CHEMILUMINESCENCE IN GASES Pertel, It. (Chicago, Ill.) Phillips, G. 0. (Salford, England) Pilette, Y. P. (Boston, ?\lass.) Pirog, J. A. (Endicott, N. Y.) Platt, J. R. (Ann Arbor, AIich.) Polanyi, J. C. (Toronto, Ont.) Porter, W. L. (Natick, Mass.) Rees, C. W. (Natick, Mass.) Reeves, 11. 11. Jr. (Troy, N. Y.) Ronayne, 31. It. (Boston, Mass.) Ituigh, W. I,. (Ailingtoti, Va.) Safrany, D. It. (Avco-Everett, Mass.) Sandiier, RI. lt. (Natick, >[ass.) Sauer, AI. C., Jr. (Argonne, Ill.) Schenck, G. 0. (Muelheim-Iluhr, Germany) Schlochauer, 31. (South Hadley, Mass.) Schmidt, W. F. (Brookhaven, N. Y.) Schneider, C. (Kohl, Germany) Schofield, K. (Buffalo, N. Y.) Scholes, G. (Newcastle-upon-Tyne, England) Schuler, R. H . (Pittsburgh, Pa.) Schwarz, H. A. (Brookhaven, N. Y.) Sears, J. T. (Brookhaven, N. Y.)
371 1 Serauskas, R. W. (Chicago, Ill.) Seris, J. L. (Pau, France) Shapiro, J. S. (Brookhaven, N. Y.) Skutnik, B. (Brandeis, Mass.) Sieling, D. H. (Natick, Mass.) Simic, AI. (Natick, .Mass.) Simpson, W. H. (Philadelphia, Pa,) Slagg, N. (Picatinny, N. J.) Smith, K. E. (Rutherford, N. J . ) Snipes, W. (University Park, Pa.) Sousa, J. A. (Natick, Mass.) Steel, C. (Brandeis, Mass.) Stevens, B. (Tampa, Fla.) Suryanarayanan, K. (Boston, Mass.) Sutherlaiid, J. W. (Brookhaven, N. Y.) Swallow, A. J. (Manchester, England) Taylor, W. D. (University Park, Pa.) Thrush, 13. A. (Cambridge, England) Tiernan, T. 0. (Wright-Patterson, Ohio) Tomlinson, hi. (Pinawa, Manitoba) Treinin, A. (Jerusalem, Israel) Tsuji, K. (Knoxville, Tenn.) Vecchi, E. (Bologna, Italy) Vermeil, C. (Paris, France) Vittimberga, B. AI. (Kingston, R. I.) Walter, D. C. (Vancouver, Canada)
Walker, J. E. (Natick, Mass.) Warneck, P. (Bedford, Mass.) Wason, S. K. (E. I. du Pont de Nemours, Del.) Weinstein, J. (Natick, Mass.) Weiss, J. J. (Newcastle-upon-Tync, England) Weiss, K. (Boston, Mass.) Wettermark, G. (Uppsala, Sweden) Wharton, D. R. A. (Natick, Mass.) Wierbicki, E. (Natick, Mass.) Wijnen, M. H. J. (Hunter College, N. Y.) Wild, U. P. (Zurich, Switzerland) Wilkinson, F. (Leeds, England) Williams, F. (Knoxville, Tenn.) Willis, C. (Chalk River, Ont.) Woods, R. J. (Saskatoon, Sask.) Wrigley, A. S. (Natick, Mass.) Wyman, G. M. (Durham, N. C.) Yamanashi, B. S. (M.I.T., Cambridge, MESS.)
Yelland, W. E. C. (Natick, Mass.) Yip, R. W. (Ottawa, Ont.) Zwolenik, J. J. (Washington, D. C.)
Chemiluminescence in Gases: Reactions Yielding Electronically Excited Sulfur Dioxide' by B. A. Thrush, C. J. Halstead, and A. McKenzie Department of Physical Chemistry, University of Cambridge, Cambridge, England
(Received M a y 7, 1968)
The mechanism of formation of electronically excited molecules in transfer reactions is discussed briefly. Kew data are presented on the energy distribution of excited SO2 molecules formed in the transfer reaction, SO O3 = SO2 02.This reaction is shown to be somewhat similar to the formation of electronically excited SOn b y the reaction of SO with chemisorbed oxygen atoms on the walls of low-pressure flow systems.
+
+
Introduction M a n y gaseous recombination reactions involving atoms and siniple molecules are known to yield electronically excited products which chemiluminesce.2 The majority are three-body processes and in most cases the radiating state does not correlate directly with the species from which it is formed. I n some such cases, a high proportion of recombinations p o p u l a t e the excited state, but little is k n o w n about the detailed mechanism of crossing between states. The pressures at which such processes are studied often make it difficult to eliminate or identify the effects of collisional processes subsequent to stabilization.
By comparison, few transfer reactions yielding electronically excited molecules are known. This arises partly because few such reactions are exothermic enough to populate excited electronic states of their products and partly from correlation rules. This communication discusses factors which govern the formation of electronically excited molecules in transfer reactions; the formation of excited atoms3is not considered specifically here. Yew data on the formation of electronically excited (1) Presented a t the Conference on Photochemistry and Radiation Chemistry, Natick, Mass., April 22-24, 1968. (2) B. A. Thrush, Ann. Rev. Phys. Chem., in press.
Volume 78, Number 11 October 1968
B. A. THRUSH, C. J. HALSTEAD, AND A. MCKENZIE
3712 SO2 in the reaction of sulfur monoxide (SO) with ozone are presented and discussed.
General A transfer reaction can yield electronically excited products in three basic ways: (a) if a potential surface leads directly from the reactants to electronically excited products; (b) if a radiationless transition to the potential surface yielding electronically excited products occurs in the region of the transition state; and (c) if a highly vibrationally excited product is converted into an electronically excited one by a radiationless transition (which is the reverse of internal conversion). Mechanism a is governed by correlation rulesa4 These show that this process will be largely confined to systems where one reactant is an atom or linear molecule, for which the possible presence of degeneracy associated with orbital angular momentum means that the reactants can correlate with more than one potential surface having a particular resultant spin. The high multiplicities of the ground states of some atoms and linear diatomics associated with partly filled degenerate orbitals also raise the possibility of reactions in which the formation of ground-state products is spin forbidden. Nonlinear molecules have lower symmetry, few of their point groups allow degenerate molecular orbitals, and such orbitals are normally full or empty in the ground states of gaseous nonlinear species. Separate potential surfaces connecting nonlinear reactants with products therefore only arise due to difference in spin coupling between the reactants which will normally have singlet or doublet ground states. Mechanism a is therefore rare in transfer reactions between nonlinear species. Where these rules allow reactants to correlate both with ground state and electronically excited states of a product, the formation of both states should involve similar frequency factors. The separation of the potential surfaces in the transition state will generally give different activation’energies for the two processes: that for the formation of electronically excited products should be higher by an obvious extension of Polanyi’s The chemiluminescent bimolecular reaction of nitric oxide with ozone NO
+
0 3
=
NO2
+
0 2
(1)
is probably the clearest example of mechanism a, where the formation of ground state (2A1) and electronically excited (2B1)NOz both have frequency factors of (6 f 2) X lo1*cm3 mol-l sec-’ and activation energies of 2.3 and 4.2 kcal/mol, respectively.6 Several processes appear to be responsible for CN chemiluminescence in the reactions of active nitrogen, but the best investigated process probably involves mechanism a. This is the strong orange-yellow flame The Journal of Physical Chemistry
reaction between halomethanes and active nitrogen, where Broida and coworkers’ have shown that levels v’ = 4-10 of the AW state of CN are populated in this reaction in preference to isoenergetic levels of the ground state (X22+)or of the B 2 2 +state. This excitation is almost certainly due to the very exothermic transfer reactions
N(%)
+ CCl(’II)
=
CN(A211)
+ C1(2P)
(2)
and
N(4S)
+ CH(211) = CN(A211) + H(%)
(3)
For reaction 3, the formation of CN(A211)cannot be regarded as an example of the selection involving conservation of orbital angular momentum in linear systems as its transition state would correspond essentially to a triplet state of HCN which would be nonlinear in its equilibrium configuration. This excitation is an example of mechanism a but only in a limited sense, because the AzII state is the lowest lying state of CN for r(C-N) > 1.5 8, and is therefore the “ground state” of CN in the region of the transition state.* The crossing of the X22 and A211states at r(C-N) ‘v 1.5 8 is unusual and arises from the decreased bonding of n,2p orbitals relative to a,2p with increasing internuclear distance. In contrast to mechanism a, mechanisms b and c should be associated with lower efficiencies of chemiluminescence due to the transmission coefficients for the crossing process. Distinction between mechanisms b and c can be difficult; evidence that mechanism c could occur in a particular reaction is provided if radiationless transitions are detected in the study of the fluorescence and phosphorescence of the appropriate product. Such studies are important in the investigation of chemiluminescent reactions, since they provide the needed relation between the rates of radiation and of populating the emitting state. Reaction between Suljur Monoxide (SO) and Ozone. This process was studied by Halstead and T h r u ~ hwho ,~ showed that it is bimolecular yielding SO2 in its ground (‘A1) and first two excited states ( 3 B and ~ ‘B1 (?)) (3) (a) M. C. Moulton and D. R. Herschbach, J . Chem. Phys., 44, 3010 (1966); (b) J. R. Airey, P. D. Pacey, and J. C. Polanyi, “XIth Symposium on Combustion,” Combustion Institute, Pittsburgh, Pa., 1967, p 85. (4) K. E. Shuler, J . Chem. Phys., 21, 624 (1953). (5) ,M.G. Evans and M . Polanyi, Trans. Faraday Soc., 35, 178 (1939). (6) (a) M. A. A. Clyne, B. A. Thrush, and R. P. Wayne, ibid., 60, 359 (1964); (b) P. N. Clough and B. A. Thrush, {bid., 63, 915 (1967). (7) (a) H. E. Radford and H. P. Broida, J . Chem. Phys., 38, 644 (1963); (b) T . Iwai, M. I. Savadatti, and H. P. Broida, ibid., 47, 3861 (1967). (8) D. W. Setser and B. A. Thrush, Proc. Roy. Soc., A288, 256 (1965). (9) C. J. Halstead and B. A. Thrush, ibid., A295, 380 (1966).
CHEMILUMINEE~CENCE IN GASES SO
+
0 3
3713
= S02X(lA1) = SOz('B1)
+ Oz
+ Oz
+
= S O % ( ~ B ~ )0%
(4)
301
1
A
1.5
(44 (4b)
Experiments, were conducted in an argon carrier at pressures between 0.3 and 3.0 mm. The long-lived 3B1 state showed a vibrational distribution corresponding to a temperature a little above ambient, but emission from the 'B1 state showed considerable vibrational excitation extending from its origin which is probably at 91 kcal/ mol up to an energy of 101 kcal/mol. With the aid of studies of SO, fluorescence, it was shown that the Arrhenius expression for population of the 'B1 state was kaa = 10" exp(-4200/RT) cm3 mol-l sec-l and that two-thirds of the triplet emission (which has an activation energy of 3.9 kcal/mol) was caused by collisional quenching froin the 'B1 to the 3B1 state. The rate constant for population of the ground state was found t o be 1c4 = 1.5 X 10l2 exp(-2100/RT) cm3 mol-' sec-'. Since SO(3Z-) and Oa(lA1) yield only one triplet surface which must correlate with ground-state products, the chemiluminescence arises through mechanism b or e. The higher activation energies of reactions 4a and 4b relative to reaction 4 suggest that mechanism b operates and that crossing to the appropriate potential surface occurs at or near the transition state with a transmission coefficient of about 0.1 for reaction 4a. Several features of this reaction require further investigation, particularly (a) the nature of the extensive vibrational distribution in the 'B1 state, and (b) the rate of population of the 3B1state for which no quenching data exist. The Singlet Emission of SO2. Providing the electronic transition moment of the lBl-lA1 system of SO2 does not change sharply with configuration, the vibrational energy distribution in the 'B1 state can be found from the vibrational sum rule. This rule establishes that a particular vibronic band occurs in the same fraction of transitions from its initial level both in emission or absorption, providing allowance has been made for the different dependence of emission and absorption intensities on frequency. The weakness of the SO 0 3 chemiluminescence makes it impossible to obtain well-resolved spectra, and only bands on the highfrequency side of the origin are resolved. These bands are the strongest ones in the absorption spectrum where they form quite a long progression with spacings of the order of 200 cm-l. It is generally agreedlo,l1that these arise from the (O,O,O) level of the ground state. Clements'O has interpreted them as a progression purely in V Z ' , whereas invokes both symmetrical stretching (vl' = 770 cm-l) and bending vibrations (v2' = 320 cm-l) with a very large interaction term xlz' = -20 cm-l. The latter interpretation gives more than one assignment t o each of the strongest bands,
+
-
0
j
I
90
2
I
I
-_I
--+i %--.
6 8 Total energy, kcal/mol. 4
B 0.5 b?
0
100
Figure 1. Vibrational energy distribution of 'BI SO2 emission from SO OSreaction. Two separate determinations are shown. The highest energy points on the linear scale refer to unresolved emission at energies between 96 and 101 kcal/ mol; such points cannot be shown properly on the logarithmic scale.
+
whereas their envelopes at moderate resolution indicate that one vibronic transition predominates in each band. The excited state of SO2 involved is probably the lB1 state derived from the same 'Ag state of linear SO2 as is the lA1 ground ~ t a t e . ~I n, ~this ~ situation excitation of vz' should predominate, and the isoelectronic species CF2 has a transition in this region of the spectrum the vibrational structure of which is now interpreted almost exclusively in terms of bending vibration~.~~ Figure 1 shows linear and logarithmic plots of the vibrational energy distribution in the 'B1 state determined from the vibrational sum rule assuming: (a) that each band is a single, vibronic band or that the same vibronic band predominates in emission and absorption as expected from the Franck-Condon principle; and (b) that each emitting band, being a transition to the (O,O,O) level of the ground state, also contributes baclrground emission to bands at longer wavelengths, the form of this contribution being determined from the overall distribution of the lB1 chemiluminescence and its intensity from the Franck-Condon factor of the band concerned. The solid line in Figure 1 corresponds to a vibrational temperature of 1750°K for the bending levels (v2'). At the pressures used, S02('B1) is removed predominantly by quenching, about one collision in ten with argon being effective. This number of collisions is probably sufficient to redistribute vibrational energy within the excited 'B1 state, and similar excitations of VI' and v3' must also be present in the emission observed. The vibrational sum rule yields percentage popula(10) J. H.Clements, Phys. Rev., 47, 224 (1935). (11) N. Metropolis, ibid., 60, 295 (1941). (12) A, D.Walsh, J . Chem. Soc., 2283 (1953). (13) D.E.Mann and B. A. Thrush, J . Chem. Phys., 3 3 , 1732 (1960).
Volume 73, Number 11
October 1968
B. A. THRUSH, C. J. HALSTEAD, AND A. MCKEKZIE
3714 tions of emitting levels directly, and the two determinations in Figure 1 account for 7040% of the total emission, providing Clements’ origin at 31,945 cm-l is accepted. This is very good overall agreement, since excitation of vl’ and v3’ must produce some small deficit despite the selection rule that Av3 = 0 predominates and the probability that transitions involving Avl = 0 are strongest. The relative dispositions of the emission and absorption show that hletropolis’ origin for the lB1 system (29,622 cm-l) is incorrect; it would also give a sum of individual percentage populations totaling several hundred per cent. The vibrational distributions in Figure 1 have therefore been normalized to 1 0 0 ~ Clements’ o origin. On this basis, the mean bending vibrational energy in the lB1 state is 2.5-3.0 kcal/ mol corresponding t o about 15% of the (heat of reaction activation energy) if electronic excitation is subtracted. Because of their larger quanta, v1 and v3 should each contain about 2 kcal/mol. Triplet Emission. In recent studies of the sulfur dioxide afterglow (due to 0 SO) using a 6-in. diameter linear flow system,14we have observed emission by the ‘B1 and 3B1states of SO2from the gases close t o the Pyrex surface at total pressures between 10 and 50 p . The spectrum of this emission closely resembles the SO 03 chemiluminescence except that: (a) the ‘B1 bands between 2950 and 3250 8 are more clearly defined, apparently because the underlying unresolved emission is weaker; (b) the maximum intensity ( I x ) of the singlet emission is shifted from 3400 8 to about 3600 A; and (c) the triplet emission is relatively somewhat weaker; all the previously reported transit i o n ~from ~ levels (O,O,O) and (0,1,0) were detected, but the 3748-A band from level (1,0,0) was not discernible, even with long exposures, indicating that the vibrational temperature was lower. These observations are consistent with the formation of electronically excited SO2 in the reaction of gaseous SO with chemisorbed oxygen atoms
+
+
+
wall - 0
+ SO +wall - + SOz*
(5)
and the intensity distribution of the emission indicates that the heat of chemisorption of the oxygen atoms responsible is 25-30 kcal/mol, that is, slightly greater than D(0-02). There is evidence for an analogous red glow above the surface when the reaction between 0 and NO is studied at low pressures.l6 This is almost certainly NOz emission due to a process analogous t o ( 5 ) and resembling the NO 0 3 chemiluminescence (reaction 1) in its spectral distribution. An analysis of the radial distribution of the 3B1 emission in the flow tube supported the view that 3B1 SO2 molecules were formed on the flow tube walls and diffused to the center, being removed by a first-order process. Numerical analysis of the spatial distribution
+
The Journal of Physical Chemistry
at total pressures between 12 and 52 p for an argon sec for the carrier yielded a life of (7.6 =k 1.6) X 3B1state, assuming its diffusion coefficient in argon to be 100 cm2 sec-I at 1 mm pressure.16 This value did not decrease with increasing pressure and can therefore be identified with the radiative life of the 3B1state. I t is ten times longer than the radiative life of the lB1 state.17 Collisional quenching from the ‘B1 to the aB1 state could not therefore affect its apparent life. Our value may be compared with a lower limit of 5 X 10-4 sec in the solid state17 and a value of 2 X sec for shock-heated SO2.l8 The lack of pressure dependence shows that the halfquenching pressure of the 3 B state ~ for argon is not less than 50 p, whereas the pressure dependence of the SO 0 3 emission shows it cannot be greater than 300 p , If these limits are applied t o the 3B1emission in the SO 0 3 reaction (allowing for population of the 3B1state by collisional transfer from the lB1 state), it can be shown that lcrb = (1 t o 6) X lo9 exp(-3900/RT) cma mol-’ sec-’ and is probably close to the center of the range quoted.
+ +
Discussion The higher activation energies (4.2 and 3.9 as against 2.1 kcal/mol) and lower frequency factors (loll, ca. 3 X log as against 1.5 X 10l2cm3 mol-l sec-l) for the formation of ’B1 and 3B1electronically excited SO2 in reaction 4 as compared with ground-state products are consistent with a crossing to the appropriate potential surface at or near the transition state. The low-frequency factor for populating the 3B1 state suggests a weaker crossing process perhaps to a singlet or quintet rather than a surface arising from S02(3Bl) 02(38,-) triplet, since triplets must be involved in the formation of both ground state and lB1 SO2. Very little is known about vibrational excitation in polyatomic reaction products, but it is highly unlikely that the observed distributions in and between the 3Bl and ‘B1 states of SO2 could arise from the formation of highly vibrationally excited ground state SO2 molecules which then cross into the emitting states. The higher activation energies for their formation argue against such a process, since there is no evidence that Ihe formation of highly vibrationally excited reaction products is greatly favored by a small increase in reactant energy.
+
(14) A. McKenrie and B. A. Thrush, to be published. (15) T. R. Rolfes, R. R. Reeves, and P. Harteok, J. Phys. Chem., 69, 849 (1965). (16). Based on unpublished measurements of T. Jones (Leeds University). (17) K. F. Greenough and A. B. F. Duncan, J . Amer. Chern. SOC., 83, 555 (1961). (18) E. P. Levitt and B. D. Sheen, Trans. Faraday SOC.,63, 540 (1967).
