J. Phys. Chem. 1991,95, 1651-1654
1651
Recoil Tritium Reaction in Solid Hydrogen at Ultralow Temperatures Y oshiteru Fujitani, Tetsuo Miyazaki,* Kenji Fueki, Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa-ku. Nagoya 464-01, Japan
Nobuyuki M. Masaki, Yasuyuki Aratono, Masakatsu Saeki, and Enzo Tachikawa Department of Chemistry, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1 1 , Japan (Received: July 6, 1990; In Final Form: September 7 , 1990)
Reactions of recoil T atoms were studied in solid H2-D2 mixtures at 4.2 K. The fraction of HT in the sum of HT and DT yields, [HT]/( [HT] + [DT]), is roughly equal to the mole fraction of H2 in the H2-D, mixtures. These yields were compared with the yields of H atoms produced by y-radiolysis of the H2-D2 mixtures at 4.2 K. From the comparison, it was concluded that formation of HT could not be due to combination of thermalized T atoms with hydrogen atoms from y-radiolysis of solid hydrogen during reactor irradiatiion but must be due to hydrogen atom abstraction by recoil T atoms from H2 and D2.In the hydrogen atom abstraction by thermal T atoms by quantum mechanical tunneling, a large isotope effect, Le., a large value of k(T + H,)/k(T + D2), is expected at 4.2 K, while k(T + H,)/k(T + D2) is about 1 .O in the hot T atom reaction. From the difference in isotope effects between the hot atom reaction and the thermal atom reaction it was concluded that more than 90% of recoil T atoms react with hydrogen by a hot atom reaction in solid hydrogen at 4.2 K before completing their thermalization.
Introduction Reaction of tritium with high energies in the solid phase is an interesting problem related to solid-state chemical kinetics as well as tritium behavior in nuclear fusion materials. The reaction of recoil tritium in solid hydrogen has been studied here for the following reasons. First, the tritium reaction in solid hydrogen can be considered as a prototype solid-state reaction that can be discussed unequivocally. Second, the reactions of recoil T atoms with H, (D2, HD) in the gas phase have been studied previously by many investigators,’ and the results in the solid phase can be compared with those in the gas phase. Third, the reactions of H (D) atoms, produced by radiolysis and photolysis, in solid hydrogen have been studied extensively by Miyazaki et a1.,2*3and the results of recoil T atoms in solid hydrogen can be compared with those of H (D) atoms. The possible elementary processes of recoil T atoms in solid hydrogen are a reaction of hot T atoms with H2 (D2, HD), moderation of hot T atoms, and a tunneling reaction of thermalized T atoms with H 2 (D2, HD). The reaction and moderation of hot H atoms, produced by the photolysis of HI, in solid D2 have been studied previously at 4.2 K.3 Miyazaki et aL2 and Shevtsov et aL4 have studied independently the tunneling reactions of thermalized H (D) atoms with H2 (D2, HD) at ultralow temperatures. The experimental evidence for tunneling reactions obtained by them has stimulated theoretical calculations by Sato et aL5 and by Truhlar et aL6 Recoil T atoms are produced by the irradiation of 6Li with thermal neutrons from a nuclear reactor. The neutron irradiation for the study of T atom reactions in solid hydrogen (melting point 14 K) are done at 4.2 K. Facilities for neutron irradiation at ultralow temperatures are limited only to a few reactors in the world. The ultralow-temperature irradiation facility of the KUR reactor at Kyoto University is operated above ca. 15 K.’ Recently technical staffs of the Japan Atomic Energy Research Institute at Tokai-mura (Ibaraki) have constructed a new neutron-irradiation port in the JRR-4 reactor of their Institute.* This irradiation facility has two characteristics: (1) Since the facility has a large space for the irradiation, we can put a large cryostat in the irradiation field. (2) Since. the intensity of neutrons relative to the intensity of y-rays is relatively high, the influence of yirradiation upon the recoil tritium reaction is minimized. In this work we have made a special cryostat, which can be cooled by liquid helium, for neutron irradiation at 4.2 K and succeeded for
* Author to whom correspondence should be addressed. 0022-3654/9 1/2095- 165 1$02.50/0
the first time in the study of recoil tritium reactions at 4.2 K. Experimental Section 6Li-enriched LiF was prepared from metallic 6Li by the following method. A piece of metallic 6Li, washed with carbon tetrachloride, is allowed to react with hydrochloric acid, forming LiC1. Then, LiCl is dissolved in hydrofluoric acid. When the acid solution is dried up, LiF is obtained as residue. The LiF residue is dissolved again into hydrofluoric acid, and the solution is dried up. Pure LiF was obtained by repeating this process. LiF formed was identified by X-ray diffraction. The 6Li/(7Li + 6Li) ratio in LiF, measured by mass spectroscopy, was 0.95. The purities of the H2, D2, and HD gases were greater than 99.999,99.5, and 8.4 X 98 mol %, respectively. The hydrogen (1.4 X or 1.4 X IO-’ mol) and 6LiF (0.0075 g) were sealed into quartz cells (6.3-cm height; 0.22-cm inner diameter; 0.23-cm3 volume) for neutron irradiation. To prevent Hz-D2 exchange when sealing off the quartz cell, the sealing part is a thin capillary tube (0.02-0.04-cm inner diameter) and the other part of the cell is immersed in liquid nitrogen during the sealing off of the cell. The LiF powder was packed at the bottom of the reaction cell. The cell was cooled at first at 77 K and then was put in the steel cryostat (95-cm height; 37-cm diameter), filled with liquid helium. (1) Malcolme-Lawes, D. J. Hor Atom Chemistry; Matsuura, T., Ed.; Elsevier Science Publishers and Kodansha: Tokyo, 1984; p 39, and related papers cited therein. (2) (a) Tsuruta, H.; Miyazaki, T.; Fueki, K.; Azuma, N. J . Phys. Chem. 1983,87,5422. (b) Miyazaki, T.; Lee, K.; Fueki, K.; Takeuchi, A. fbid. 1984, 88, 4959. (c) Miyazaki, T. Bull. Chem. SOC.Jpn. 1985, 58, 2413. (d) Miyazaki, T.; Lee, K. J . Phys. Chem. 1986,90,400. (e) Lee, K.; Miyazaki, T.; Fueki, K.; Gotoh, K. Ibid. 1987,91, 180. (f) Miyazaki, T.; Iwata, N.; Lee, K.; Fueki, K. Ibid. 1989,93, 3352. (9) Miyazaki, T.; Iwata, N.; Fueki, K.; Hase, H. Ibid. 1990, 94, 1702. (3) Miyazaki, T.; Tsuruta, H.; Fueki, K. J . Phys. Chem. 1983,87, 1611. (4) (a) Ivliev, A. V.; Katunin, I. I.; Lukashevich, V. V.; Skylylarevskii, V. V.; Suraev, V. V.; Filippov, V. V.; Filippov, N. I.; Shevtsov, V. A. Pis’ma Zh. Eksp. Teor. Fiz. 1982,36,391. (b) Ivliev, A. V.; Iskovskikh,A. S.;Katunin, A. Ya.; Lukashevich, I. I.; Sklyarevskii, V. V.; Suraev, V. V.; Filippov, V. V.; Filippov, N. I.; Shevtsov, V. A. Pis’ma Zh. Eksp. Teor. Fir. 1983, 38, 317. (c) Ivliev, A. V.; Katunin, A. Ya.; Lukashevich, I. I.; Sklyarevskii, V. V.; Suraev, V. V.; Filippov, V. V.; Filippov, N. I.; Shevtsov, V. A. Zh. Eksp. Teor. Fiz. 1985, 89, 2197. ( 5 ) (a) Takayanagi, T.; Masaki, N.; Nakamura, K.; Okamoto, M.; Sato, S.; Schatz, G. C. J. Chem. Phys. 1987,86, 6133. (b) Takayanagi, T.; Nakamura, K.; Sato, S. Ibid. 1989, 90, 1641. (6) Hancock, G. C.; Mead, C. A.; Truhlar, D. G.;Varandas, A. J. C. J . Chem. Phys. 1989, 91, 3492. (7) Shibata, T.; Iwata, S.; Yoshida, H.; Nakagawa, M.; Okada, M. Annu. Rep. Res. React. Inst. Kyoto Uniu. 1969, 2, 89. (8) Shida, S., private communication.
0 1991 American Chemical Society
1652 The Journal of Physical Chemistry, Vol. 95, No. 4, 1991
Hydrogen is condensed at the surface of the LiF powder, where the thickness of the 1.4 X 10-5-molhydrogen deposit was estimated as ca. 0.01 cm. The recoil T atoms produced in the LiF powder by the 6Li (n, a ) T reaction enter solid hydrogen to produce HT and DT. To examine the effect of the amount of hydrogen, three different amounts were sealed into the cells. The relative yields of total tritiated products (HT DT) in 1.4 X 8.4 X and 1.4 X mol of solid hydrogen are 1.4, 1.0, and 0.7, respectively. Though the amount of 1.4 X mol of hydrogen is 17 times larger than that of 8.4 X mol, the yield of the tritiated products in the 1.4 X mol of hydrogen is only 1.4 times as large as that in the 8.4 X mol of hydrogen. Thus, most of the recoil T atoms injected into the solid hydrogen may react inside the solid of 1.4 X mol. As shown later in Figure 2, the fraction ([HT]/([HT] [DT])) of HT in the total products in the H2-D2 (50 mol %) mixture of 1.4 X mol is similar to that in the 8.4 X 10-7-mol mixture. Therefore, the amount of 1.4 X mol of hydrogen was used for the study of tritium reactions in solid hydrogen. The cells in the cryostat was irradiated in the special neutron-irradiated field of the JRR-4 reactor. The thermal neutron flux and the dose rate of y-rays at the irradiation port were 1.7 X IO9 n cm-2 s-l and 1.1 C kg-l h-l (4.3 X IO3 rad h-I), respectively. The irradiation time was 7 h. The ratio of thermal neutron flux to the dose rate of y-rays at the special irradiation field of the JRR-4 reactor is about 6 times larger than the ratio in the ultralow-temperature irradiation facility of the KUR reactor. After neutron irradiation at 4.2 K, the samples were warmed to room temperature for the analysis of HT and DT. The tritiated products were analyzed by means of radiogas chromatography (1.2-m-long activated alumina column containing 10 wt % manganese chloride) at 77 K. The irradiated quartz cell was put in an injection apparatus of the radiogas chromatograph, where tritiated products can be transferred by crushing the cell to the analytical column. Figure 1 shows a radiogas chromatogram of products by tritium reactions in a H2-D2 (20 mol %) mixture at 4.2 K. H T and DT peaks are well resolved.
Fujitani et al. T*
4
T*
Results and Discussion Isotope Effect on Recoil T Atom Reaction in Solid Hydrogen at 4.2 K . Figure 2 shows fractions (depicted by open circles for 1.4 X I O-s mol of hydrogen; closed triangles for 8.4 X of hydrogen; closed circles for 1.4 X I 0-7 mol of hydrogen) of H T yields in total yields of H T and DT, produced by recoil tritium reaction in the solid H2-D2 mixtures at 4.2 K. The fraction of the HT yield, produced by recoil tritium reaction in solid H D at 4.2 K, is shown by an open triangle. Fractions of H atoms yields in the total yields of H and D atoms, produced by y-radiolysis of H2-D2 mixtures at 4.2 K, were studied previously by ESR spectroscopy2a-cand are also shown by squares. The initial energy of the recoil T atoms produced by the 6Li(n, a ) T reaction is 2.7 MeV. Most of them are trapped in solid LiF, while a part of them comes out of LiF and enters solid hydrogen or the walls of the quartz cell. The ratio of tritium recovered in solid hydrogen relative to that produced by the (n, a) reaction was about 2%. There is the possibility that tritiated products may be released from the walls of the cell or the LiF powder on warming the irradiated cells to room temperature. Though DT is produced only by reactions in solid deuterium, a part of HT may be released from the walls or LiF. The fractions ([HT]/ ([ HT] + [DT])) of H T yields in pure solid D2 of 1.4 X 8.4 X lo-', and 1.4 X mol are 0.050, 0.053, and 0.13, respectively (cf. Figure 2).9 Thus, the contribution of the wall (or LiF) reaction to HT yields is small in the solid hydrogen above 8.4 X mol. The possible reactions of recoil T atoms in the H2-D2 mixtures at 4.2 K can be represented as follows: 6Li + n
-
T*
+a
(1)
(9) Since D2contains ca. 0.4%HD as an impurity, a part of HT may be produced also by an abstraction from H D by thermal T atoms in solid D2.
+
T
+ D2
+ H2 T + D2
T
H2
T
HT
+H
+ Hz* 4
T
+
+
+ Hz
DT
(3)
+D
+ D2*
-
(2)
(4) (5)
+H DT + D
(7)
(Dz) --* H (D)
(8)
+ H (D)
HT
H T (DT)
(6)
(9)
Hot T atoms (T*), produced by reaction 1, react with H2 or D2 (reactions 2 and 4). A part of the hot T atoms is thermalized by collisions with H2and D2 (reactions 3 and 5). The thermalized T atoms can react with H2 or D2 by quantum mechanical tunneling even at 4.2 K (reactions 6 and 7).1° In fact the tunneling reactions D HD D2 H has been observed clearly at 4.2 and 1.9 K, where the rate constant for this reaction at 4.2 K is 2 X cm3 mol-' s - I , ~ ~ . ~ Since samples in a nuclear reactor are irradiated by y-rays as well as by neutrons, H and D atoms are produced by y-radiolysis of solid hydrogen (reaction 8). Combination reactions of thermalized T atoms with these H and D atoms may take place during warming the irradiated sample to room temperature (reaction 9) for analysis of the products. To discuss the possibility of the combination reactions of thermal T atoms with H and D atoms (reaction 9), amounts of H and D atoms trapped in the y-irradiated solid hydrogen were determined by ESR spectroscopy. As shown by squares in Figure 2, H atoms are produced very selectively in the H2-D2 mixtures at 4.2 K through quantum mechanical tunneling reactions.h*c The number of H and D atoms produced by the radiation is greater than the number of T atoms from the (n, a) reaction, and the trapped H and D atoms have relatively long half-life (>20 h) in the solid hydrogen.2f If the combination reaction between thermal T atoms and H (D) atoms could take place during warming the reactor irradiated sample to room temperature (reaction 9 ) , H T would be produced selectively relative to DT because of the preferential formation of H atoms by the radiolysis. The fraction of HT in the total yields of HT and DT, however, is roughly equal to the mole fraction of H2 in the H2-D2 mixtures (cf. Figure 2). Thus, the combination of thermal T atoms with H (D) atoms produced by radiolysis plays a minor role in the H T formation. HT and DT are formed mainly by hydrogen atom abstraction by T atoms from H2 and D2. Next we will discuss the relative probability of hydrogen atom abstraction by hot and thermal T atoms. Recently the isotope effect on tunneling abstraction reactions of thermal T atoms with H2 and D2 (reactions 6 and 7 ) has been calculated theoretically as 7.5 X los at 4.2 K (cf. Table I).Io A very large isotope effect has been observed experimentally in tunneling reactions of thermal D atoms at 4.2 K.2aqc The selective formation of H atoms in the radiolysis of the H2-D2 mixtures at 4.2 K, shown by squares in Figure 1 , is caused by the tunneling reaction of D atoms with H2, and the isotope effect for k(D + H2)/k(H + D2) has been estimated as > 3 X IO4 (cf. Table I).2a,c The isotope effects on reactions of recoil hot T atoms with H2 and Dz in the gas phase have been reported as 1.15" and 1.0l2respectively (cf. Table I), which are much smaller than the values expected for the tunneling abstraction reaction of thermal T atoms. We will estimate the hot product yield v) of recoil tritium reaction in solid hydrogen as a function of the [H2J/([H2] + [DZ]) ratio by use of the difference in isotope effects between the hot atom reaction and the thermal atom reaction. The hot atom
+
-
+
(IO) Takayanagi, T.; Sato, S.; Tsunashima, S.Proceedings of 32nd Japanese Conference on Radiation Chemistry; Hiroshima, Oct 1989; p 37. ( 1 1 ) Seewald, D.; Gersh, M.; Wolfgang, R. J. Chem. Phys. 1966.45, 3870. (12) Malcome-Lawes, D. J . J . Chem. SOC.,Faraday Trans. I 1983, 79, 2735.
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 1653
Recoil Tritium Reaction in Solid Hydrogen TABLE I: Isotope Effect on T-H2 (T-D,) Reactions
condition k(T + H 2 ) / W + D2) solid H2 (D2) at 4.2 K -1.0 H 2 (D2) at 4.2 K 7.5 x 105 gaseous H 2 (D2) at r m m temperature 1.15,” 1.0l2 > 3 x 104 solid H 2 (D2) at 4.2 K
reactive species recoil T thermal T recoil hot T thermal D (H)
’The value was obtained by assuming k(T
I
remarks exptl value in this work‘ calcd value quoted from ref IO exptl value quoted from refs 1 1 and 12 exptl value for k(D + H2)/k(H + D2) quoted from refs 2a.c
+ H2)/k(T + D2) = [HT][D,]/[DT][H,]. ([HT]/[DT]),h, by the thermal atom reactions is given by eq 11. Then, the fraction ([HT]/([HT] [DT])) of HT yields in total yields is given by eqs 12 and 13. [HT] and [DT] in the left-hand
+
HT
lHTl
’$ 300-
2 2 200>
. .
.-
c
W
60 80 100 Retention time I min Figure 1. Radiogas chromatogram of H T and DT produced by recoil T atom reaction in the H2-D2 (20 mol 7%) mixture at 4.2 K.
Figure 2. Fractions of H T yields produced by recoil T atom reaction and those of H atom yields produced by y-radiolysis in the solid H2-D2 A, a) H T yields in the H2-D2 mixtures of 1.4 mixtures at 4.2 K. (0, X mol (0),8.4 X IO-’ mol (A),and 1.4 X lo-’ mol (a);(A)H T yields in solid HD; (0)H atom yields in the radiolysis of the H2-D2 mixtures quoted from refs 2a,c. Curves are H T yields calculated for hot product yields of I .O (-); 0.9 (- - -); 0.8 (- -); and 0.7 (. -) (see text).
-
reactions in the H2-D2 mixtures are represented by reactions 2 and 4
T*
+ D2-
(kD)h
DT
+D
(4) where (kH)h and (kD)h are relative probabilities of reactions 2 and 4, respectively. The ratio of HT yield to DT yield by the hot atom reactions, ([HT]/[DT])h, is given by eq 10 where m is the ratio
of H2 concentration to D2 concentration. Similarly, the thermal atom reactions are represented by reactions 6 and 7, and the ratio, T
+ Ht-HT(kdh
+H
(6)
(7)
side of eq 12 represent the yields formed by both hot and thermal reactions. If (kH/kD)h and (kH/kD)th are known, [HT]/( [HT] + [DT]) ratios can be estimated by eq 13 as a function off and m. Since it seems that hot atom reactions are not affected by phase and temperature, (kH/kD)h for hot T atoms at 4.2 K is taken here as 1.0 (cf. Table I). (kH/kD)th for tunneling reactions of thermal T atoms at 4.2 K is taken here as 200.13 If (kH/kD)rh is varied from 200 to 3 X lo4 (cf. Table I), [HT]/([HT] + [DT]) ratios for (kH/kD)th = 3 x 104 coincide with those for (kH/kD)th = 200 within a difference of 5%. In Figure 2 are shown [HT]/([HT] + [DT]) ratios estimated by eq 13 with the assumed hot product yields u> as 1.0,0.9,0,8, and 0.7. The estimated HT yields for f = 0.9 and 1.O agree roughly with the experimental HT yields. Therefore it is concluded that more than 90% of recoil T atoms react with the hydrogen by hot atom reaction in solid hydrogen at 4.2 K before completing their thermalization. It was reported that the hot product yield of recoil tritium reaction in gaseous H2 and D2 is 0.7-0).8.14 The hot product yield (>0.9)obtained in solid hydrogen is higher than the value in the gas phase. Among the possible explanations for the present high hot yield in the solid phase, the following two could be the most probable. First, the probability of thermalization relative to hot reaction should be less in the solid than in the gas phase since in the gas phase the energy of the atom can be removed in a series of glancing collisions, whereas in the solid the molecules are tightly packed, resulting in the high hot yield in the solid. Second, a “hot zone” must be produced toward the end of the trajectory of hot atoms in a solid,I5 and this must be populated by H and D atoms with which any thermalized T must react with equal probability, giving a result indistinguishable from an abstraction hot reaction. A similar model has been proposed previously for explaining the reaction of recoil T atoms in neopentaneI6 and decane1’ in the solid phase. Acknowledgment. This work was done in part as the Collab(13) The ratio of rate constants (k(D + H2)/(k(H + D2)) in H2;D2 mixtures depends upon the concentration of H2, probably due to a diffusioncontrolled The ratio of the H2 ( 5 mol %)-D2 (95 mol %) mixture is ca. 200,while the ratio of the H2 (>2S mol %)-D, ( 3 X lo4. Thus, (kH/kD)thfor thermal T atoms was changed here from 200 to 3 x 104. (14) Malcolme-Lawes, D.J.; Oldham, G.; Ziadeh, Y. Z. J. Chem. Soc., Faraday Trans. 1 1982, 78, 961. (IS) Mozumder, A. Advances in Radiation Chemistry; Burton, Magee, J. L. Eds.; Wiley-Interscience: New York, 1969;Vol. 1; p 1. (16) Aratono, Y.;Tachikawa, E.; Miyazaki, T.; Sakurai, M.; Fueki, K. Bull. Chem. SOC.Jpn. 1981, 54, 1627. (17) (a) Aratono, T.;Tachikawa, E.; Miyazaki, T.; Kawai, Y.;Fueki, K. J. Phys. Chem. 1982, 86, 248. (b) Aratono, Y.; Tachikawa, E.; Miyazaki, T.; Fueki, K. Bull. Chem. Soc. Jpn. 1982,55, 1957.
J. Phys. Chem. 1991, 95, 1654-1658
1654
oration Program between the Japan Atomic Energy Research Institute (JAERI) and Nagoya University. The tritium analysis was performed at the Isotope Center of Nagoya University. This work was supported in part by the Grant-in-Aid for Scientific
Research from the Japanese Ministry of Education, Science, and Culture. We thank Mr. S. Shida and Mr. S. Ichimura of JAERI for their assistance in the neutron irradiation. We thank Mr. C. Sagawa of JAERI for his preparation of 6LiF.
A Kinetic Study of the Recombination Reaction Na
+ SO, + Ar
Youchun Shi and Paul Marshall* Department of Chemistry, University of North Texas, P.O. Box 5068, Denton, Texas 76203 (Received: July 17, 1990; In Final Form: September 25, 1990)
The recombination of atomic sodium, Na(32S), with SO2has been investigated at 787 K, in a bath of Ar at pressures from I .7 to 80 kPa. Na was generated by pulsed excimer laser photolysis of NaI vapor at 308 nm and monitored by time-resolved resonance absorption of the D lines at 589 nm under pseudo-first-order conditions. The measured pseudo bimolecular rate constants lie in the third-order and falloff regions and were fitted well by using either a Lindemann mechanism or an empirical cm6molecuk2 RRKM expression. The parameters are discussed in the text. The low-pressurelimit is about (2.4-2.7) X s-l. By use of RRKM theory, combined with a lower limit based on the absence of any observed equilibration, the Na-S02 cm3molecule-’ bond energy is estimated as 190 f 15 kJ mol-I. Extrapolation to the high-pressurelimit yields k , = (1-3) X s-I, in accord with a harpoon model for electron transfer to form an ionic adduct.
1. Introduction There have been several recent investigations of recombination reactions of sodium atoms, for example with 0214and OH.5 In all these studies the measurements have been made in the lowpressure third-order regime, and it has been assumed that the high-pressure limit is gas kine ti^.^^^ The purpose of the study presented here is to check this assumption experimentally. The reaction Na + SO2 (+Ar) N a S 0 2 (+Ar) (1)
-
is selected because molecular beam experiments demonstrated that alkali metals form long-lived collision complexes with S02,6and the N a S 0 2 adduct is known as an ion-pair species from matrix isolation studies.’ This adduct has also been the subject of an a b initio investigation* and is a dominant product in lean flames containing sodium and sulfur? The greater number of vibrational degrees of freedom in this adduct, as compared to e.g. NaOz, is expected to lower the falloff pressure region to an experimentally accessible range. Here we present measurements on reaction 1 in the third-order and falloff regions and extrapolate the data to the high-pressure limit. A second area of interest with this class of reaction is the bond energy of the adduct formed. We quantify this in two ways, by fitting the third-order rate constant to an RRKM expression and by setting a lower limit from the absence of observed equilibration.
2. Experimental Technique The concentration of atomic sodium, Na(32S), is monitored by time-resolved atomic resonance absorption spectroscopy following its generation by pulsed UV photolysis of sodium iodide ( I ) Marshall, P.; Narayan, A. S.; Fontijn, A. J . Phys. Chem. 1990, 94, 2998. (2) Husain, D.; Marshall, P.; Plane, J. M. C. J . Chem. SOC.,Faraday Trans. 2 1985, 81, 301. (3) Silver, J. A.; Zahniser, M. S.; Stanton, A. C.; Kolb, C. E. In Proceedings of the 20th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1984; p 605. (4) Plane, J. M. C.; Rajasekhar, B. J . Phys. Chem. 1989,93, 3135. ( 5 ) Husain, D.;Plane, J. M. C.; Xiang, C. C. J . Chem. Soc., Faraday Trans. 2 1984, 80, 1619. (6) Ham, D. 0.;Kinsey, J. L. J . Chem. Phys. 1968, 48, 939. (7) Milligan, D. E.;Jacox, M.E. J . Chem. Phys. 1971, 55, 1003. (8) Ramondo, F.; Bencivenni, L. Mol. Phys. 1989, 67, 707. (9) Steinberg, M.; Schofield, K. Prog. Energy Combust. Sci. 1990, 16, 31 1.
0022-3654/91/2095-1654$02.50/0
vapor in the presence of a large excess of SO2. The main components of a new experimental system are now described. High-Temperature Reactor. The reactor is based on a six-way stainless steel cross, and a schematic diagram of the apparatus is shown in Figure 1 (two unused side arms are not shown). The intersection of the three cylinders forms a roughly cubic region, about 2 cm on a side. Each side arm is 11 cm long with a 2.2-cm i.d. The inner 7 cm of each side arm is wrapped in nichrome resistance heating wire, electrically insulated with ceramic beads. The reactor is housed in a thermally insulating box, 20 cm on a side, made of 2.5-cm-thick alumina boards (Zircar Products ZAL-50). The terminal 1.5 cm of each side arm outside the insulation is water-cooled, and connections to the end of each side arm are by standard IS0 NW25 K F fittings. The intersection region of the side arms defines the reaction zone, where transient species are generated and detected. A combustion boat containing solid NaI is placed in a heated side arm of the reactor. NaI vapor is entrained by the gas flow and swept through the reaction zone. An Omega C N 3910 KC/S temperature controller monitors the reactor temperature with a sheathed type K (chromel/alumel) thermocouple inside the insulating box and operates a solid-state relay to control the current through the heating wire (maximum power 650 W at 1 15 V). This arrangement provides a temperature stability of about f l K, from rmm temperature up to about 900 K. A second sheathed type K thermocouple can be slid to the center of the reaction zone to monitor the gas temperature, which is displayed on an Omega DP 285 readout. A separate moveable radiation shield was employed in preliminary experiments, to test for possible radiation errors in the thermocouple measurements. We found that, for the conditions used in this work, such errors are less than 2 K and are therefore neglected. Optical System. A Questek 21 10 XeCl excimer laser provides pulses of radiation (308 nm, fwhm duration 15 ns) for photolysis of the NaI vapor. The UV beam is diverged slightly with a concave mirror before entering the reactor through a Vycor window. The maximum intensity incident on the window is about 10 mJ cm-2. The actinic intensity, and thus the initial Na concentration [Na],,, is varied by changing the excimer gas fill and by placing neutral density filters in the UV beam. [Na] in the reaction zone is monitored by resonance absorption of the C W radiation from a hollow cathode lamp (the unresolved doublet at 589.0 and 589.6 nm, Na(32S) Na(32P3,2,,,2)), approximately focused into a parallel beam perpendicular to the photolysis ra-
-
0 1991 American Chemical Society
