J . Phys. Chem. 1986, 90, 3346-3353
3346
Quenching Processes in Electronically Exclted SO, Generated by Chemical Reactiont Robert J. Glinski* Department of Chemistry, Tennessee Technological University, Cookeville, Tennessee 38505
and David A. Dixon* E . I . DuPont de Nemours & Co., Central Research and Development Department, Experimental Station, Wilmington, Delaware I9898 (Received: September 27, 1985)
Luminescence from SO2was studied as the molecule was formed in the steady-state reactions of ozone with methyl mercaptan and with dimethyl disulfide. The bulk of the electronically excited SO2 was generated in the elementary reaction SO + 03.A chemical timing method was used to analyze the quenching of SO2*. Effects of increasing noble gas pressure on the fluorescenceand phosphorescence were observed. By use of a presently accepted photophysical model for SO2,the quenching of the fluorescence allowed the spectroscopically unresolvable long-lived and short-lived fluorescent singlet states to be distinguished. The 3B1state that yields the phosphorescence was not observed to be created in the elementary reaction of SO with 03.It was also difficult to account for the formation of the 3B1state by a collisionally induced intersystem crossing from one of the singlet states. Instead, the participation of an unspecified intermediate state producing the 3Bl state on collision is proposed. The photophysicaleffect and the participation of the unspecified state are suggested to be similar to those discussed by other workers.
introduction Review of SO2Spectroscopy and Photophysics. Sulfur dioxide has continued to be of great interest because of the complexity and the as yet unresolved questions that this triatomic molecule presents. The subject of the spectroscopy and photophysics, with all its nuances, has been reviewed in detail by Heicklen, Kelly, and Partymiller.' Lee and Loper2 have also presented an excellent overview regarding electronic relaxation in SO2. Only the details of the spectroscopy and photolysis of SO2 from the very many experimental and theoretical studies relevant to this work are summarized here. Five excited states of SO2 are predicted to lie below 35 000 crn-l. Spectroscopic techniques have yielded definitive data on the energies of only three of these state^.^-^ The first weak absorption is into the 3B1state which lies at 25766 cm-'! The singlet absorption feature commences at the origin of the weak 'A2 IAl transition lying near 27 930 crn-I?v5 The exact location of the origin of the allowed singlet absorption corresponding to the 'BI 'Al transition is as yet unassigned but is suggested to lie in the area of 31 OOO ~ m - ' .Above ~ ~ this energy the spectroscopy and photophysics of SO2become especially complex due to strong interactions of the excited states with each other and with the high vibrational levels of the ground state.'.2 The existence of two other triplet states has been inferred for gaseous SO2 from indirect evidence6 and theory.' The 3A2state is predicted to lie very near the 3Bl energy based on its vibronic interaction with the 3B, state6 and spectroscopic studies of crystalline SO2 below 20 K.* The +-
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t DuPont contribution no. 3874. (1) Heicklen, J.; Kelly, N.; Partymiller, K . Reu. Chem. Intermed. 1980, 3, 315-404. (2) Lee, E. K. C.; Loper, G. L. In Radiationless Transitions; Lin, S. H., Ed.; Academic: New York, 1980; p 21. (3) (a) Brand, J. C. D.; Hardwick, J. L.; Humphrey, D. R.; Hamada, Y.; Merer, A. J. Can. J. Phys. 1976, 54, 186-196. (b) Hamada, Y.; Merer, A. J. Can. J . Phys. 1975, 53, 2555-2576. (4) Hallin, K. E. J.; Hamada, Y.; Merer, A. J. Can. J . Phys. 1976, 54, 2118-2127. ( 5 ) (a) Watanabe, H.; Tsuchiya, S.; Koda, S. J. Mol. Spectrosc. 1985, 110, 136-140. (b) Suzuki, T.; Ebata, T.; Ito, M.; Mikami, N. Chem. Phys. Letf. 1985, 116, 268-212. (6) Brand, J. C. D.; Jones, V. T.; dilauro, C. J. Mol. Spectrosc. 1973,45, 404-411. Heicklen (ref 1) also cites an unpublished study of Merer. (7) (a) Hillier, L. H.; Saunders, V. R. Mol. Phys. 1971,22, 193-201. (b) Bendazzoli, G. L.; Palmieri, P. Int. J . Quantum Chem. 1975, 9, 537-544. (8) (a) Snow, J. B.; Hovde, D. C.; Colson, S. D. J . Chem. Phys. 1982.76, 3956-3959. (b) Avouris, P.; Demuth, J. E.; Schmeisser, D.; Colson, S . D. J. Chem. Phys. 1982, 77, 1062-1063.
0022-3654/86/2090-3346$01.50/0
3B2state is predicted by a b initio calculations to be the lowest triplet state and to have an origin in the region of 16 700 cm-l.' Assignment of this state is subject to the most uncertainty.' Fluorescence from SO2 has been observed by direct photoexcitation into the singlet state^,^^'^ in shock tubes,l' or by chemiluminescence.12J3 Fluorescence, excited by absorption of broad-band radiation at energies greater than 30 553 cm-l, exhibits biexponential d e ~ a y . ~Collision-free ,~~~ lifetimes of 100-300 ps for the long-lived species and 17-43 I.LS for the short-lived species are observed when the broad-band fluorescence is detected from excitation by even a relatively narrow band (-0.1 nm) by Brus and McDonaldg" and by Su et al.9b The collisional quenching rate constants have also been measured by the Stern-Volmer approach for each fluorescent component.'-9 Holterman et al.'4b have observed resonance fluorescence that exhibited single-exponential decay upon excitation into single rovibronic levels of the 'Az state. The collision-free lifetimes of 10-40 ps indicate that the "pure" short-lived rovibronic levels account for some of the fluorescence on direct excitation into the IA2 state. Phosphorescence from the 3Bl state can be detected after direct photoexcitation of the 3Bl,'5after excitation into the singlet state followed by a collision-induced intersystem crossing to the )B, ~ t a t e , in ' ~ chemiluminscent reaction^,'^,'^ or by electron bombardment.I6 Su and co-workers" have determined the colli(9) (a) Brus, L. E.; McDonald, J. R. J . Chem. Phys. 1974, 61, 94-105. (b) Su,F.; Bottenheim, J. W.; Sidebottom, H. W.; Calvert, J. G.; Damon, E. K. Inr. J . Chem. Kine?. 1978, 10, 125-154. (10) (a) Shaw, R. J.; Kent, J. E.; ODwyer, M . F. Chem. Phys. 1976,8, 155-173. (b) Kimel, S.; Feldmann, D.; Laukcmper, J.; We&, K. H. J. Chem. Phys. 1982, 76, 4893-4903. (c) Lalo-Kourilsky, C.; Vermeil, C. J. Photochem. 1982, 19, 109-121. (1 1) Levitt, B. P.; Sheen, D. B. Trans. Faraday SOC.1967,63,540-548. (12) (a) Pitts, J. N., Jr.; Kummer, W. A,; Steer, R. P.; Finlayson, B. J. Adu. Chem. Ser. 1972, No. 113,246-254. (b) Becker, K. H.; Inocencio, M. A,; Schurath, U. Int. J. Chem. Kine?. 1975, Symp. 1,205-220. (c) Halstead, J. C.; Thrush, B. J. Proc. R. Sot. London, A 1966, 295, 380-398. (13) (a) Glinski, R. J.; Sedarski, J. A.; Dixon, D. A. J. Phys. Chem. 1981, 85, 2440-2443. (b) Glinski, R. J.; Dixon, D. A. J. Phys. Chem. 1985,89, 33-38. (14) (a) Watanabe, H.; Hyodo, Y.; Tsuchiya, S.; Koda, S . Chem. Phys. Lett. 1981,81,439-442. (b) Holtermann, D. L.; Lee, E. K. C.; Nanes, R. Chem. Phys. Lett. 1980, 75,91-93. Holterman, D. L.;Lee, E. K. C.; Nanes, R. J. Chem. Phys. 1982,77, 5327-5339. Holtermann, D. L.; Lee, E. K. C.; Nanes, R. J. Phys. Chem. 1983,87, 3926-3933. (15) Su,F.; Bottenheim, J. W.; Thorsell, D. L.; Calvert, J. G.; Damon, E. K. Chem. Phys. Lett. 1977,49, 305-311. (16) Caton, R. B.; Duncan, A. B. F. J. Am. Chem. SOC. 1968, 90, 1945-1 949.
0 1986 American Chemical Society
Quenching of Electronically Excited SO2 sion-free lifetime, quenching rate constant, and quantum yield for excitation into both the singlet and triplet absorption features at energies below 33 058 cm-I. Two anomalous effects are notable regarding SO2 phosphorescence.' Rudolph and Strickler" have made observations, since confirmed by several other studies,I5J8 of a pronounced pressure saturation effect above 20 Torr on the quenching efficiency of the phosphorescence. Recently, Strickler and Ito18 have bene successfully able to model this effect using either a photophysical or radiationless transition model. Caton and Duncan16have measured an anomalously long quenching rate when the phosphorescence is excited by electron bombardment of SOzvapor at total pressures on the order of 100 mTorr. The two effects do not appear to be quite the same in that they are observed a t very different pressures. The Caton and Duncan16 effect is a more acute anomaly and has not been dismissed as being in err0r.l This result may suggest a more subtle feature in the photophysics of the 3B1state. In order to account for all of the above observations, Heicklen et al.' have made the most recent attempt to model the complete photophysics of SO2. The details of the Heicklen mechanism are to be discussed later in this paper. Chemiluminescencefrom SO2 Another technique for exploring excited-state chemistry is to generate the excited species by a chemical reaction and observe the chemiluminescence. Both fluorescence and phosphorescence have been observed in chemiluminescence from SOz. Halstead and Thrushlzc studied the reaction of SO O3in the total pressure range of 0.3-3.0 Torr. The reaction was reported to yield both 3BI and lB1 states which emit light. The biexponential character of the SO2 fluorescence was not known at the time; therefore, Halstead and Thrushlzc assigned the fluorescence to one excited singlet state. The observation of phosphorescence under these conditions led Halstead and ThrushIzcto suggest that the 3Bl state was produced both by a collision-induced intersystem crossing from the singlet state and in the elementary reaction of O3 with SO radical. Glinski, Sedarski, and Dixon13, have studied the chemiluminescence from SO2 produced in the reactions of O3with small organic sulfides at pressures less than 10 mTorr. Electronically excited SOz was found to be produced for the most part by the J~ et al.13a reaction of SO O3in these reaction s y ~ t e m s . ' ~Glinski were able to observe fluorescence from both the short- and long-lived components and were able to demonstrate that the 3B1 state was not created in the elementary reaction of O3 with SO radical. Instead, it was shown that a collision-induced intersystem crossing from an excited singlet state probably produced the 3Bl state. It was suggested then that 'A2, i.e., the short-lived component, was the source of the 3B1state based on preliminary data. The apparent conflict between the two chemiluminescence investigations regarding the production of the 3Bl state in the elementary reaction can be resolved by considering the pressure conditions of each experiment. The lowest pressure used in the Halstead and Thrush12cexperiments of 300 mTorr required that even the short-lived state of SO2had undergone a collision before it would fluoresce. Therefore, the SOz 3B1state will be formed in a fast secondary process that will appear to be a primary process. In the work of Glinski et al.I3, at much lower pressures no phosphorescence was observed even when substantial fluorescence was present until a pressure was reached where excited singlet SOzwould undergo rapid quenching, 25-mTorr total pressure. Therefore, production of 3B1SO2 in the elementary reaction of SO with O3contributes very little to the observed phosphorescence. Photophysics by Chemical Timing. Coveleskie, Dolson, and ParmenterIg have developed and applied the technique of chemical timing to the study of intramolecular vibrational relaxation that occurs on picosecond time scales. We will show how a similar approach can be applied to the analysis of the emission spectra of SOzproduced in a steady-state chemiluminescence system. The
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(17) Strickler, S. J.; Rudolph, R. N. J . Am. Chem. SOC.1978, 100, 3326-3331. (18) Strickler, S. J.; Ito, R. D. J . Phys. Chem. 1985, 89, 2366-2372. (19) Coveleskie, R. A.; Dolson, D. A.; Parmenter, C. S.J . Phys. Chem. 1985,89, 645-654.
The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 3341 method exploits the natural gating effect of the average gascollision lifetime. Whereas the average lifetime of 20 ps required for observing intramolecular vibrational relaxation processes necessitates working a t p 10 kTorr, fluorescence lifetimes of 100 NS require much lower pressures, in the range of 5-50 mTorr, in order to time the'fluorescence. To illustrate the approach, consider a steady-state system where emitters A and B are produced. The fluorescent species can radiate with reciprocal lifetimes k, and kb or be quenched to the ground state with frequencies dependent on pressure of kqa[M] and kqb[M]. If the emission occurs at the same wavelengths, and therefore cannot be resolved spectroscopically, the expression for the fluorescence intensity as a function of total pressure, [MI, will be
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where I , and are the intensities of each component at a steady-state concentration of each component in the limit of zero pressure, Le., zero quenching. Depending upon the relative values of the rate constants,a plot of I vs. 1/[M] will possess two separate linear regions. In the pressure range where k, C k ,[MI and kb > kqb[M], the plot will be linear with slope kaIa/?zq,. At high pressures where kb C kqb[M] the plot will be linear with slope kaIa/kq, + kdb/kqb. The two fluorescent components can therefore be distinguished by an integrated intensity measurement without being spectroscopically resolvable. In this paper we present the results of our analyses of SO2 chemiluminescence employing this chemical timing approach. We have used the method to distinguish the two spectroscopically unresolved components of SO2 fluorescence and to investigate the mechanism of the collisionally induced intersystem crossing leading to production of the 3B1 state. The data are analyzed in the framework of the Heicklen et al.' photophysical mechanism, and the results of our analysis serve as a test of that mechanism.
Experimental Section As described p r e v i o ~ s l y , 'an ~ ~effusive ~~ beam of ozone was injected into a pumped, large-volume ( V = 60 L) chamber filled with a low pressure of the fuel. Pumping was accomplished by a mechanical pump and a gated oil diffusion pump. Background pressures of C 1O4 Torr were obtained before each run. Pressures were measured with a Granville-Phillips Convectron gauge. The Convectron gauge was calibrated against an MKS Baratron capacitance manometer for ozone pressures of 1-100 mTorr and was found to have a linear response that was within 25% of the Baratron reading. The Convectron gauge response was also linear for the noble gases used in this study, and the readings were corrected to true pressures. The gas inlet lines were stainless steel. The flow rates were controlled by fine-control needle valves. Use of the fuel as the beam had no significant effect on the data. Chemiluminescence generated in the chamber was viewed through a fused silica window positioned parallel to the effusive beam. A lens of radius 25 mm and focal length 30 cm was set one focal length from the entrance slit of a 0.75-m scanning monochromator. A grating blazed for 300 nm was used. Photons were detected with an EM16256 photomultiplier (PMT) cooled to C-50 OC. The spectra reported here are uncorrected for the efficiency of the optical detection system. The PMT signals were sent into a picoammeter and recorded on a chart recorder. Ozone was prepared dilute in dry O2by electric discharge and was collected on silica gel in a Pyrex trap cooled to -59 OC in a slurry of trichloroethane and dry ice. The trap was flushed with helium to eliminate as much 0,as possible, pumped on slowly to evacuate the trap of helium, and then gradually warmed to room temperature. Dimethyl disulfide (99+%) was used as obtained from Aldrich Chemical Co. after considerable vacuum degassing. Methyl mercaptan (Matheson, 99.5% purity) was used without further purification. (20) Glinski, R. J. Ph.D. Thesis, University of Minnesota, 1983.
3348
The Journal of Physical Chemistry, Vol. 90, No. 15, 1986
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Glinski and Dixon
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Figure 1. Chemiluminescence spectrum produced in the reaction of 40 mTorr of ozone and 20 mTorr of (CH,S)2. Note the pronounced triplet features between 380 and 460 nm. Spectrum is uncorrected for spectrophotometer response. A
Experimental Results Chemiluminescence from SO,. Intense emission in the wavelength region 268-550 nm is observed in the reactions of ozone with many organosulfur c o r n p o ~ n d s . ~A~ typical ~ ' ~ ~ ~chemilu~ minescence spectrum generated in the reaction of dimethyl disulfide with ozone at moderate pressures is shown in Figure 1. Spectra, entirely congruent to Figure 1, can also be obtained in the reaction of CH3SH with ozone under similar condition^.'^^^^ We now demonstrate that the spectra generated in the reactions of ozone with CH3SH and CH3SSCH3are due primarily to SO2 chemiluminescence created in the reaction of O3with SO radical and that SO,.generated in this manner is suitable for study of its photophysics. Pitts and co-workersi2a,22 have shown that the reactions of ozone with H,S, CH3SH, and CH3SCH3yield spectra nearly identical with the chemiluminescence spectrum obtained in the reaction of ozone with SO radical. Halstead and ThrushlZcobtain a chemiluminescence spectrum in the reaction of SO with 0, in which all of the observable features are nearly identical with the spectra obtained in the reactions of (CH3S), and CH3SH with 0,. In addition, spectra like that of Figure 1 obtained in the reactions of all the organosulfur compounds and SO radical with ozone show emission extending into the blue to only 268 nm.23 This corresponds very closely to the exoergicity of 106 kcal mol-' for the reaction of SO with O3 to produce SOz and 0,. The observations, together with the observations of Pitts and coworkers,12a,22 strongly suggest that the emission such as that in Previous inFigure 1 is due to the reaction of SO with 03.23 vestigations by Martinez and H e r r ~ nGlavas , ~ ~ and Toby,25and Glinski and D i ~ o n lsuggest ~ ~ , ~ that SO is formed in the reaction of O3with simple organosulfides by a radical chain mechanism involving C H 3 S 0 radicals. Therefore, a plausible and consistent mechanism for production of SO radicals can be proposed even though the following modeling results are not strongly dependent on the mechanism for creating SO. Three features of these chemiluminescence (CL) spectra can be analyzed as they relate to the identification of the emitter as SO, and the emitting states. The most dominant feature in the CL spectra is a broad-band emission feature extending from 268 to 550 nm. Superimposed on this broad band is a moderately intense set of bands in the wavelength region 380-470 nm (Figure 1). In addition, very weak structure can be observed in the region 295-320 nm in Figure 2. The identities of these features are now discussed, while their dependence on reactant and noble gas pressures is analyzed in the next section. The underlying broad-band emission observed in Figure 1 and in the reaction of SO with O3 is strongly similar to the spectra obtained by direct excitation of SO2by monochromatic light (of (21) Toby, S . Chem. Rev. 1984, 84, 277-285. (22) Akimoto, H.; Finlayson, B. J.; Pitts, J. N., Jr. Chem. Phys. Lett. 1971, 12, 199-202. (23) We have previously determined that the contribution of the reaction CH,SH + O3 SO2* CH,OH to the total SO2* will be very small at the pressures of this investigation. Glinski, R. J.; Sedarski, J. A,; Dixon, D. A. J . Am. Chem. SOC.1982, 104, 1126-1128. (24) Martinez, R. I.; Herron, J. T. Int. J. Chem. Kinet. 1978,10, 433-452. (25) Glavas, S.; Toby, S . J . Phys. Chem. 1975, 79, 779-782.
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Figure 2. Spectrum obtained in the reaction of 30 mTorr of ozone with 20 mTorr of CH3SH. Spectral slit width is -0.4 nrn. The Clements bands lettered A-F correspond to vibrational progressions in the 'AZ ' A , transition. The wavelength assignments are due to Mettee.27 +-
relatively wide bandwidth) at wavelengths between 265 and 305 nm as found by both Strickler and and Mettee.27 Su et al.9a and Brus and McDonaldgbhave determined that broadband excitation in this wavelength region leads to fluorescence having both long-lived and short-lived components. Heicklen et a1.I label these states 'B,(L) and IB,(S), respectively, and we follow this nomenclature. In the next section we will show that the broad-band chemiluminescence also consists of emission from states having two different lifetimes. The vibrational structure in the region of 380-470 nm seen in Figure 1 is identical with that observed in the chemiluminescent reaction of O3 with Solzcand in the direct excitation of We have shown previously'3a that this is unambiguously SO2 phosphorescence from the 3Bi state by comparison with all the lines observed by GaydonZ8and by Halstead and Thrush.lzCOur assignments of the bands correspond with those of the other investigators, and we were able to identify additional lines in the phosphorescence spectrum.13a In addition, the strength of the assignment of all of the emission in Figure 1 to SO2* is increased by our observations of weak vibrational structure between 295 and 320 nm in the reaction of 0, with CH3SH. Figure 2 shows these weak bands in the chemiluminescence spectrum along with the assignments of the clement^^^ bands and the band positions of Mettee.27 The bands labeled A-F correspond to those that can be observed in the absorption29into the 'A2state of SO,. The vibrational structure in Figure 2 is identical with, but somewhat weaker than, the structure of the fluorescence when SO, was excited at 296 nm in the work of MetteeSz7This structure, with the same relative intensity is observed in the reaction of CH3SH with 03,was also observed by Halstead and ThrushIzCin the chemiluminescent reaction of O3with SO. We therefore conclude that electronically excited SO, accounts for the largest part of the emission between 270 and 550 nm produced in the reactions of O3 with CH3SH and CH3SSCH3 and that most of the SO2* is formed in the reaction of O3with SO. The contributions of each of the excited states of SO2to the total chemiluminescence are analyzed in the following sections in terms of the pressure dependencies of the emission and a photophysical model of SO,. Figure 3 summarizes in a schematic the formation of SO, in the reaction of SO + O3 and the location of the electronically excited states in the energy range of the exothermicity of this reaction. The figure shows the density of vibrational levels in the fluorescent 'B, state of SO, pf, and the approximate distribution of energies with which SO, could be formed in the reaction of O3 with SO, N/N,(SO,). A product of the two functions, N / N0(SO2),,corresponds to the distribution of energy SO2 will (26) (27) (28) (29)
Strickler, S J , Howell, D B J Chem Phys 1968, 49, 1947-1951 Mettee, H D J Chem Phys 1968, 49, 1784-1793 Gaydon, A G Proc R SOC London,A 1934, 146, 901-910 Clernents, J H Phys Reu 1935, 47, 224-232
The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 3349
Quenching of Electronically Excited SO2
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ENERGY (ern-'1 Figure 3. Representation of the energy distribution of fluorescing states of SO2,N / N o ( S 0 2 ) Fcreated , in the reaction of SO with 03.pF is the density of fluorescing SO2 states. N / N o ( S 0 2 )is the distribution of energies of SO2 produced in the reaction of SO with 03.AH,(SO+O,) = 106 kcal mol-'.
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Figure 5. Dependence of SO2 fluorescence (385 nm) and phosphorescence (396-, 407-, and 414-nm bands) on noble gas (He) pressure. First points on each plot at P = 20 mTorr represent the point of zero added He pressure. Curves drawn do not represent fits of these data. Intensity scales are unrelated.
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Figure 4. Dependence of the chemiluminescence intensity on ozone pressure. Fluorescence was measured at 385 nm; phosphorescence was measured under the 407- and 414-nm bands. Curves drawn do not represent fits of the data. Fluorescence and phosphorescence intensity scales are unrelated.
possess when formed in a fluorescent state in the reaction of SO with 03.We see that the SO2 that fluoresces in this reaction is most likely formed with a relatively narrow distribution of energy between about 32 500 and 34 500 cm-'. Dependence of the Chemiluminescence on Ozone and Noble Gas Pressures. The effects of increasing ozone and noble gas pressures on the luminescence are described here. Figure 4 shows the ozone pressure dependence for both the fluorescence and phosphorescence at one constant fuel pressure. The plotted fluorescence magnitude corresponds to the intensity near the band maximum. The spectral slit width at that point is about 5 nm. The phosphorescence magnitude is an integrated intensity under several of the strongest lines. The low-pressure region of the fluorescence curve reflects the formation of kinetics of the SO2* and has been discussed in detail in a previous p~blication.'~"-~ The maxima and high-pressure regions reflect the point where quenching becomes important in the chemistry of the emitter. It is the high-pressure behavior of the fluorescence that contains information about the photophysics of SO2. It is seen in Figure 4 that the fluorescence curves begin to turn over and reach a maximum at the total system pressure of 15 mTorr. In other experiments at higher initial CH3SH pressures the turnover point also corresponds to a total pressure of 15 mTorr. Simultaneously, at the pressure where the turnover of the fluorescence curve occurs, the phosphorescence becomes observable. The phosphorescence then rises to a maximum and falls off when quenching of the triplet by the added gas becomes important. Fluorescence produced by broad-band photoexcitation at wavelengths greater than 250 nm is known to arise from a singlet state with both a short-lived, 'B,(S),and a long-lived component, 'B1(L). We therefore assume that this is the character of the SO2chemiluminescence observed in these studies. This assumption is justified in the next section. The correlation between the loss
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Figure 6. Dependence of SO2 fluorescence (385 nm) and phosphorescence (396-, 487-, and 414-nm bands) on noble gas (Ne) pressure. First points on each plot at P = 89 mTorr represent the point of zero added Ne pressure. Curve drawn is a linear least-squares fit of the phosphorescence data; correlation coefficient = 0.260. Intensity scales are unrelated.
of fluorescence intensity and the rise of phosphorescence intensity at the same total systems pressure implies that one of the emitting components, 'B1(S) or 'B1(L), is being collisionally quenched to the triplet manifold. The lack of phosphorescence at the lowest pressure where fluorescence is observed but not yet undergoing rapid quenching is direct evidence that the 3B1state is not produced in a significant yield directly in the reaction of SO with 03.The behavior of the fluorescence and phosphorescence with added noble gas pressures at fixed reactant pressures was investigated to study the complex relationships between the excited states. In order to eliminate complex reactions of the quench gases with the intermediates and to circumvent other complications, noble gases were used in order to ensure pure quenching. The results of the study of the variation of the fluorescence an phosphorescence intensities with added noble gas pressure are shown in Figures 5 and 6 at relatively high reactant pressures. The fluorescence curves show pure quenching. It is difficult to tell, by simple inspection, the nature of the states being quenched or the processes involved. The phosphorescence curves show a region of creation at lower pressure followed by an approximate zero-order dependence at the highest pressures. The poor linear correlation of the phosphorescence curve in Figure 6 suggests that the curve may be slightly concave downward with a turnover point at about 300 mTorr. The high reactant pressures of Figures 5 and 6 allow us to make only a semiquantitative analysis of these data. The result to be drawn from these phosphorescence data is that a zero-order dependence on quench gas of the phosphorescence is observed between about 60 and 400 mTorr. We have obtained more precise pressure dependencies for argon a t the lowest possible reactant pressures. These data are plotted in Figures 7 and 8 and are more thoroughly analyzed in terms of the photophysical model. We feel that the analysis of these data is valid because of the
Glinski and Dixon
3350 The Journal of Physical Chemistry, Vol. 90, No. 15, 1986
Fluorescence 385 NM A 320 NM
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0 10 0 15 0 20 I / P (Total) MTORR-' Figure 8. (a) SO2fluorescence (385 nm) intensity vs. argon pressure at low reactant pressure, 7 mTorr (Ar equivalent). (b) Fluorescence data 0
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replotted as a function of 1/P. Curve is a linear least-squares fit of the data below 0.09 mTorr-I; correlation = 0.987. I
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Figure 7. (a) Dependence of SO2 fluorescence and phosphorescence (396-, 407-, and 414-nm bands) on added argon pressure at very low
reactant pressure, -4.5 mTorr (argon equivalent). Curve drawn represents a linear least-squaresfit of the data below 40 mTorr; correlation = 0.981. (b) Fluorescence data of (a) is replotted as a function of l / P . Curve is a linear least-squares fit of the data below 0.1 mTorr-I; correlation = 0.984.
5
as Ifvs. l/P(noble gas). The initial reactant pressures have been converted to effective noble gas pressures based on the approximate relative quenching efficiencies. The fluorescence data of Figure 7a have been replotted as 1 / P in Figure 7b. One other run was carried out with argon; those data are plotted in Figure 8a,b. The run was carried out at a slightly greater initial reactant pressure to demonstrate that the results, Le., the position of the change in slope of the plot of Ifvs. 1/P, occur at the same place. The plots clearly show two regions of linear 1/[MI dependence. The data of Figures 5 and 6 are not replotted vs. 1/P because of the high reactant pressures which prohibit clear analysis of the data. These are the two observations which must be accounted for by an accurate photophysical model of the electronic excited states of SO2: (1) the 1/P dependence of the fluorescence that has two distinctly linear regions with a clear transition at about 10-mTorr total pressure and (2) the first-order pressure dependence of the phosphorescence which becomes zero-order at high quenching pressures without discrete transition.
low initial reactant concentrations. Since the quenching efficiencies of each of the reactants could be estimated relative to the quenching rates for argon, we could convert the initial reactant pressures into an effective noble gas pressure. This has been done for the data in Figures 5-8. Furthermore, any error in this procedure becomes less significant as the noble gas pressure becomes much greater than the reactant pressure. In addition, the reactions of ozone with organosulfur compounds are rather slow. This means that the concentrations of reactive intermediates are Modeling Results and Discussion quite small. We can estimate that the ratio of the concentrations The essentials of a photophysical mechanism are those put forth of reactants to reactive intermediates is greater than lo6. by Heicklen et al.' This mechanism maintains that the triplet Therefore, quenching of the SO2* by the reactive intermediates state is formed through a collision-induced intersystem crossing can be neglected. The mechanism that leads to the production ~ ~ ' the long-lived fluorescent state. This is reasonable because of SO2 chemiluminescence is also relatively p r e d i ~ t a b l e . ' ~ ~ ~ ~from the coupling between the 'B1 and 3B1states would be expected There does not appear to be a chemical reaction in the chemito be strong,' and the long-lived fluorescence possesses the pholuminescence mechanism that would be predicted to have a strong tophysical characteristics of the 'B, state. Therefore, the long-lived third-body pressure dependence of its rate constant. Therefore, component of the fluorescence is expected to be the source of the we believe that any participation of the noble gas in the chemistry phosphorescence. Because direct experimental verification is can be assumed small since the range of noble gas pressure was lacking, a mechanism involving a collision-induced intersystem always varied by about 1 order of magnitude. crossing from the short-lived fluorescent state will also later be Figure 7, obtained for argon as the quench gas, presents data examined, with all other features of the mechanism remaining obtained a t the lowest possible reactant pressures and shows a the same. phosphorescence dependence that begins as a first-order curve In our experiment the singlet excited states were produced in which turns into a zero-order dependence without a sharp trana chemical reaction which occurs at steady state. sition. Figure 7 also shows the response of the fluorescence monitored a t two different wavelengths demonstrating that the fuel + O3 SO,('B,(L)) Y (la) emission across the fluorescence band has virtually the same quenching characteristics. A similar observation was reported S02('A2) Y (1b) by Brus and McDonald,% who collected fluorescence through two different filters in their direct photoexcitation studies. The photophysical mechanism according to Heicklen' that acSince, in the chemical timing analysis, intensity is a function counts for everything that happens to these excited states is given of 1/[M], the noble gas fluorescence quenching data are replotted by reactions lc-9.
---
+
+
The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 3351
Quenching of Electronically Excited SO2
SO2(lBl(S)) + M
+ -
+M
'Bl(S)
'B,(L)
-
-
X'AI
'B,(L)
M
X
+M
+M
+M
3B, + M
3Bl + M
-
3Bl
X
-
X
+M
+ hvp
(2)
TABLE I: Quenching Rate Constants and Radiative Rate Constants for Electronically Excited SOz" Quenching Coefficients, cm3 molecule-' s-I
Ar k* + k , kS + k6 ks
(3)
(8)
Ne 2.5 X 1.8 X lo-'' - 1 x 10-13
He 1.7 X 1 . 1 x 10-11 - 1 x 10-13
Reciprocal Zero-Pressure Lifetimes,bs-l k9 = 1.2 x 102 k7 = 6.6 X lo3
k4 = 2.2 x 1 0 4
(5)
(6)
3.9 x 10-10 2.4 X lo-'' 9.5 x 10-14
"Taken from the review and evaluation of Heicklen, Kelly, and Partymiller.' bExcitation = 295 nm. TABLE 11: Quenching Frequencies of Electronically Excited SOz (M = ArP
(9)
Reaction I C represents the strong coupling between the nonradiative 'A2 levels and the radiative 'B,(S) levels. The rate of reaction IC is very fast, and thus the rate constant does not appear in the following rate expressions-it is as if the 'B1(S)state were created directly in the chemical reaction.30 If the 'Bl(S) and 'Bl(L) states were to emit before any other process occurred, the intensity of their emission would be azo for the 'B,(S) and bZo for the 'B,(L). The quantum yields of reactions 4,7, and 9 are taken to be unity.' The mechanism allows each singlet state to fluoresce or to be collisionally quenched to the ground state and allows the long-lived state to be quenched to the 'B,(S) state and the 3Bl state. The triplet is allowed to phosphorescence or to be quenched to the ground state. W e can now analyze the mechanism into standard rate expressions. The expressions for the fluorescence and phosphorescence intensities are
If = dhY,/dt = k4[IBI(S)] + k7[IBI(L)]
(10)
zp = dhvp/dt = kg[3Bl]
(11)
and
Examining first the fluorescence, steady states are taken in both ['B,(S)]and ['Bl(L)] giving a10
10 20
3.24 6.48
31 61
13 25
7.8 16
k4 = (k2 + k3)[M] at 1.7 mTorr k, = ( k , + ks)[M] at 8.5 mTorr k9 = k8[M] at 39 mTorr "Taken from the review and evaluation of Heicklen, Kelly, and Partymiller.'
lifetime of the short-lived state is k4 = 2.2 X lo4 s-l, and that of the long-lived state is k7 = 6.6 X lo3 s-I. Above about 20 mTorr k7 < (k, k6)[M] and k4 < (k2 + k3)[M], and eq 15 reduces to
+
- (k, + k6)k4aZo+ (k2 + k3)k7bZo+ k3aZo
(17) (k2 + k3)(k, + k 6 w 1 Below about 5 mTorr, k7 > (k, + k6)[M] and k4 < (k2 + k,)[M]. Hence, eq 15 reduces to
['B'(S)l = k4 + (k2 + k3)[M] Therefore, a plot of fluorescence intensity vs. 1 / P should commence from the origin and be linear to approximately P = 15-20 mTorr. The plot should then turn into a linear function at P = 5 mTorr with a slope of -k4aIo/(k2 k3) and a nonzero intercept. Inspection of Figures 7b and 8b shows that this simple model describes those data quite well. As a greater test of this model, we turn now to the expression for the phosphorescence intensity, eq 11; the steady-state approximation can be imposed on the 3Bl concentration.
+
k7
+ (k, + k6)[Ml
The expression for the fluorescence intensity can then be written as If = k4az0
+
(15) k7 + (k, + k 6 ) [ ~ 1 k4 + (k2 + k3)[MI The form of eq 15 can be examined at various pressures. The values of the rate constants from the literature are tabulated in Table I. Table I1 can be constructed to determine how the quenching collision frequencies compete with the reciprocal lifetimes over a range of pressures. It is best to examine what happens to eq 15 from 1/P = 0, high pressures, to 1 / P being large, low pressures. The reciprocal
The phosphorescence intensity is
(30) Calvert and co-workers' (ref 9b) data do not rule out the possibility of reaction 3 being reversible.
We can again refer to Table I1 which also contains the phosphorescence quenching frequencies.
3352 The Journal of Physical Chemistry, Vol. 90, No. 15, 19186
At about 5-mTorr pressure where k7 > (k5 + k & [ M ] ,k4 < (k2
+ k 3 ) [ M ]and , kg > k s [ M ] ,the expression for the phosphorescence intensity reduces to
"(
Returning to the first mechanism that contains reaction 6 'B,(L)
+M
-
3B1+ M
(6)
we focus on the difficulty in interpreting the intermediate- and high-pressure behavior of the phosphorescence dependencies. Recall that eq 21 reduced to
Ip = kl bIo + - ) [ M I The mechanism predicts first-order kinetics at pressures around 5 mTorr. At the intermediate pressure of 20 mTorr, k7 < ( k , k 6 ) [ M ] ,k4 < ( k 2 k 3 ) [ M ] and , kg > k 8 [ M ] .Equation 21 now becomes
+
Glinski and Dixon
+
k.( 610 + Ip =
k5
-) (23)
+ k6
+
The plot of Zp vs. [MI is then predicted to be zero-order near 20 mTorr. At pressures of greater than 40 mTorr, this mechanism predicts that quenching of the triplet state become dominant, kg < kg[M], and the phosphorescence intensity now shows a 1 / [ M ] dependence:
Inspection of Figures 5, 6,and 7a shows that the data indicate first-order kinetics at low pressure and zero-order kinetics at high pressure with an indistinct transition region from 50 to 90 mTorr. The mechanism and data agree in the low-pressure region, but the mechanism ceases to fit the data in the transition region and at high pressure. Before we can make a final analysis regarding the fitness of the first mechanism, we can check the alternate mechanism which allows the short-lived singlet to make the collision-induced intersystem crossing to the triplet state. To do this, insert the step 'B,(S)
+M
IBI(L)
+M
and delete the step
-
3B, + M 3B, + M
(2a)
(6)
The fluorescence kinetics are not changed by this substitution, in that the total fluorescence quenching rate constants are not altered. The phosphorescence intensity can now be written
which means that the phosphorescence dependence is predicted to be zero-order in M a t total pressures as low as 5-10 mTorr. This is clearly not the case in view of Figure 7a. Equation 26 describes a first-order dependence only very near the origin, at - 5 mTorr, where the short-lived state undergoes quenching. Since the linear region in Figure 7a extends out to -50-90 mTorr-a region far beyond the point where the short-lived singlet is completely quenched-the short-lived state cannot contribute significantly to the creation of the triplet, apart from the contribution by the reaction 'B,(S)
+M
-
'B,(L)
+M
(3)
Hence, we have demonstrated that using the short-lived component as the source of the triplet provides a poor description of the photophysics as presumed by Heicklen et al.'
at pressures near 20 mTorr, when k7 < ( k , k s ) [ M ] ,predicting a zero-order dependence. But above 4 mTorr, the quenching frequency of the triplet, ks[M],becomes greater than the reciprocal lifetime, k9, and a 1 / [ M ] dependence is predicted. Notice, however, that the phosphorescence data plotted in Figure 7a show a clear linear dependence to a pressure of at least 40 mTorr. The zero-order dependence also does not commence until at least 80 mTorr, well beyond the point where the model predicts a transition to a l / P dependence should occur, 40 mTorr. In addition, the approximately zero-order dependence of Figure 6 at pressures from 100 to 400 mTorr indicates that either the model or the kinetic parameters of the tables are inadequate. But recall that the rate constants k5,k6, and kl gave a quantitatively correct prediction of the fluorescence dependences. Also, the likelihood of the phosphorescence constants k8 and k9 being significantly incorrect is not very high, given the amount of agreement in the most recent measurements.' W e therefore suggest that the model might be amended to try to account for our observations. The Heicklen' mechanism, reactions lc-9, was proposed as the simplest means of describing the results of a large array of photophysical and photochemical experiments. The above analysis shows that the mechanism is qualitatively correct in fitting the chemiluminescence data, although the 1 / [ M I dependence is not clearly outlined in the data. But the poor quantitative fit decreases our confidence in the application of this mechanism to the description of these data. Several other apparently anomalous results of phosphorescence studies, however, have led Heicklen et al.' to consider minor but important amendments in their mechanisms. Recall that, in their work, Caton and Duncan16produced SO2CBI) by electron bombardment at total pressures from 70 to 800 mTorr and found that the phosphorescence quenching constant was as much as 2 orders of magnitude smaller than that found by other investigators. Heicklen' attempts to explain this anomaly by proposing an unspecified excited state which undergoes collisional quenching to 3B1. The work of Strickler and Rudolphr7also indicates that the quenching of 3B1will appear to be very slow if a very long lived intermediate state is supposed. The two anomalous effects are probably not the same since Strickler and c o - w o r k e r ~ ' observed ~~'~ their effect at quite high pressures, greater than 20 Torr. Since our observations are at relatively low pressures, it is possible that we are observing an effect similar to that observed by Caton and DuncanI6 regarding the jB1 state. To test this hypothesis, we insert into the model the formation of an unspecified excited state and the quenching of it to the 3BIstate. Assuming that the contribution of eq 6 to the total 'B1 state produced is small, we delete eq 6-though it may occur when S02('B1) is created by direct p h o t o e x ~ i t a t i o n . ~The * ~ ~added steps are as Heicklen et a1.I suggest fuel
-- + + - + + O3 T
T
T
removal
M
3Bl
M
y
(14
(28) (29)
Here too, SO,(T) is probably formed in the elementary reaction The T state is destroyed in a unimolecular reaction of SO with 0,. which could be either by collision with the chamber walls (if very long lived) or by conversion to noninteracting vibrational levels
J. Phys. Chem. 1986, 90, 3353-3359 of the ground state. The steady-state concentration of the T state is
kld = kzs + k29[M] and the steady-state concentration of 3B1 is
If we assume that collisions are relatively inefficient at producing the 3B1state, then kz8> k2,[M] and the phosphorescence intensity is
This expression predicts a transition region between the initial first-order and final zero-order dependencies at about 40 mTorr, where k9 = ks[M]. This is a quantitatively better fit to the phosphorescence data of Figures 5 and 7a and does not require a reevaluation of the rate constants. Our findings therefore support the idea that the 3B1state can be formed through an unspecified intermediate state. Candidates for this long-lived state are one of the low-lying triplet states, 3A2 or 3Bz,or the high vibrational levels of the ground electronic state.’
3353
The problem of identifying the unspecified state and the precise nature of its quenching to the phosphorescent state is difficult because the photophysics of the electronic states are complex in this energy region, and it does not seem possible to access the unspecified state by optical excitation of ground-state SOz.31 In any case, the results of this paper, together with those of Caton and DuncanI6 and Strickler,”Js indicate that further investigations into the photophysics of SO2and the better characterization of the low-lying triplet states are well worthwhile.
Acknowledgment. W e thank Dr. Calvert for reading the manuscript and helpful comments. This work was supported in part by the Graduate School of the University of Minnesota. We also thank the National Center for Atmospheric Research for the use of their facilities in preparing this manuscript. Registry NO. SO*,7446-09-5; 03,10028-15-6; SO, 13827-32-2; methyl mercaptan, 74-93-1; dimethyl disulfide, 624-92-0.
(31) Dunning and Raffenetti (Dunning, Jr., T. H.; Raffenetti R. C.-j. Phys. Chem. 1981,85, 1350-1353) have presented strong theoretical evidence for the existence of isomeric SO2states at about 4 eV. Martinez and Herron (Martinez, R. I.; Herron, J. T. Chem. Phys. Lett. 1980,72,77-82) also present experimental evidence of a long-lived neutral metastable state of SO2of similar energy. We note that states as these could possibly be formed by chemical
reaction.
SURFACE SCIENCE, CLUSTERS, MICELLES, AND INTERFACES Thermal Agglomeration Kinetics of Low Nuclearity Molybdenum Clusters in Liquid Poly(d1methylslloxane-co-methylphenylslloxane): Stability of Metal Clusters in AreneContaining Liquid Reaction Media. 3 Mark P. Andrews? and Geoffrey A. Ozin* Lash Miller Chemical Laboratories, Chemistry Department, University of Toronto, Toronto, Ontario, Canada M5S 1 A1 (Received: January 30, 1985: In Final Form: October 21, 1985) As seen in parts 1 and 2 of this study, bis(arene)M complex formation in poly(dimethylsi1oxane-co-methylphenylsiloxane), DC510, necessarily cross-linked the polymer. This feature is exploited in this study to damp those polymer micromotions required to assist mass transport. By increasing the cross-link density in DC510, the thermally activated bimolecular loss of the Mo dimer complex is reduced. Diffusion coefficients extracted ranged from 0.34 X to 32.5 X cm2 s-l, depending on the temperature and Mo atom loading. These low values of the diffusivity are considered to reflect the highly restricted mobility of the dimers, bound as they are to the actual cross-links. An accompanying study of the EPR-active V/DC510 system confirmed that motion in the vicinity of the cross-link is extremely damped. Together these studies enlighten certain aspects of the physical properties of polysiloxanes relevant to metal vapor chemistry and provide an interesting view of the stabilization of metal clusters in arene-containing liquid reaction media.
Introduction Throughout our studies of the reactions of metal atoms with arene-functionalized liquid oligomers and polymers it was often noted that all species absorbing to the red of the bis(arene)-metal complex MLCT band were thermally unstable-the order of appearance of each of the bands was opposite to the order of disappearance.’ We reason here that the gradual decay of these species might somehow be connected with the resistance of the Current Address: AT&T Bell Labs, Murray Hill, NJ 07974.
complexes toward thermally induced diffusional agglomeration, presumably to form colloid. It is pointed out that diffusion of (1) (a) Francis, C. G.; Timms, P.L. J. Chem. Soc., Chem. Commun. 1977, 466. J . Chem. SOC.,Dalton Trans. 1980, 1401. (b) Francis, C. G; Huber, H. X.;Ozin, G. A. US.Patent, 4292253, Sept 1981. Inorg. Chem. 1980, 19,219. J . A m . Chem. Soc. 1979,10, 1250. Angew. Chem., Int. Ed. Engl. 1980, 19, 402. (c) Francis, C. G.; Ozin, G. A. J . Mol. Srrucr. 1980, 59, 55. J. Macromol. Sci., Chem. 1981, 16, 167. (d) Ozin, G. A.; Andrews, M. P. Angew Chem. 1982,94,219. Angew. Chem. Suppl. 1982, 1255. Inorg. Synrh. 1983, 22, 116. Ozin; G. A. CHEMTECH 1985, 488.
0022-3654/86/2090-3353$01.50/00 1986 American Chemical Society
