] obtained Similar results in the product
analysis,
and have positively
’
identified
the presence of H,O, in the condensable products. The reasons for.the absence of 02 as a major product in these two photolysis
systems
3.2. Hz yield as a function
are probably
of presstire
tion rate decreases and approaches a plateau value at long irradiation times. Since the percentage of conversion near the plateau region is only 0.25%, the
similar.
of water
vapor
In this.se&s of experiments the irradiation time was kept constant a: 15 mm, and the Hz yield was. .’ determined for-various pressures bf water vapor. Fig. ::
504
,. ,.
The pressure of water vapor was kept constant at 18 ton- throughout this series of experiments. The dependence of Hz yield on irradiation time is shown in fig. 2. As may be noted the H2 yield rises rapidly and almost linearly at short irradiation times with an initial rate of 5.8 X lo-lo mole/s, but the H2 produc-
“inner-filter’,‘.effect is not important. The dependence of Hz ;lield on irradiation time is also similar to that observed by Ung and Back in their study-of the photolysis at 1849 A. The interpretations of these kinetic
behaviors offered in ref., [l] should be equally applic-able in the present photolysis system.
:, ‘.
,,
.’ ,. :
” .:‘,,’ _::
.’
“.
,,:
‘.
0
.O
I
I
I
I
1
20
40
60
80
100
Irradiation
Time
I
120
Irradiation
(min )
Fig. 2. Hydrogen yieid a~ a function of irradiation mpor pressure: 18.0 toir; temperature: 25°C.
2.0
15 October 1974
CHEMICAL PH-YSI’CSLETl-kRS
Volume 28, number 4
time. Water
r
Calculaled
O2 Conaumplion
Fig. 4. Photclysis of water vapor with consLant amount of 02 initially added. Water vapor pressure: 16 torr; Oz added: 1.26 pmoles; temperature: 25°C. amount of 0, (1.26 X 10m6 mole) was added initially, and the amounts of Hz and O2 were measured aftervarious times of irradiation. Both series Were carried out at 25°C and at a p&-sure of water vapor of 18 torr. Fig. 3 shows that H, yields decrease rapidly when small quantities of 0, were added, but remain constant and are independent of 0, as larger amounts of 02 were added. This constant H2 yieId, ever more than a lo-fold increase of 02 is equal to about 23% of the total
H2 field
pure water vapor tions. Considering 0,
Added
Time (min 1
obtained
k the photoIysis
under identical experimental the large concentration of
of condi-
added
02 and the efficiency of the reaction
(gmole)
H+02 Fig. 3. Photolysis of water vapor with various amounts of 02 initially added.-Water vapor pressure: 18 ton; irdiation time: 15 min; temperature: 25°C.
3.4, Photolysis of water vapor in rhe presence of added 02
Two series of experiments were performed to study the photolysis of water vapor in the presen.ce of added 0,. In the first s&S (fig. 3), various amounts.of 02 were added initially, and tl-kquantities of I$ and 92 were determined-after a constant irradiation time of _. 15 min. In’the second series (fig. 4), a constant
+M-+H02
+M
(1)
H-atoms are expected to be scavenged quantitatively, thus no molecular Hz can be formed via atom recombinations H+HtM+H2+M.
(2)
Furthermore, dn account of the product formation observed both in the present investigation and in the pho tolysis at. 1849 A [ 11, secondary rehct ions involkg OH, HO2 and Hz02 would not produce significant amour@ of Hi. The constant Hz yield cbserved must be due to the direct production of molecular I& from the primary process of the photolysis, namely : .’
605 .-
-Volume 28, number ;1
CKEMCAL kYSICS LEVERS
15 diober
1974
‘.
This conclusion is strongly supported by,the results obtained in th: photolysis.at 1849 A, in which the &I2 yield is reduced to a negligible value by the addition of small q,u@ties of’C2_ The ,reduction of the initial Hz production rate in the presence of a cbnstant amount of added O2 as shown in fig. 4 also substantiates the above mterpretation. Ti-te~initialHz production rate in the presence of added 0, is approximateIy 23% of that in the absence of added 02, which is in quantitative agreement ‘with results derived from t
+M.
(4)
The overall process is W+‘02
-+H,02,
(5)
suggesting that 02 and Hz are consumed at about the same rite if the H-atoms are ultimately converted to krolecular H,. However, as [O,) increases, so does BOz], the rates of the following reactions
were.assmned to be responsible for the consutiption of molecu?ar Hz and the published value of kg [4] is accepted,?he steady-state concentration of OH required to produce the observed H reduction rate ?J _. will be 6 X 1Or3 molecules per: cm . This IShrghly improbable in view of the light intensity used for the photolysis and the lifetime of OH radicals in the. system. Kinetic analysis suggests that the Hz consumption is due primarily to the reaction of O(l Dj-atoms @D)+H2’OH+H.
(9)
This conclusion is again compatible with the results
obtained in the photolysis at 1849 A, in which the corresponding H, production rate exceeds the iktial rate when the 0, concentration is sufficiently reduced at long irradiation times, because there are no O(lD)atoms available from 1849 A photolysis to consume the molecular Ha. ?he present interpretation is also consistent with the observation of Stuhl and Welge [5] .,They found a distinct increase of the initial OH concentration in the flash photolysis of water vapor with large ~:xcess of added ,H,, which was attributed to the presence of reaction (9). The reaction of O(LD)-atoms with H202 via O(lD) + HZ02 + OH + HO,
(10)
is an unknown entity. Its presence in the system may enhance the rates of reactions involving OH and HO2 radicals, but is not expected to alter the overall mechanism significantly.
QH+H02‘+H20+02
(61
3.5. Quantum yield measurements
Ho2 f HO2 + H202 + O2
(7)
Using the photolysis of mixtures of 02 and CO as an actinometer and assuming a quantum yield for CO2 equal to 2 [2], the effective intensity of the Xe Iamp employed in the‘presdnt work was estimated to be IOl5 photons per second. The initial total Hz, production rate of 5.8 X lO-‘O mole/s (fig. 2) is equivalent .to an ‘overall H2 quantum yield of 0.35 in the photoI@ ofpure’water vapor at 14’70 A. Regarding fig. 3, the constant H2 yield of 0.12 X 10B6 mole for a reaction time of 15 mm is kquivalent to a pryductiori rate of 0.80 X 1014 molecules/s. This gibes a quulturnyield of.O.08 for the primary process whkh produces molecular H2 directly, i.e.;’ reaction (3:). These.estimates are in good agreement
will thus increase with the subsequent regeneration of .O,. This is reflected in the gradual reductions in the Gz consumption rates at hi&h 02 ccncentrations ‘(fig. 3) The variation of H2 yields with irrarliation time as .shq~ynin fig. 4 requires some comment. The observed falloff in the Hi production rate from the initial linear-dependence indicatea that molecular H2 is consumed xith a constant rate of about IO-r0 mole per second-after about 15 min of irradiation time. If rhe
:
,..
15 October 1974
CHEMICAL PHYSICS LETTERS
Volume 28, number 4
with the results derived from figs. 3 and ‘4 that the Hz yield from the primary process is about 23% of the total Hz yield obtained in the photolysis of pure water vapor. As a consequence, a Hz quantum yield of 0.27 may be assigned for the primary process which produces H and OH, namely, H,O + /zujh = 1470 A) +H + OH .
(11)
This value appears to be quite reasonable as 2 Hatoms are taken up by reaction (2) to form Hz, and only 1 H-atom is consumed through the reaction H+OH+M-+HH20+M
(12)
for the reformation of H,O. The deviation from the stoichiometric quantum yield of 0.33 can be attributed to the fact that, the rate of reaction (12) is greater than that of reaction (2). Assuming the quantum yield for the photodecomposition of water vapor equal to unity, the quantum yields at 1470 A for reactions (3) and (11) are 0.08 and 0.92, respectively. Very little quantitative measurements have been made on the quantum yields of the primary processes in the photolysis of water vapor. In a study of flash photolysis of DzO/Hz mixtures at wavelengths between 1300 and 1900 A, Black and Porter [6] concluded that reaction (3) represents less than 10% of the primary processes in the first continuum. However, there are objections to the arguments used to arrive at this conclusion [7]. Stief studied the photolysis of D,O at 1470 A [S] using ethylene as scavenger for D-atoms and concluded that in the presence of 5% and 20% of C,H,, the rate of D2 production was only 6% of that obtained in the absence of C2Hq, which is considerably lower than the present estimates. However, it should be noted that a 6% D, yield of the total D2 production is not equivalent to the estimate that molecular formation of D, accounts for 6% of the primary process as implied, be&se it takes 2 D-atoms to form one mclecule of D2 and
that not every D-atom produced in the primuy process will result in the formation of D, _On the other hand, Stuhl and Welge [S] have presented rather convincing evidence for the ooc’urrence of reaction (3) in the photolysis at I470 A, but the quantum yield was not estimated. The occurrence of reaction‘(3) in the photolysis of water vapor with a quantum yield of 0.08 at 1470 A possesses ATecial aeronomic
sign&axe
as it
indicates that molecular II2 can be formed from +Jle primary process in the first continuum. This will account for at least partly the photochemicaI formation of molecular H, in the atmosphere, especiaUy at the mesospheric levels. However, if the primary process in the first continuum were.due solely to reaction (1 l), the rapid reaction of H-atoms with atmospheric 02 would prevent a significant formation of H, from the photolysis ofrwater vapor in the atmosphere.
Acknowledgement The author wishes to express his sincere gratitude to Professor H.I. Schiff for his support and interest in this work.
References [l] A.Y.-11. Un,g and R.A. Back, Can., J. Chem. 42 (1964) 753. [2] A.Y.-M. Ung and H.I. Schilf, CUI..J. Chem. 44 (1966)
1981. [3] M. Zelikoffond K. Watanabe, J. Opt. Sot. Am. 43 (1953) 753. N.R. Greiner, J. Chem. Fhys. SL (i969) 5049. F. Stuhl and K.H. We&e, I. Chem. ,Dhp. 47 (1967) 332. C. Black and G. Porter, Proc. Roy. Sot. A266 (1962) 185. J.R. McNesby and Ii. Okabe, Advan. Photochem. 3 (1964) 157. [8] L-J. Stief, J. Chem. Phys. 44 (146.5) 277.
[4] [5] ,[6] [7]
‘.
607 .
CHE?&J CAL PHYSICSLETTERS
28, number 4
t5 October
1974
‘.
THE PHOTOLYSIS._OF WATER VAPOR AT 1470 A. H2 PRODWCTION INTHE PR%ARY PROCESS*
’
A-Y.-M,UNG Institute of Spgce nrx’dt?nospheric Studies, U~~iversity of Saskatchewan. Saskatoon, Saskatchewan S7N OJVR, Canada Received 24 June 1974 TIE primary pro&es
in the photolysis
Hz0 + izu(h = 1470 ii, --) HZ + O(‘D) ,
of water vapor at
1470 A ace.due to
Hz0 + It&‘, = 1470 A) + H + OH
with the Hz yield of the fust process accounting for 23% of the overall H2 prodxtion. ike qwntum yield of this process is estimated to be 0.08 by using 02 as a scavenger for H-atcns. Secondary re;lctioris involving the photo-
lytic pioducts and added 02 are discussed.
1. Introduction Watei vapor possesses two distinctive absorption features in the vacuum ultra~olet spectrum. In tile
spectral region 1430-1860 a, there is a continuum whose maximum lies at about 1650 A. The second continuum, 1250-1430 W, is cIearly associated with a series of broad diffuse bands. It is generally believed that the photodissociation of water vapor in the second continutim will produce a variety of primary processes, whereas the photo&is in the first continuum involves only one primary prpcess which yields H and OH. However, based on energetic considerations and the spin selection rules, the follow.’ ing primary processes are possible in the fust continuum.
H20f1AI) + hv(h G 2420 a) --f H(! 2S) + OH(X21-I) m @I
-Fe-questions of-wether.process (A) can occur in.‘the fiist coritinuum, and, if it does,.to @at extent‘it.may occu;, ,remain uncertain. The results of,Ung and Back ‘The ~xpe~~n~i’~~r~ &as tied of Chemistry,
out’& the Depakknt McGill University, Montreal, P.Q_, Canada.
.(, ., ‘.
..:..
-’
of tile first continuum.
2. Experimental The PhotoIysis was conducted in an all glassquartz-sapphire system similar. to that described previously [2]_ The Xe resonance lamp was,excited with a microwave discharge operating at 2450 MHz and was separated from the phoiol@s ceflby a sapphke double-window set-up to’reduce the effect oFlight divergence. This ~ran~eme~t provided esseptiairy a tionochromatic radiation tit 1470 A. The reaction vessel, t0.cm’i.n length and 34 mm in-diameter, was a quartz cylinder and was comected ~0 a circulatory system comprising a glass-enclosed paddle wheel pvrnp actuated by an external magnetic field to mix and. circulate reagents and products in the system. This @rev&ted the a~~~~~a~~on of reaction products ‘- neai.the entrance window and thus Feduced the “irmei-filter” effect and the &mplicatiotis of.secon.. .&try reactions iii the iytiem. .,.
,::
,.
:
. ‘.. ._
[l] suggest that the primary process in the photoIysis of water vapcr at 1849 A which ~es~ust~~vith~ the sharply rising edge of the first continuum, is entirely due to reaction(B). It is therefore of interest to examine the p&nary processes in the water photolysis at 1470 A which lies wifiin *Jle descending edge
:
,’ ,’
‘. -.._
‘,
., ‘, ‘.‘. ‘.,-...
.., ‘. .’
-603
,Volume 28,
.:
nlimb&
CHEMICALPIZYSICSLE’I-IERS
4
:
:
.
:.
15 October 1974
.
:
-D&Bed water tias used without special purificaOXfjo3 except for vigorous degas&g and several trap-toPap, distillations undervacuum. The experimental pro: _&dure was also similar to that described before [z] _ Conderisable gases were removed at -196°C. Non.condens,able gases were transferred to an analytical ,.system by the use of a Toepler pump, and the whole $ osvple was analyzed by an absolutely @librated gas chromatography containing a 7 ft. column of 54 molecular sieve. Blank experiments showed that Hz, 5 O2 and several other simple gases couId be measured E OJ-accurately to a cprantity as little as -lo-lo mole. Ali -. experiments were carried out in a Hg-free system and at room temperature.,.
.’
ofpizotolysis
prod&
of pure
4
6
Water
16
‘2
Vapor
Prissure
20
(torr)
Fig. 1. Hydrogen yield LLT a function of water pressure. hadiation time: 15 min; temperatuG: 25°C.
,3. Results and discussion 3.1. Reqtiorr
-,
water
1 sh0n.s the effect of water.vapor pressure on the HZ yield at 25°C. It may be observed that H, yield increases with increasing pressure initiali~ and ap-
vapor
In a large number of preliminary experiments under a wide variety of conditions of radiation intensity, reaction time (7.5 to 120 min) and pressure of water vapor (3.5 to 20 torr), Hz was found to be the -only non-condensable product. No 02 could be detected in any of these experiments, although the sensitivity for its detection was at least lo-lo mole khich is several orders of magnitude lower than the .H, yields obtained in these experiments.. Condensable product was not subject to analysis due to dif-
proaches saturation at high pressures. This indicates that complete absorption of the incident radiation is achiev:d at a pressnre,of about 18 torr. Using the published value of 18.1 cm-r atm-l for the absorption coefficient at 1470 8, 131, a simple calculation sug gests that the absorption is about 99% at 18 torr, which is in very good agreement with observations. 3.3. ff:? yield as a fimction
of irradiation
time
ficulties in analyzing smah quantities of H202. However, according to the stoichiometric requirement, 2H20 G Hz + H202, HzOz must be the condensable product, and the ratio of H,/H,O, equal to unity.
In the investigation of the photolysis of water vapor at 1849 A, Ung and Back [l] obtained Similar results in the product
analysis,
and have positively
’
identified
the presence of H,O, in the condensable products. The reasons for.the absence of 02 as a major product in these two photolysis
systems
3.2. Hz yield as a function
are probably
of presstire
tion rate decreases and approaches a plateau value at long irradiation times. Since the percentage of conversion near the plateau region is only 0.25%, the
similar.
of water
vapor
In this.se&s of experiments the irradiation time was kept constant a: 15 mm, and the Hz yield was. .’ determined for-various pressures bf water vapor. Fig. ::
504
,. ,.
The pressure of water vapor was kept constant at 18 ton- throughout this series of experiments. The dependence of Hz yield on irradiation time is shown in fig. 2. As may be noted the H2 yield rises rapidly and almost linearly at short irradiation times with an initial rate of 5.8 X lo-lo mole/s, but the H2 produc-
“inner-filter’,‘.effect is not important. The dependence of Hz ;lield on irradiation time is also similar to that observed by Ung and Back in their study-of the photolysis at 1849 A. The interpretations of these kinetic
behaviors offered in ref., [l] should be equally applic-able in the present photolysis system.
:, ‘.
,,
.’ ,. :
” .:‘,,’ _::
.’
“.
,,:
‘.
0
.O
I
I
I
I
1
20
40
60
80
100
Irradiation
Time
I
120
Irradiation
(min )
Fig. 2. Hydrogen yieid a~ a function of irradiation mpor pressure: 18.0 toir; temperature: 25°C.
2.0
15 October 1974
CHEMICAL PH-YSI’CSLETl-kRS
Volume 28, number 4
time. Water
r
Calculaled
O2 Conaumplion
Fig. 4. Photclysis of water vapor with consLant amount of 02 initially added. Water vapor pressure: 16 torr; Oz added: 1.26 pmoles; temperature: 25°C. amount of 0, (1.26 X 10m6 mole) was added initially, and the amounts of Hz and O2 were measured aftervarious times of irradiation. Both series Were carried out at 25°C and at a p&-sure of water vapor of 18 torr. Fig. 3 shows that H, yields decrease rapidly when small quantities of 0, were added, but remain constant and are independent of 0, as larger amounts of 02 were added. This constant H2 yieId, ever more than a lo-fold increase of 02 is equal to about 23% of the total
H2 field
pure water vapor tions. Considering 0,
Added
Time (min 1
obtained
k the photoIysis
under identical experimental the large concentration of
of condi-
added
02 and the efficiency of the reaction
(gmole)
H+02 Fig. 3. Photolysis of water vapor with various amounts of 02 initially added.-Water vapor pressure: 18 ton; irdiation time: 15 min; temperature: 25°C.
3.4, Photolysis of water vapor in rhe presence of added 02
Two series of experiments were performed to study the photolysis of water vapor in the presen.ce of added 0,. In the first s&S (fig. 3), various amounts.of 02 were added initially, and tl-kquantities of I$ and 92 were determined-after a constant irradiation time of _. 15 min. In’the second series (fig. 4), a constant
+M-+H02
+M
(1)
H-atoms are expected to be scavenged quantitatively, thus no molecular Hz can be formed via atom recombinations H+HtM+H2+M.
(2)
Furthermore, dn account of the product formation observed both in the present investigation and in the pho tolysis at. 1849 A [ 11, secondary rehct ions involkg OH, HO2 and Hz02 would not produce significant amour@ of Hi. The constant Hz yield cbserved must be due to the direct production of molecular I& from the primary process of the photolysis, namely : .’
605 .-
-Volume 28, number ;1
CKEMCAL kYSICS LEVERS
15 diober
1974
‘.
This conclusion is strongly supported by,the results obtained in th: photolysis.at 1849 A, in which the &I2 yield is reduced to a negligible value by the addition of small q,u@ties of’C2_ The ,reduction of the initial Hz production rate in the presence of a cbnstant amount of added O2 as shown in fig. 4 also substantiates the above mterpretation. Ti-te~initialHz production rate in the presence of added 0, is approximateIy 23% of that in the absence of added 02, which is in quantitative agreement ‘with results derived from t
+M.
(4)
The overall process is W+‘02
-+H,02,
(5)
suggesting that 02 and Hz are consumed at about the same rite if the H-atoms are ultimately converted to krolecular H,. However, as [O,) increases, so does BOz], the rates of the following reactions
were.assmned to be responsible for the consutiption of molecu?ar Hz and the published value of kg [4] is accepted,?he steady-state concentration of OH required to produce the observed H reduction rate ?J _. will be 6 X 1Or3 molecules per: cm . This IShrghly improbable in view of the light intensity used for the photolysis and the lifetime of OH radicals in the. system. Kinetic analysis suggests that the Hz consumption is due primarily to the reaction of O(l Dj-atoms @D)+H2’OH+H.
(9)
This conclusion is again compatible with the results
obtained in the photolysis at 1849 A, in which the corresponding H, production rate exceeds the iktial rate when the 0, concentration is sufficiently reduced at long irradiation times, because there are no O(lD)atoms available from 1849 A photolysis to consume the molecular Ha. ?he present interpretation is also consistent with the observation of Stuhl and Welge [5] .,They found a distinct increase of the initial OH concentration in the flash photolysis of water vapor with large ~:xcess of added ,H,, which was attributed to the presence of reaction (9). The reaction of O(LD)-atoms with H202 via O(lD) + HZ02 + OH + HO,
(10)
is an unknown entity. Its presence in the system may enhance the rates of reactions involving OH and HO2 radicals, but is not expected to alter the overall mechanism significantly.
QH+H02‘+H20+02
(61
3.5. Quantum yield measurements
Ho2 f HO2 + H202 + O2
(7)
Using the photolysis of mixtures of 02 and CO as an actinometer and assuming a quantum yield for CO2 equal to 2 [2], the effective intensity of the Xe Iamp employed in the‘presdnt work was estimated to be IOl5 photons per second. The initial total Hz, production rate of 5.8 X lO-‘O mole/s (fig. 2) is equivalent .to an ‘overall H2 quantum yield of 0.35 in the photoI@ ofpure’water vapor at 14’70 A. Regarding fig. 3, the constant H2 yield of 0.12 X 10B6 mole for a reaction time of 15 mm is kquivalent to a pryductiori rate of 0.80 X 1014 molecules/s. This gibes a quulturnyield of.O.08 for the primary process whkh produces molecular H2 directly, i.e.;’ reaction (3:). These.estimates are in good agreement
will thus increase with the subsequent regeneration of .O,. This is reflected in the gradual reductions in the Gz consumption rates at hi&h 02 ccncentrations ‘(fig. 3) The variation of H2 yields with irrarliation time as .shq~ynin fig. 4 requires some comment. The observed falloff in the Hi production rate from the initial linear-dependence indicatea that molecular H2 is consumed xith a constant rate of about IO-r0 mole per second-after about 15 min of irradiation time. If rhe
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15 October 1974
CHEMICAL PHYSICS LETTERS
Volume 28, number 4
with the results derived from figs. 3 and ‘4 that the Hz yield from the primary process is about 23% of the total Hz yield obtained in the photolysis of pure water vapor. As a consequence, a Hz quantum yield of 0.27 may be assigned for the primary process which produces H and OH, namely, H,O + /zujh = 1470 A) +H + OH .
(11)
This value appears to be quite reasonable as 2 Hatoms are taken up by reaction (2) to form Hz, and only 1 H-atom is consumed through the reaction H+OH+M-+HH20+M
(12)
for the reformation of H,O. The deviation from the stoichiometric quantum yield of 0.33 can be attributed to the fact that, the rate of reaction (12) is greater than that of reaction (2). Assuming the quantum yield for the photodecomposition of water vapor equal to unity, the quantum yields at 1470 A for reactions (3) and (11) are 0.08 and 0.92, respectively. Very little quantitative measurements have been made on the quantum yields of the primary processes in the photolysis of water vapor. In a study of flash photolysis of DzO/Hz mixtures at wavelengths between 1300 and 1900 A, Black and Porter [6] concluded that reaction (3) represents less than 10% of the primary processes in the first continuum. However, there are objections to the arguments used to arrive at this conclusion [7]. Stief studied the photolysis of D,O at 1470 A [S] using ethylene as scavenger for D-atoms and concluded that in the presence of 5% and 20% of C,H,, the rate of D2 production was only 6% of that obtained in the absence of C2Hq, which is considerably lower than the present estimates. However, it should be noted that a 6% D, yield of the total D2 production is not equivalent to the estimate that molecular formation of D, accounts for 6% of the primary process as implied, be&se it takes 2 D-atoms to form one mclecule of D2 and
that not every D-atom produced in the primuy process will result in the formation of D, _On the other hand, Stuhl and Welge [S] have presented rather convincing evidence for the ooc’urrence of reaction (3) in the photolysis at I470 A, but the quantum yield was not estimated. The occurrence of reaction‘(3) in the photolysis of water vapor with a quantum yield of 0.08 at 1470 A possesses ATecial aeronomic
sign&axe
as it
indicates that molecular II2 can be formed from +Jle primary process in the first continuum. This will account for at least partly the photochemicaI formation of molecular H, in the atmosphere, especiaUy at the mesospheric levels. However, if the primary process in the first continuum were.due solely to reaction (1 l), the rapid reaction of H-atoms with atmospheric 02 would prevent a significant formation of H, from the photolysis ofrwater vapor in the atmosphere.
Acknowledgement The author wishes to express his sincere gratitude to Professor H.I. Schiff for his support and interest in this work.
References [l] A.Y.-11. Un,g and R.A. Back, Can., J. Chem. 42 (1964) 753. [2] A.Y.-M. Ung and H.I. Schilf, CUI..J. Chem. 44 (1966)
1981. [3] M. Zelikoffond K. Watanabe, J. Opt. Sot. Am. 43 (1953) 753. N.R. Greiner, J. Chem. Fhys. SL (i969) 5049. F. Stuhl and K.H. We&e, I. Chem. ,Dhp. 47 (1967) 332. C. Black and G. Porter, Proc. Roy. Sot. A266 (1962) 185. J.R. McNesby and Ii. Okabe, Advan. Photochem. 3 (1964) 157. [8] L-J. Stief, J. Chem. Phys. 44 (146.5) 277.
[4] [5] ,[6] [7]
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607 .