Diode Laser Study of the Product Branching Ratio ... - ACS Publications

J. Phys. Chem. 1996, 100, 9407-9411

9407

Diode Laser Study of the Product Branching Ratio of the NH2(X 2B1) + NO2 Reaction Robert W. Quandt and John F. Hershberger* Center for Main Group Chemistry, Department of Chemistry, North Dakota State UniVersity, Fargo, North Dakota 58105 ReceiVed: February 13, 1996; In Final Form: April 4, 1996X

The reaction of NH2(X 2B1) with NO2 was studied at room temperature using time-resolved infrared diode laser spectroscopy to detect N2O, H2O, and NO products. The N2O + H2O product channel was found to have a branching ratio of 0.14 ( 0.02. Large amounts of NO were detected, but secondary sources of this species prevent a quantitative determination of the branching ratio into the H2NO + NO channel. These results are in contrast to previously published branching ratio data.

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Introduction The kinetics of the NH2(X 2B1) radical are important because of the role this species plays in a variety of combustion environments. NH2 radicals are key intermediates in NOx control strategies such as thermal de-NOx,1,2 as well as the chemistry of nitramine propellants.3 Because of its importance in the thermal de-NOx mechanism, the NH2 + NO reaction has been very extensively studied.4-14 There is now relatively good agreement on the rate constant and room temperature branching ratio, although there is still substantial uncertainty regarding the branching ratio at high temperatures. The reaction NH2(X 2B ) + NO , however, has been substantially less well charac1 2 terized. This reaction has several thermodynamically accessible channels:

NH2(X 2B1) + NO2 f N2O + H2O f H2NO + NO

∆H ) -385 kJ/mol

(1a)

∆H ) -57.7 kJ/mol (1b)

f N2 + H2O2

∆H ) -364 kJ/mol

(1c)

f N2 + 2OH

∆H ) -146 kJ/mol

(1d)

f 2HNO

∆H ) -25.1 kJ/mol

(1e)

where the thermochemical data were taken from ref 15, except for ∆H for channel 1b, which was taken from the QCISD calculations of Mebel et al.16 The total rate constant of reaction 1 has been measured by several groups.15,17-20 Most measurements at 298 K have been in the range k1 ) (2.1-2.3) × 10-11 cm3 molecule-1 s-1, although one study20 reported a substantially lower value of k1 ) 9.3 × 10-12 cm3 molecule-1 s-1. Measurements at elevated temperatures15,17,20 have all shown that this reaction has a negative temperature dependence, although there is substantial disagreement on the rate constants at high temperatures. Bulatov et al. observed no dependence of the total rate constant on pressure over the range 10-650 Torr.15 Information on active product channels in this reaction is more limited. Ab initio calculations have suggested that channels 1a and 1b are most likely.16 Hack et al. used mass spectrometry to detect N2O and H2O products.20 Although no quantitative branching ratio was quoted, they suggested that channel 1a X

Abstract published in AdVance ACS Abstracts, May 15, 1996.

S0022-3654(96)00432-7 CCC: $12.00

dominates the reaction. A recent flow reactor and kinetic modeling study of the NH3/NO2 system suggested that channel 1a is dominant at low temperatures, but that 1b becomes important at high temperatures (∼800-1300 K).21 In this work, we report a direct determination of the quantitative branching ratio into channel 1a at 298 K. NH2(X 2B ) radicals were produced by the 193 nm excimer laser 1 photolysis of ammonia. This dissociation has been well studied and represents a relatively clean source of NH2 radicals and hydrogen atoms.22,23 NH2(X 2B1) is produced with a near unity quantum yield, with insignificant amounts of NH2(A 2A1) and NH also produced. Experimental Section The experimental apparatus has been discussed in detail elsewhere,24-27 so only a brief description will be given here. The photolysis source was an excimer laser (Lambda Physik COMPex 200) operating at 193 nm. The infrared probe beam was a lead-salt diode laser (Laser Photonics) operating in the 80-110 K temperature range. Photolysis and probe beams were made collinear using a dichroic mirror and transmitted through a 146 cm single-pass absorption cell with CaF2 windows. Iris diaphragms (6 mm diameter) were placed at each end of the reaction cell in order to ensure reproducible beam overlap. After the cell, the UV light was removed with a second dichroic mirror, and the IR probe beam was focused through a 0.25 m grating monochrometer and onto a 1 mm InSb detector (∼1 µs rise time). Transient adsorption signals were averaged on a LeCroy 9310A digital oscilloscope and stored on a personal computer for further analysis. Typical experimental conditions were 0.1 Torr NH3, 0.05 Torr NO2, and 1.0 Torr SF6 buffer gas. SF6 buffer gas was chosen because it is efficient at relaxing vibrational excitation of the N2O and NO products. It is also expected to relax internal excitation of the photolytically produced NH2 radical. The reaction mixture was allowed to stand for ∼5 min in the cell in order to ensure complete mixing. Only 1-2 shots per transient signal were obtained in order to prevent depletion of reactants or buildup of products. All experiments were performed at room temperature. Since only some of the possible products are observable, the initial NH2(X 2B1) concentration must be determined in order to measure the branching ratio. To measure these concentrations, it was necessary to determine the incident photolysis laser power as well as the absorption coefficient at 193 nm. A Molectron joulemeter was used to measure the photolysis power © 1996 American Chemical Society

9408 J. Phys. Chem., Vol. 100, No. 22, 1996


Figure 1. Transient infrared absorption signals for N2O, NO, and H2O product molecules. Reaction conditions: PNO2 ) 0.05 Torr, PNH3 ) 0.10 Torr, PSF6 ) 1.0 Torr.

immediately after the second iris. These measurements were taken under the following conditions: with the evacuated reaction cell in place, with the evacuated reaction cell removed from the optical path, with the second iris opening set at 6 mm, and with the second iris set fully open at 25 mm. Due to window losses with the cell in place and the slight beam divergence of the photolysis laser, there was about a 30% spread in the energy measurements. Therefore, the average of the four measurements was taken as the best estimate of the photolysis power. The photolysis power was varied over the range 0.512.0 mJ in different experiments. NH3 and SF6 (Matheson) were purified by repeated freezepump-thaw cycles at 77 K. NO2 (Matheson) was purified at 223 K to remove NO and N2O impurities. The following transitions were used to probe NO, N2O, and H2O products:

NO (V)0) f (V)1)

R(7.5) line at 1903.133 cm-1

N2O (0000) f (0001)

R(3) line at 2227.039 cm-1

H2O (000;J,Ka,Kc)4,3,1) f (010;J,Ka,Kc)5,4,2) at 1867.853 cm-1 The HITRAN molecular absorption data base28 was used as an aid in the location and assignment of spectral lines. Results Typical transient signals for N2O, NO, and H2O product molecules are shown in Figure 1. Off-resonant signals obtained with the infrared laser detuned ∼0.02 cm-1 off of the probed absorption lines were found to be negligible. The rise at early times is due to formation of the product molecules. The rise time is slower than would be predicted by the rate constant of the title reaction. This phenomenon has been observed in our previous product yield measurements24-27 and is attributed to nascent formation of highly vibrationally excited product molecules. Since only the vibrational ground states of the various product molecules are probed, the transient rise times are primarily indicative of a convolution of vibrational relaxation rates rather than a reaction rate. The SF6 buffer gas included in the reaction mixture vibrationally relaxes the nascent products to a Boltzmann distribution on a time scale that is fast compared to removal of products by diffusion from the probed volume, which appears as a slow decay in Figure 1.

Quandt and Hershberger

Figure 2. Absorption coefficient of NH3 at 193 nm as a function of incident photolysis laser pulse energy.

In this study N2O, H2O, and NO products were detected. No attempts were made to detect H2O2 and OH, as they do not absorb infrared light in the range of available diode lasers. Absolute number densities of N2O, H2O, and NO products were calculated from the peak-to-peak transient signal amplitudes in a manner previously described, assuming Doppler line shapes and a temperature of 298 K. Pressure broadening is negligible at the pressures used, and transient heating effects due to the absorption of excimer laser light are minimized by probing low rotational states, which are relatively insensitive to temperature. For NO and N2O, tabulated line strengths from the HITRAN molecular absorption data base28 were used. The accuracy of these line strengths was verified to within ∼10% by measuring the infrared absorption of known pressures of static gas. For H2O, the infrared absorption coefficient of the probed spectral line was determined by measuring the absorption of known pressures of static H2O vapor in the reaction cell and performing a standard Beer-Lambert plot. The absorption coefficient of the H2O line at 1867.853 cm-1 was measured to be 0.207 ( 0.014 cm-1 Torr-1, where the uncertainty represents two standard deviations. To determine the product branching ratio in these experiments, the initial radical reactant concentration [NH2]0 must be known. [NH2]0 was determined from the measured 193 nm absorption coefficient and the incident photolysis laser power upon the reaction mixture. A photolysis quantum yield of unity was assumed. The adsorption coefficient (R) at 193 nm was determined by measuring transmitted excimer laser power as a function of [NH3] and making a standard Beer-Lambert plot. The absorption coefficient of NH3 was found to have a slight dependence on excimer laser pulse energy, as shown in Figure 2. This effect is attributed to bleaching of the sample at the higher laser energies due to the relatively large absorption coefficient of ammonia. In the limit of zero excimer energy, R approaches an intercept of 0.399 ( 0.014 cm-1 Torr-1, which is in good agreement with a previously reported value23 of 0.406 cm-1 Torr-1. To properly calculate [NH2]0 for each transient experiment, a value of the NH3 absorption coefficient was obtained from the best fit line in Figure 2 for the particular laser power used in each transient. Typical [NH2]0 values ranged from 1013 to 1014 molecules cm-3. If secondary reactions are neglected, the branching ratio for channel 1a can then be obtained by simply dividing the N2O yield by the [NH2]0 concentration calculated for the measured photolysis laser power. The branching ratio into the NO channel 1b is calculated in a similar fashion. Figure 3 shows the measured ratios of [N2O]/[NH2]0 as a function of photolysis laser pulse energy. Similarly, Figure 4 shows the measured

Product Branching Ratio of the NH2(X 2B1) + NO2 Reaction

J. Phys. Chem., Vol. 100, No. 22, 1996 9409 Discussion

Figure 3. Apparent branching ratio, [N2O]/[NH2]0, as a function of incident photolysis laser pulse energy. [N2O] was obtained from transient signals, while [NH2]0 was calculated from the NH3 absorption coefficient, as described in the text.

Several secondary reactions can potentially complicate these measurements. One such complication is the generation of NO from the photodissociation of NO2 at 193 nm. The magnitude of this effect on the NO transient signals was determined by measuring NO transients with NH3 precursor omitted from the reaction mixture. The NO transients obtained under these conditions are due to NO2 dissociation and possibly the O + NO2 f NO + O2 reaction. The number density of NO obtained in the absence of NH3 was subtracted from that obtained with NH3 present. This was typically a ∼10% correction. Another indication of secondary chemistry is the observation that the [N2O]/[NH2]0 is moderately dependent on the initial radical concentration. The slope of Figure 3 suggests that at high laser energies, additional NH2 removal processes other than the title reaction become significant, resulting in a lower than expected N2O yield. Possible secondary reactions that remove NH2 radicals include


NH2 + NH2 f NH + NH3 f N2H4 NH2 + NO f H2O + N2 f OH + HN2

Figure 4. Apparent branching ratio, [NO]/[NH2]0, as a function of incident photolysis laser pulse energy.

[NO]/[NH2]0 ratio. The dependence of these quantities on photolysis laser energy indicates that secondary reactions are present in this system, as will be discussed below. For H2O products, no strong energy dependence was observed, but signal/ noise considerations limited measurements to higher laser energies, where a ratio of [H2O]/[NH2]0 of 0.67 ( 0.15 was observed. To verify the accuracy of our calculated [NH2]0 values, the yield of H2O from the NH2 + NO reaction was determined. This reaction has two channels:

NH2(X 2B1) + NO f N2 + H2O f N2H + OH

∆H ) -522 kJ/mol (2a) ∆H ) -4.2 kJ/mol (2b)

Although this reaction has been the subject of controversy for many years, most recent reports indicate a branching ratio into channel 2a of 0.8-0.9 at 298 K.8-10,12,13 In our experiment, we obtained a [H2O]/[NH2]0 ratio of roughly 0.7 ( 0.2. Although this value is somewhat lower than literature values, it must be emphasized that our measurement required quite high excimer laser energies due to the weak absorption coefficients of the H2O lines obtainable with our laser diodes. Under these conditions, radical-radical reactions may affect the result. There is also substantial uncertainty in the H2O infrared absorption coefficient. In any case, this calibration suggests that our calculation of [NH2]0 is accurate to about (15%.

(3a) (3b) (2a) (2b)

Note that reaction of NH2 with ammonia precursor is slow and would result in no chemical change.29 Reaction 3a would be expected to become more important at high radical densities, but an upper limit for k3a of 3.3 × 10-15 cm3 molecule-1 s-1 at 298 K has been estimated.30 Furthermore, formation of NH radicals would actually be expected to cause an increase in the N2O yield, as our previous study of the NH + NO2 reaction has shown that N2O + OH is a significant product channel, with a branching ratio of 0.41 ( 0.15.27 The recombination reaction 3b31 is not expected to be significant at the low pressures used in these experiments. Reaction 2, however, is fast, with k ) 1.8 × 10-11 cm3 molecule-1 s-1, and is known to not produce N2O in significant yield. Several sources of NO are present in our experiment: a nearly insignificant impurity of ∼0.1% in the NO2 sample, a small amount of NO produced by photodissociation of NO2, and more substantial amounts produced by channel 1b of the title reaction, and the reaction

H + NO2 f NO + OH

(4)

where k4 ) 1.4 × 10-10 cm3 molecule-1 s-1,3 and the hydrogen atoms originate from the photolysis of the NH3 precursor. Except for the small impurity, all of these processes become more pronounced as the laser energy and therefore radical density are increased. For example, the fraction of NO2 dissociated by 193 nm light under our conditions varies from [N2O]. In fact, we observed formation of H2O in excess of N2O by a surprisingly large ratio of approximately 7 ( 2. Unfortunately, this measurement was restricted to high photolysis laser energies (5-10 mJ) because our diode laser system is not optimized for sensitive detection of water (the strongest H2O lines lie outside the wavelength range of our available laser diodes). In any case, this result demonstrates that secondary sources of H2O are present and that the measured H2O yields cannot be used to obtain reliable estimates of the contribution of reaction 1a. In addition to reaction 2a, water can be formed by secondary reactions such as

OH + NH3 f NH2 + H2O

(5)

OH + OH f H2O + O

(6)

where the OH can originate from channel 1d of the title reaction or from reaction 4. Reaction 5 is quite slow at room temperature,32-34 but reaction 6 is fairly rapid35,36 and may contribute substantially at high radical densities. As is apparent in the above discussion, secondary sources of NO products are also possible in this reaction system. NO produced from NO2 dissociation is corrected for in the analysis as decribed above. Some of the remaining NO undoubtedly originates from channel 1b of the title reaction; however, reaction 4 is fast and is expected to produce large amounts of additional NO. In fact, if all the hydrogen atoms react via reaction 4, one expects [NO]/[NH2]0 g 1.0 since an H atom is produced for every NH2 radical in the photolysis. Thus, our NO yield is surprisingly low, suggesting that alternate H atom removal routes other than reaction 4 may be present. H + NH3, H + N2O, and H + H2O are all slow at room temperature. The radical-radical reaction

H + NH2 f H2 + NH

(7)

is fairly fast, with an early estimate37 of k7 ) 4.81 × 10-12 cm3 molecule-1 s-1, but both reactants are present in insufficient concentrations in our experiments to compete effectively for H atoms with reaction 4. One possibility is that the hydrogen atom is produced translationally hot and that this translational energy enhances the rate of some of the above reactions as well as diffusive loss from the probed reaction zone. Such hot atom effects are common occurrences in photolytically produced hydrogen atoms due to momentum conservation considerations.38 This explanation is not completely satisfactory, as one would expect collisions with the buffer gas to quickly relax translationally hot atoms. It is possible, however, that SF6 is not completely inert toward hot H atoms. An early report39 of a slow but measurable rate of the H + SF6fHF + SF5 reaction over the range 1460-1700 K lends support to this hypothesis. If this reaction occurs to some extent in our system, it would represent an H atom depletion route in addition to reaction 4 and could account for the lower than expected NO yield. Kinetic modeling simulations using standard software40 were performed in order to examine the effects of the secondary reactions described above. Table 1 shows the reactions used in the calculations. The model successfully predicts that H2O is formed in substantial excess over N2O at high laser energy due to reactions 2, 5, and 6 and that the [N2O]/[NH2]0 ratio

TABLE 1: Reactions Used in Kinetic Modeling Simulations

reaction

rate (cm3 molecule-1 Torr-1)

ref

NH2 + NO2 f H2O + N2O NH2 + NO2 f NO + H2NO NH2 + NO f N2 + H2O NH2 + NO f N2H + OH NH2 + H f H2 + NH OH + NH3 f H2O + NH2 H + NO2 f OH + NO OH + OH f H2O + O

2.86 × 10-12 1.76 × 10-11 1.57 × 10-11 2.34 × 10-12 4.81 × 10-12 1.54 × 10-13 1.40 × 10-10 2.0 × 10 -12

this work, 14, 16-18 this work, 14, 16-18 5-7, 9, 10, 12 5-7, 9, 10, 12 36 33 3 34, 35

does decrease somewhat as the laser pulse energy is increased. The model includes reactions 4 and 7 as H atom removal routes, but makes no attempt to take hot atom effects into consideration. Reaction 7 accounts only for about ∼1% of the H atom removal even at the highest radical concentrations used. The model predicts more NO formation than was actually observed, as will any model that contains no additional H atom removal routes. The possible complications regarding hot atom effects described above make reliable modeling of the various sources of NO difficult. An experiment was attempted that would form NH2 radicals by an alternate route that would not form H atoms or any other radical species that could react with NO2. Dissociation of S2Cl2 at 248 nm produces chlorine atoms, which can react (rather slowly)41 with NH3 to form HCl and NH2. Unfortunately, dark reactions (probably between S2Cl2 and NH3) corrupted the reaction mixture on a time scale of seconds, preventing any reliable measurements. It is of interest to compare our results with previous measurements. Hack et al., using mass spectrometry to detect N2O and H2O, suggested that channel 1a dominates.20 No quantitative calibration was reported, however, and NO would be undetectable in their system because of a large background from NO2 electron impact fragmentation. In a recent flow reactor and kinetic modeling study, Glarborg et al. reported that channel 1a predominates at low temperatures, while the H2NO + NO channel becomes more important at high temperatures around ∼1000 K.21 Clearly, our results reported here are in disagreement with both of these studies in that we find N2O + H2O to be only a minor channel of the NH2 + NO2 reaction at room temperature. The recent results of Park and Lin,42 however, indicate a branching ratio for channel 1a of 0.19 at room temperature, in reasonable accord with our results. In a detailed ab initio study of this reaction using QCISD and Gaussian-2 methods, Mebel et al. found energetically accessible pathways to channels 1a and 1b.16 Their calculations indicate that an initially formed H2NNO2 adduct can readily isomerize via a nitro-nitrite rearrangement to form the H2NONO complex, which may dissociate with no potential energy barrier into H2NO + NO. Alternatively, H2NNO2 may undergo a pair of hydrogen atom migrations from N to O, leading to N2O + H2O products. Routes to N2 + 2OH were found to have high energy transition states, and no route to N2 + H2O2 was found. Our experimental results are consistent with these calculations, but we cannot rule out the possibility of some contribution from channels other than 1a and 1b. Conclusions The product branching ratio of the reaction NH2(X 2B1) + NO2 was measured using time-resolved infrared diode laser spectroscopy. The N2O + H2O product channel was found to have only a minor contribution to the total reaction rate, with a branching ratio of 0.14 ( 0.02 determined from the observed

Product Branching Ratio of the NH2(X 2B1) + NO2 Reaction

J. Phys. Chem., Vol. 100, No. 22, 1996 9411

N2O yield. NO products were also detected with a yield of 0.77 ( 0.1 per NH2 radical in the limit of low radical concentration, but part or all of this yield may originate from secondary chemistry. We therefore cannot at this time unambiguously identify the dominant product channel of this important reaction.

(16) Mebel, A. M.; Hsu, C.-C.; Lin, M. C.; Morokuma, K. J. Chem. Phys. 1995, 103, 5640. (17) Kurasawa, H.; Lesclaux, R. Chem. Phys. Lett. 1979, 66, 602. (18) Whyte, A. R.; Phillips, L. F. Chem. Phys. Lett. 1983, 102, 451. (19) Xiang, T.-X.; Torres, L. M.; Guillory, W. A. J. Chem. Phys. 1985, 83, 1623. (20) Hack, W.; Schacke, H.; Schroter, M.; Wagner, H. Gg. Symp. (Int.) Combust. 1979, 17, 505. (21) Glarborg, P.; Dam-Johansen, K.; Miller, J. A. Int. J. Chem. Kinet. 1995, 27, 1207. (22) Donnelly, V. M.; Baronavski, A. P.; McDonald, J. R. Chem. Phys. 1979, 43, 271. (23) Kenner, R. D.; Rohrer, F.; Stuhl, F. J. Chem. Phys. 1987, 86, 2036. (24) Cooper, W. F.; Park, J.; Hershberger, J. F. J. Phys. Chem. 1993, 97, 3283. (25) Park, J.; Hershberger, J. F. J. Chem. Phys. 1993, 99, 3488. (26) Park, J.; Hershberger, J. F. J. Phys. Chem. 1993, 97, 13647. (27) Quandt, R. W.; Hershberger, J. F. J. Phys. Chem. 1995, 99, 16939. (28) Rothman, L. S.; et al. J. Quantum Spectrosc. Radiat. Transfer 1992, 48, 469. (29) Leroy, G.; Sana, M.; Tinant, A. Can. J. Chem. 1985, 63, 1447. (30) Davidson, D. F.; Dohse-Hoeinghaus, K.; Chang, A. Y.; Hanson, R. K. Int. J. Chem. Kinet. 1990, 22, 513. (31) Khe, P. V.; Soulignac, J. C.; Lesclaux, R. J. Phys. Chem. 1977, 81, 210. (32) Silver, J. A.; Kolb, C. E. Chem. Phys. Lett. 1980, 75, 191. (33) Stephens, R. D. J. Phys. Chem. 1984, 88, 3308. (34) Diau, E. W.-G.; Tso, T.-L.; Lee, Y.-P. J. Phys. Chem. 1980, 94, 5261. (35) Trainor, D. W.; Rosenberg, C. W., Jr. J. Chem. Phys. 1974, 61, 1010. (36) Wagner, G.; Zellner, R. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 1122. (37) Boyd, A. W.; Willis, C.; Miller, O. A. Can. J. Chem. 1971, 49, 2283. (38) Flynn, G. W.; Weston, R. E., Jr. Annu. ReV. Phys. Chem. 1986, 37, 551. (39) Fenimore, C. P.; Jones, G. W. Combust. Flame 1964, 8, 231. (40) Braun, W.; Herron, J. T.; Kahaner, K. Int. J. Chem. Kinet. 1988, 20, 51. (41) Westenberg, A. A.; DeHaas, N. J. Chem. Phys. 1977, 67, 2388. (42) Park, J.; Lin, M. C., in press.

Acknowledgment. The authors would like to thank M. C. Lin of Emory University for valuable discussions and for making a manuscript available prior to publication. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences of the Department of Energy, Grant DE-FG0693ER14390. Acknowldgement is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work.


References and Notes (1) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287. (2) Lyon, R. K. EnViron. Sci. Technol. 1987, 21, 231. (3) Tsang, W.; Herron, J. T. J. Phys. Chem. Ref. Data 1991, 20, 609. (4) Roose, T. R.; Hanson, R. K.; Kruger, C. H. Symp. (Int.) Combust. Proc. 1981, 18, 853. (5) Andresen, P.; Jacobs, A.; Kleinermanns, C.; Wolfrum, J. Symp. (Int.) Combust. Proc. 1982, 19, 11. (6) Silver, J. A.; Kolb, C. E. J. Phys. Chem. 1982, 86, 3240. (7) Dreier, T.; Wolfrum, J. Symp. (Int.) Combust. Proc. 1984, 20, 695. (8) Hall, J. L.; Zeitz, D.; Stephens, J. W.; Kasper, J. V. V.; Glass, G. P.; Curl, R. F.; Tittel, F. K. J. Phys. Chem. 1986, 90, 2501. (9) Silver, J. A.; Kolb, C. E. J. Phys. Chem. 1987, 91, 3714. (10) Atakan, B.; Jacobs, A.; Wahl, M.; Weller, R.; Wolfrum, J. Chem. Phys. Lett. 1989, 155, 609. (11) Pagsberg, P.; Sztuba, B.; Ratajczak, E.; Sillesen, A. Acta Chem. Scand. 1991, 45, 329. (12) Stephens, J. W.; Morter, C. L.; Farhat, S. K.; Glass, G. P.; Curl, R. F. J. Phys. Chem. 1993, 97, 8944. (13) Wolf, M.; Yang, D. L.; Durant, J. L. J. Photchem. Photobiol. A: Chem. 1994, 80, 85. (14) Park, J.; Lin, M. C. J. Phys. Chem. 1996, 100, 3317. (15) Bulatov, V. P.; Ioffe, A. A.; Lozovsky, V. A.; Sarkisov, O. M. Chem. Phys. Lett. 1989, 159, 171.

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Diode Laser Study of the Product Branching Ratio ... - ACS Publications

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