J . Phys. Chem. 1990, 94, 7200-7205
7200
have significantly different characteristics than most other solid polymer reactions that have been previously described. The sensitivity of these data to chemical structure suggests that this may be a fruitful system for the study of polymer physics and chemistry.
Acknowledgment. This work could not have been carried out without the supportive atmosphere provided by Bob Gleason, Geraint Owen, John Moll, and Len Cutler. I am also grateful to Dave Ylitalo and Professor Curt Frank for assistance in obtaining the fluorescence data.
Excitation-Transfer Reactions from N,(A3E,+) and CO(a3n) to OH S. J. Wategaonkar and D. W. Setser* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: April 12, 1990)
Efficient excitation transfer from N2(A) and CO(a) to OH has been observed in a room-temperature flow reactor. The excitation-transfer rate constants for O H ( A 2 P ) formation are (9.5 & 1.9) X lo-" and (4.0 & 2.0) X IO-" cm3 molecule-' s-I for N2(A) and CO(a), respectively. These values suggest that excitation transfer makes the dominant contribution to the total quenching of N2(A) and CO(a) by OH. The OH(A) molecules are formed with high rotational energy. Preliminary experiments show that excitation transfer from N2(A) to CH,O occurs,but the rate constant is smaller than for OH. Quenching of N2(A) by SH and SF gave no SH(A-X) or SF(A-X) emission. The excitation mechanism and the potential surfaces for the OH(A) excitation are qualitatively discussed.
Introduction
Excitation-transfer reactions between N2(A3Z,+) and other diatomic molecules provide an excellent way to study such energy-transfer events at the quantum-state level by analysis of the fluorescence of the product molecule. Some examples are the reactions with N0,I-j SO: S2: CS? and IF.6 For favorable cases, it may even be possible to utilize the energy storage properties of metastable N2(A3Z,+) to develop short-wavelength lasers. Unfortunately, the number of stable diatomic molecules that are good energy acceptors is limited, and attention is being turned to molecules that must be generated in situ. In the present work we have examined the reaction with O H radical by observing the OH(A-X) emission intensity. N2(A3Z,+)
+ OH(X2n)
-
-
N2(X) + OH(A2Z+); AHo = -2.15 eV ( l a )
N 2 + 0 + H; AHo = -1.78 eV
-
(1b)
N 2 0 + H; AHo = -3.45 eV
An interesting aspect of this system is the possibility of reactive quenching, as well as excitation transfer. Reaction 1b could represent formation of OH(A) above the predissociation limit, as well as other processes. An expected difference for N2(A) + radicals, relative to N2(A) + closed-shell molecules, is more attractive potentials for the N2(A) + R entrance channels. The potentials needed to describe reaction 1 correspond to excited-state potentials of the extensively studied reaction of H atom with N,0.7-9 The exit channel for ( l a ) is relevant to the potential ( I ) Piper, L. G.; Cowles, L. M.; Rawlins, W. T . J . Chem. Phys. 1986, 85, 3369. (2) (a) Thomas, J. M.; Kaufman, F.; Golde, M. F. J. Chem. Phys. 1987, 86, 6885. (b) Bott, J. F.; Heidner, R. F.; Holloway, J. S.; Koffend, J. B. Aerospace report #ATR-89(8438)-1, (3) (a) Clark, W. G.; Setser, D. W. J. Phys. Chem. 1980, 84, 2225. (b) Callear, A. B.; Wood, P. M. Trans. Faraday SOC.1971, 67, 272. (4) Cao, D. 2.; Setser, D. W. J . Phys. Chem. 1988, 92, 1169. (5) Richards, D. S.; Setser, D. W . Chem. Phys. Lett. 1987, 136, 215. (6) Piper. L. G.; Marinelli, W. J.; Rawlins, W. T.; Green, B. D Chem. Phys. 1985.83, 5602. (7) Hoffmann, G.; Oh, D.; Wittig, C. J . Chem. SOC.,Faraday Trans. 2 1989, 85, 1141. (8) Ohoyama, H.; Takayanagi, M.; Nishiya, T.; Hanazaki, 1. Chem. Phys. Letf. 1989. 162. I .
0022-3654/90/2094-7200$02.50/0
needed to describe quenching of OH(A) by N2.10 The rate constant for formation of OH(A) was conveniently observed by comparing the OH(A-X) emission intensity to the NO(A-X) emission intensity; the H NO2 reaction was used to generate equal concentrations of O H and N O in a flow reactor containing N,(A). Since the rate constant for NO(A) excitation is well-e~tablished'-~(kNo. = (6.6 f 0.8) X 10-I' cm3 molecule-' S K I ) ,the rate constant for O H can be obtained from the ratio of the steady-state emission intensities according to (2). The [NO] will be equal to [OH], except for long reaction time for which some O H may be removed by the disproportionation reaction or reaction with excess H. I(OH(A)) [NO1 kOH* = kNo* (2) I(NO(A)) [OH] Since the rate constant for (la) was found to be large, similar experiments were done with the metastable CO(a311) molecule. c o ( a 3 n ) + OH(XQ) c o ( x ) OH(A~Z+); AHo = -1.99 eV (3a) C O + 0 + H; AHo = -1.62 eV (3b)
+
--
-
+
C 0 2 + H; AHo = -7.07 eV (3c) HCO + 0; AHo = -2.25 eV (3d) Since the excitation energies of CO(a) and N2(A) are nearly equal, the excess energies for (la) and (3a) are similar. The rate constant for (3a) was measured vs that for excitation of NO(A+B) by CO(a). The excitation-transfer reaction between CO(a) and N O is not so well-characterized as for N2(A) + NO, and the current view is that only 15% of the quenching proceeds by excitation transfer.13 The excited-state potentials for reaction 3a have a ground-state counterpart in the extremely important and well' ~ quenching characterized O H + CO reaction ~ y s t e m . ' ~ - The (9) Marshall, P.; Fontijn, A.; Melins, C. J . Chem. Phys. 1987, 86, 5540. (10) (a) Copeland, R. A.; Crosley, D. R. J . Chem. Phys. 1986,84,3099. (b) Copeland, R. A.; Dyer, M. J.; Crosley, D. R. J . Chem. Phys. 1985, 82, 4022. ( I I ) Wategaonkar, S. J.; Setser, D. W. J . Chem. Phys. 1989, 90, 251. (12) (a) Imine, A. M. L.; Smith, 1. W.; Tuckett, R. P. J . Chem. Phys., in press. (b) Sauder, D. G.; Dagdigian, P. J. J. Chem. Phys. 1990, 92, 2389. (13) (a) Taylor, G.W.; Setser, D. W. J. Chem. Phys. 1973,58,4840. (b) Taylor, G. W.; Setser, D. W. Chem. Phys. Left. 1971, 8, 51. (c) Balamuta, J.; Golde, M. F. J. Chem. Phys. 1982, 76, 2430. (14) Brunning, J.; Wyn Derbyshire, D.; Smith, I. W. M.; Williams, M. D. J. Chem. SOC.,Faraday Trans. 2 1988, 84, 105.
0 1990 American Chemical Society
Excitation Transfer from N2(A) and CO(a) to O H
The Journal of Physical Chemistry, Vol. 94, No. 18. 1990 7201
of OH(A) by CO1’ also should be considered when trying to develop a model for the exit channel of (3a). Searches also were made for the excitation-transfer reactions between N2(A) and C H 3 0 , SH, and SF. These radicals were generated by F atom reactions with C H 3 0 H , HzS, and GO;, respectively. Weak emission was found from the CH30(A-X) transition, which is analogous to the OH(A-X) transition. However, no emissions from SH(A) or SF(A) were observed, probably because the excited molecules were formed above their predissociation limits, which are 4.0 and -3.5 eV, respectively. The OH(A) predissociation limit is 4.74 eV, which suggests that good acceptor states for Nz(A) should have dissociation limits above -4.7 eV. This is consistent with the Franck-Condon favored release of energy (-4.5 eV) from Nz(A,o’=O) to N2(X,o”=6,7). However, the very efficient excitation of IF(A;I .93 eV), which also has a low dissociation energy, provides a reminder that exceptions do exist.6 Although the excitation of OH(A) by N2(A) and CO(a) is efficient, the OH(A-X) transition is not a good laser candidate because of a propensity for facile electronic quenching.1°J7 Reactions la and 3a could be important excitation mechanisms for OH(A) chemiluminescence in the oxidation of certain fuels. Experimental Methods Experiments were done in a 4-cm-diameter, 60-cm-long Pyrex flow reactor. The reactor was pumped with a Roots blower backed by a mechanical pump. The maximum flow velocity was 115 m s-’ for 0.9-Torr pressure of Ar. However, most of the work was done under throttled conditions at a velocity of 35 m s-I and 3.0-Torr pressure. The entrance end of the reactor was fitted with an aluminum flange, which supported an H or F atom inlet, an inlet for the Ar buffer gas, and a movable reagent inlet. The H atoms were produced by passing H2 mixed with Ar through a microwave discharge (2450 MHz). The O H radicals were produced by adding NO2 to the H / H 2 flow. H
+ NO2
+
OH(o13)
+ NO(oS2)
(4)
The reaction time between H and NOz could be varied by moving the reagent (NO,) inlet. The window for observing the interaction of Nz(A) with the radicals present in the main flow reactor was 20 cm downstream of the H atom inlet and was fitted with a quartz flat. The N2(A) and CO(a) metastable molecules (the radiative lifetimes are -2 s and 7.5 ms, respectively) were generated in a side reactor from the reactions of N2 or C 0 2 with Ar(3Po,2) metastable atoms, which were produced by passing an Ar flow ( ~ 1 5 %of the total Ar flow) through a low-power dc discharge located in a 2.5-cm-diameter tube. The N 2 or C 0 2 was added to the Ar(3Po,2)flow immediately after the discharge and before this gas flow entered the main reactor. Sufficient N2 or C 0 2 was added to completely remove the Ar(3Po,2)atoms before the flow entered the main reactor; the delay time from the N2 (or C 0 2 ) inlet to the entrance to the main reactor was = I O ms. The N2(A3Z,+) and CO(a3n) flow was injected into the main reactor via a 9-mm-diameter inlet tube that terminated 2 cm in front of the observation window. The CO(a-X) emission could be observed directly; however, the N2(A+X) emission was too weak to be recorded for these operating conditions. The NO, NOz, and other reagents were purified by the freeze-pumpthaw method several times and stored as 5% mixtures in Ar. The N,, CO,, and H2 were taken directly from the commercial cylinders. The At carrier gas was purified by passing it through 5-A molecular sieve traps cooled to -77 OC. The reagent flows were metered by capillary oil flow meters, and the Ar flow rates were measured by floating ball type flow meters. All flow meters were calibrated so that the flow rates could be (IS) Radhakrishnan, G.; Buelow, S.;Wittig, C. J . Chem. Phys. 1986,84, 727. (16) Shutz, G.C.; Fitzcharles, M. S.; Harding, L. B. Faraday Discuss Chem. SOC.1987.84, 359. (17) Crosely, D. R. J . Phys. Chem. 1989, 93, 6273.
TABLE I: NAA) Exoeriments“ - . . Enerev-Transfer -_ At,b ms
2.25
0.27d 0.86 2.25
4.29
[NO] [H2IC [NO2] NO(ul:uo) OH(u,:uo) IoH*/INo*c 0.40 0.65 0.55 0.90 0.48 6.60 6.60 0.76 0.76 0.77 0.77 0.77 0.76 0.40 0.40 0.40
0.06 0.35 0.41 1.90 1.56 1.80 0.86 0.42 1.08 0.90 0.70
0.14 0.13 0.13 0.13 0.13 0.19 0.18 0.19 0.17 0.14 0.18 0.15 0.10 0.14 0.1 1 0.14
0.34 0.34 0.23 0.27 0.22 0.28 0.23 0.23 0.24 0.18 0.24
1.69 1.68 1.52 1.54 1.31 1.28 1.36 1.58 1.07 (1.23) 1.10 (1.26) 1.16 (1.33)
‘Concentrations in 10I3 molecules cm-]. is the reaction time between H and NOz. [HI = [H2] based on 50% dissociation of H,; ref I I . dThe pressure for the Af = 0.27 ms experiments was I Torr; all other experiments were done at 3 Torr. eThe ratio of the integrated emission intensities; the numbers in parentheses for the last three entries have been corrected for loss of OH(X) (see text). converted to partial pressures of the components in the reactor. The spectra were collected by a 0.5-m monochromator (Minuteman) with a 1200 groove mm-’ grating blazed at 300 nm and a Hammamatsu R212UH photomultiplier tube (PMT). The monochromator and the PMT response curve was calibrated by using standard halogen and D2 lamps. The calibration was further checked by comparison with the NO(A2Z++X2n) band intensities, since these branching ratios are known.l8 A few experiments were done with SH, SF, and C H 3 0 radicals. These radicals were generated in the main reactor by the F atom reactions with H2S, COS, and CH30H, respectively. The F atoms were produced by passing a 33% mixture of CF, in Ar through the microwave discharge. Experimental Results Characterization of the N , ( A ) and CO(a) Sources. Since the direct observation of N2(A-+X) bands was not possible in this particular reactor, the N,(A) source was characterized by monitoring the NO(A-X) emission. The energy-transfer rate constants to N O from N2(A,o’) and the NO(A) vibrational distributions have been studied by several Clark and S e t ~ e r ~ ~ reported a NO(A) vibrational distribution of uo:ul = 88:12 for a N2(A) source with u I / u o == 0.6. Table I lists the experiments with NO, and the average of five experiments gives for NO(A) uo:ol = 88:12, which indicates that our N2(A) vibrational distribution was u I / u o = 0.6 with negligible u2. This conclusion is consistent with the NO(A,u’) distributions reported by Piper et al.’ for the isolated N,(A,u’=O) and N2(A,u’=I) reactions. The effective excitation-transfer rate constant for a u I / u o= 0.6 mixture was taken as 6.6 X IO-” cm3 molecule-’ s-l from consideration of the results in refs 1 and 2a. The Ar(3Po,2) C 0 2 reaction was used as a source of CO(a,o’);I3 however, the branching fraction for formation of CO(a311) is approximately 0.2,13cand the [CO(a)] was smaller than the [N2(A)]. At a flow velocity of 115 m s-I and 0.9-Torr pressure, the CO(a-X) emission could be observed ( T ~ ~=(7.5 ~ )ms)I9 and was used to monitor the [CO(a,u’)]. For a slit width of 0.5 mm, the (1,3) and (1,4) bands could be separated from the (0,2) and (0,3) bands. The integrated band intensities, after correction for the detector response and the Franck-Condon factors, gave a uo:uI ratio of 100:7. The emission from CO(a,u’=2) could not be detected. This vibrational distribution is consistent with earlier reports13 for the Ar(3Pp,2)+ C 0 2 reaction. To further characterize the CO(a) source, a few experiments were done by adding N O to the reactor. Wider slits were used
+
Piper, L. G.; Cowels, L. M. J . Chem. Phys. 1986, 85, 2419. (19) Schofield, K. J . Phys. Chem. Ref, Data 1979, 8, 723.
(18)
1202
Wategaonkar and Setser
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990
than for the N2(A) experiments to compensate for the lower concentration of the CO(a) molecules. An intrinsic problem with the CO(a) experiments is that the reactions of Ar(3Po,z)and CO(a) with the N, impurity in Ar generate some N,(A). In the absence of added C 0 2 , the NO(A-X) emission arises from the residual N2(A) generated by the reaction of the N 2 impurity with Ar(3Po.2). When COz was added to the Ar metastable atom flow, the NO(A-X) intensity increased by a factor of 2 and NO(B-X) also was observed. After correcting for the NO(A-X) emission in the background spectrum, the NO(A) to NO(B) ratio was 2. This ratio for NO(A) to NO(B) is consistent with the results of Taylor and Setser,” who made efforts to remove the N, impurity in a CO(a) flow reactor. Thus, subtraction of the background emission, Le., the signal without CO,, should remove the contribution from N2(A) and should give results that are due to [CO(a)]. The vibrational distributions of the NO(A or B) from excitation by CO(a) could not be determined due to the poor signal-to-noise ratio and overlapping bands; however, earlier work showed that the distributions were concentrated in u’ = 0 for both states.I3 The fractional dissociation of H2 by a microwave discharge has been reported to be 88% for a flow velocity of 110 m s-l and 0.9-Torr pressure.” For our conditions the dissociation was conservatively taken to be 4 0 % . The N2(A,u’=O) quenching rate constant for H2 is small (viz., 3.8 X cm3 s-1),20,21 but that for H is large (5.1 X IO-” cm3 We confirmed that the H atom quenching rate constant was indeed large by adding N O to monitor [N2(A)] and observing a large reduction in [N,(A)] when the microwave discharge in H 2 was activated. Most of the experiments were done with an excess of NO2 over H to avoid loss of N2(A) from quenching by H atoms. The N,(A) quenching rate constant by NO2 seems not to have been reported. The addition of NO2 to the reactor did not significantly reduce the and [N2(A)]for NO2 concentrations of 51 X IOl3 molecules a reaction time of 0.5 ms. Thus, the rate constant must be C2.0 X cm3 s-l. For the experiments reported here, the quenching of N2(A) by NO2 was not important. N , ( A ) + O H ( X )Energy-Transfer Experiments. The H + NO2 reaction produces equal amounts of O H and NO. For most experiments the reaction time, At, between H and NO2 was chosen such that both O H and NO were relaxed to their ut‘ = 0 levels. Thus, the measurements are for the excitation rate constant of OH(o”=O) relative to NO(u”=O). A few experiments were carried out for a short H NO2 reaction time in an attempt to observe the effect of vibrational excitation in OH(X) or NO(X) on the vibrational distributions in OH(A) and NO(A). Unfortunately, experiments with short H + NO2 reaction time required larger concentrations of H atoms to observe the OH(A) and NO(A) signals. Since H atoms cause rapid vibrational relaxation of OH, these experiments did not give any greater vibrational excitation in OH(X). Figure I shows the NO(A-X) and OH(A-X) emission spectra from reaction with N2(A). At a resolution of 0.4 nm, the OH(0,O) and ( 1 , I ) bands were not resolved. Although the (1,O) band of OH(A) was overlapped by the (0,5)and (1,6) bands of NO(A), the individual band areas were deconvoluted as follows. The (1,O) and (0,l) bands of N O were used to calculateI8 the uI:uo ratio, and this ratio was used to find the relative areas of the other NO(A-X) bands. The areas of the (0,5) and (l,6) bands of NO(A) were subtracted from the sum of the (0,5) and (1,6) band of N O plus the (1,O) band of OH(A) to obtain the relative area for the (1,O) band of OH(A). This value was scaled to obtain the relative area of the ( 1 , l ) band, which was subtracted from the combined (0.0) and ( 1 , l ) band area to obtain the OH(A) vibrational distribution. The total OH(A) excitation rate constant was obtained by comparing the ratio of the integrated band areas from OH(A) and NO(A) according to eq 2. Since both species have short lifetimes, 693 and 215 ns for OH(A) and NO(A),
+
(20) Hack, W.: Kurzke, H.; Ottinger. Ch.; Wagner, H. Gg. Chem. Phys. 1988, 126, I 1 1. (21) Golde. M . F. Inr. J . Chem. Kiner. 1988, 20, 75. (22) Hovis, F. E.: Whitefield, P. D. Chem. Phys. Lerr. 1987, 138, 162.
NO(A+X) 1
v-ro r V”=O
200
I 1
225
V’. 0 1
1
1
I
1
1
2
3
4
5
i 2,
1 3
V’. 4
250
1
1
1
I 6
5
275
300
325
Wavelength (nm)
Figure 1. Emission spectrum from the reaction of N,(A) with equal concentrations of OH and NO. The OH and NO were generated from the H NO, reaction; [HI, = 7.7 X IO1, and [NO,], = 1.6 X 10I2 molecules ~ m - ~The . spectrum has not been corrected for the response
+
of the detection system.
respectively, electronic quenching need not be considered under our low-concentration conditions. Table I lists the results obtained from four reaction times with different [HI, and [N0210. The NO(A) vibrational distribution was invariant for change of operating conditions and uI:uo = 0.15 f 0.03. For the longest reaction time, 4.3 ms, there is a 13% loss in total [OH(X)] due to reactions which remove OH, mainly the disproportionation reacti0n.2~ If the observed [OH(A)]/[NO(A)] ratios for the 4.3-111s reaction time (last three entries in Table I) are scaled by this amount, they nearly agree with the ratio for the other reaction times. The average of all the observations gave [OH(A)]/[NO(A)] = 1.44 f 0.17. Based on the N O excitation-transfer rate constant1 of (6.6 f 0.8) X cm3 s-I, an OH(A) excitation-transfer rate constant of (9.5 f 1.9) X IO-” cm3 s-l was obtained via eq 2. An uncertainty of 20% was assigned based upon the combined uncertainty in the OH(A)/NO(A) intensity ratio and the value of kNO* for our operating conditions, Le., the uncertainty in the N2(A) vibrational distribution. The reaction time, A f , was varied from 0.27 to 4.3 ms in an attempt to change the vibrational distributions of OH(X) and NO(X). However, neither the observed OH(A,ul:uo) and NO(A,u,:uo) distribution nor the [OH(A)]/[NO(A)] ratio changed appreciably with At. The slight apparent increase in OH(u,:uo) from 0.23 to 0.34, as the reaction time decreased from 4.3 to 0.27 ms, is associated with the reduction in Ar pressure; see below. The vibrational excitation provided by the H + NO2 reaction is not very high even for the nascent distributions (Po-P4 = 44:3 1:19:06 for OH” and Po-P, = 89:lO:Ol for N0I2). A kinetic simulation was done to estimate the OH(X) vibrational distribution using the known (H + NO2) reaction rate constant” and the nascent vibrational distributions for OH” and NO,I2 as well as the quenching rate constants23of OH(u> 1) by H, NO, and NO2. The calculation indicated that more than 80% of the OH(u) had relaxed to uo even for the most favorable reaction time and concentrations. (23) (a) Spencer, J. E.; Glass, G. P. Chem. Phys. 1976, IS, 35. (b) Smith, I . W . M.; Williams, M. D.J. Chem. SOC.,Faraday Trans. 2 1985.81, 1849.
Excitation Transfer from N2(A) and CO(a) to O H
The Journal of Physical Chemistry, Vol. 94, No. 18, I990
7203
TABLE 11: CO(r) Energy-Transfer Experiments'
At, ms 1.0
Ion:!
[NO] [H2]* [NO2] NO(A/B) OH(ol:oo) INo 0.95 2.30 0.51 2.00 0.15 1.54 1.22 1.81 0.18 1.18 1.54 0.51
@Allmeasurements were at I Torr; a 1.0-ms reaction time was used to maximize the concentration of CO(a). Concentrations are in 10" molecules ~ m - ~b[H] . = [H2],based on 50% dissociation of H2. CThe ratio of the OH(A-X) and NO(A,B-X) emission intensities.
The NO(v) relaxation was not included in the simulation, since
vo is ~ 9 0 % of the total NO(X) formed in reaction 3, and the N O was taken to be in vo for all reaction times. The vI:uo ratio for OH(A) was nearly constant over the entire range of operating conditions, and the average was 0.25 f 0.05. The apparent increase in the vI/uo ratio for the 0.27-ms experiments, estimated by using the deconvolution procedure described above, probably was due to the changes in the band shapes (and in turn band areas) caused by the less extensive rotational relaxation at I-Torr Ar pressure used in these experiments. The simulated OH(0,0+1,1) band with ul/uo = 0.25 fitted the observed spectrum very well, which supports this explanation. CO(a) + OH(* Energy-Transfer Experiments. The CO(a) experiments were carried out at 0.9-Torr pressure with the highest pumping speed to minimize CO(a) quenching by CO, (kQ = 1.4 X 10-" cm3 s-l) and radiative decay. The NO2 inlet was adjusted so as to allow maximum time ( A f = 1 .O ms) for the H NO2 reaction. Simulation of the kinetics for OH(X,u) indicated more than 90% of the OH(X) was in the u = 0 level. The NO(A-+X) and OH(A-+X) intensities from this reaction were about an order of magnitude smaller than the N2(A) experiments for the same [NO] and [OH]. Table I1 lists the results from the CO(a) experiments. The apparent OH(A) to NO(A+B) ratio was 1.8 f 0.3, and the OH(ul:oo) ratio was 0.17 for excitation by CO(a), which is somewhat lower than for N2(A). However, these results must be corrected for contributions from N2[A]. The extent of the [N2(A)] impurity was estimated by adding N O to the reactor and observing the N O emissions with and without the added COz. The I N o ( A ) observed without the CO, was multiplied by [koH(~)/kNo(~)] to account for OH(A) from N,(A). After applying the correction, the [OH(A)] to [NO(A+B)] ratio from excitation by CO(a) became 1.2 f 0.2. We expect that this method of adjustment for the residual N2(A) contribution overestimates the relative importance of N2(A). The CO(a) quenching rate constant13J9for N O has been reported as (2.0 f 0.3) X cm3 s-l, but only = I 5% of the quenching corresponds to excitation of NO(A and B) states. For kNo. = 0.3 X 1O-Io cm3 s-l the excitation-transfer rate constant for OH(A) formation is (4 f 2) X IO-'' cm3 s-l. This rate constant is somewhat uncertain because kNO. is not well-established and because of the residual [N,(A)] in the reaction system. An alternative estimate of the rate constant was obtained by comparing the OH(A+X) steady-state intensity with that of CO(a-+X). The two can be related as follows:
+
IOH'
= kOH*[OHl[CO(a)l
= ~oHo[OH]ICO*/TCO-'
(5)
The [OH] was taken as equal to [NO,] since the reaction goes nearly to completion for the conditions used here. The excitation-transfer rate constant, koH., was determined as (3.1 f 0.2) X IO-'' cm3 s-I for T~~ = 7.5 ms, using the relative IOH. and Icoo intensities. This value is within the error limits of the result determined from comparison with the NO(A) formation rate constant. This rate constant is sufficiently large that (3a) must be an important component to quenching of CO(a) by OH. Further study of reaction 3 should involve measuring the total quenching rate constant and comparison of koH. to the total rate constant. O H ( A ) Rotational Distribution. The (0,O) and ( I , 1) bands of OH(A) were not resolved, but the overall emission extends to 320 nm (see Figure 2), suggesting high rotational excitation. A
305
315 Wavelength,
325 nm
Figure 2. Comparison of the simulated and experimental 0-0 and 1-1 emission bands of the OH(A-X) transition from N2(A) + OH at 1 Torr of Ar. The inset shows the overall distribution for u' = 0 and u' = I , which consisted of a 500 K Boltzmann component and a Gaussian component located at J = 15.5 for u' = 0 and J = 12.5 for u' = I . The spectrum was taken at 0.4-nm resolution.
simulation of the OH(A-X) spectrum from the 0-0 and 1-1 bands was done to assign the rotational distribution; the calculation included the band-pass of the monochromator and the line strengths of the rotational transitions. The OH(A,vl:uo) ratio was held constant at 0.25, and the rotational distribution was varied to produce a fit to the experimental spectrum obtained at 1 Torr. The distribution was described by a Boltzmann component and a Gaussian component that was centered at selected high J levels. The Gaussian distribution were centered at J = 1 and J = 121/2with widths of 5000 and 3000 cm-' foro'= 0 and o'= 1, respectively. The highest J levels were those permitted by the predissociation for the respective o f states. The best fit to the I-Torr spectrum was with a 2:l mixture of components with the Gaussian being the larger. The distributions that were used for v' = 0 and u ' = 1 are shown in'the inset of Figure 2. The best Boltzmann temperature was 500 K for the I-Torr spectrum, and this temperature decreased to approximately 300 K for the spectra at 3 Torr. Steady-state rotational distributions with a minimum at intermediate J levels are characteristic of high rotational-state distributions that have undergone partial rotational relaxation. The rotational relaxation rate constants become smaller as J increases, which leads to a Boltzmann component for low J levels and a second component at high J levels after a few collisions with the buffer gas.]' The 500 K Boltzmann assignment should not be taken too seriously, although a lower temperature was required to fit the low Jcomponent for the spectrum at 3 Torr. At 1 Torr the OH(A) molecules experience about five collisions with Ar before decaying radiatively. The two-component distribution obtained from the simulated results indicates that the initial rotational distribution for o f = 1 must have been approximately the distribution with a maximum at J 15.5. The spectrum from OH(A,u'=O) molecules formed by excitation transfer with CO(a) had a similar rotational distribution. Energy Transfer to SF, S H , and CH,O. These experiments were carried out only with N2(A) because of its higher concentration. The F C H 3 0 H reaction gives about equal amounts of C H 3 0 and C H 2 0 H radicals.24a Weak emission from the CH,O(A-X) transition which extended up to 400 nm could be observed when C H 3 0 H was added to the flow of F atoms; bands originating from vj' = 0, 1, 2 could be assigned. The intensity was distributed over a long progression in ui', the CH,-O stretch mode. Quantitative measurements of the CH30(A-X) intensity could not be made due to the weak signal and several overlapping background atomic lines from the Ar metastable discharge. However, the intensity was =lo% of that from OH(A) for the
+
(24) (a) Khatoon, T.; Hoyermann, K. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 669. (b) Schofield, K. J . Phys. Chem. ReJ Data 1973, 2, 2 5 .
7204 The Journal of Physical Chemistry, Vol. 94, No. 18, 1990
same [N2(A)] and [OH] vs [CH30]. Attempts to observe the SH(A-X) emission were made using the F + H2Sas the source reaction. The experimental conditions including the CF4 and H2S concentrations, the reaction time between F and H2S, and the pumping speed were varied over a broad range without observing the SH(A-X) emission. However, the S2(B-+X)emission was observed for high F atom concentrations and long F + H2S reaction times. The excitation of S2(B) from N2(A) + S2(X) has been characterized: and the observation of the S2(B-X) emission confirms that SH must have been present, since S2(X)arises from secondary reactions of the S H radicals.24b Thus, we conclude that the N2(A) + S H reaction does not produce SH(A) efficiently. Similar experiments were done with the F + COS reaction system, which quantitatively yields S F + C0.25 No emission could be found that could be attributed to the SF(A-X) transition.26 For long reaction times the S2(B+X) emission again was observed; the S2(X) is produced by secondary reactions of the S F radicals.25
Discussion The excitation rate constants for OH(A,u”=O,I) measured in this work are for 300 K Boltzmann rotational and vibrational distributions of the OH(X) and CO(a) reactants, but the u,’/ud ratio for N2(A) was -0.6. Based upon analogy with other N2(A) and CO(a) systems with large rate constant^,^.'^ it is unlikely that the rate constants for ( l a ) or (3a) depend strongly upon the CO(a,u’) or N2(A,u’) level. Since the k o ~ ( Avalues ) were measured relative to the rate constants of NO, the reliability depend upon the k N ~ ( Avalues. ) Recent work2bsuggests that the k N ~ ( Avalue ) of ref 1 should be increased by 25%. If true, our k o ~ (from ~) N2(A) also should be increased. The k o H ( A ) value from CO(a) is less certain because of the complication associated with the N2(A) impurity and the poorly documented CO(a) + N O reaction. We were unable to study the effect of vibrational energy of OH(X) or NO(X) upon the excitation-transfer reactions. The OH(A) molecule is not ideal for such a study, because u i is largely predissociated and because the ul’ population from a 300 K sample of OH(X) is already larger than the predicted Franck-Condon distribution, which is ud:uI’:ui= 100:8:
