FOREIGN GAS QUENCHING OF SO2 VAPOREMISSION
1071
Foreign Gas Quenching of Sulfur Dioxide Vapor Emission
by H. D. Mettee’ Department of Chemistry, The University of Texas at Austin, Austin, Texas
(Received October 1 6 , 1 9 6 8 )
Quenching probabilities per gas kinetic collision for 15 common collision partners with electronically excited SO:! are presented and compared to those found independently for electronically excited 1 2 , 502, and OH. In each of the latter cases, and for excited singlet SO2, the quenching process is efficient, usually requiring from only one to ten collisions. The single parameter which best correlates most of the data is the polarizability of the quencher.
Introduction
Experimental Section
Callear’s recent reviews2 summarize the fragmented character of the theoretical and experimental states of knowledge of electronic energy quenching processes. Most of the present information deals with excited atoms, and their usually great reactivity precludes a meaningful physical description if the collision partner is a polyatomic molecule. The situation is even less clear for the case of quenching the excited states of simple molecules by either atoms or other simple molecules. The iodine and SOz systems have been studied with a fairly wide variety of collision partners. Steinfeld and Klemperer3 and Brown and Klemperer4 found a quenching dependence which varied with the product of the reduced mass of the collision complex and the polarizability of the partner. Apparently no contribution arose from a permanent dipole. This finding confirmed an earlier expectation of Rossler.6 On the other hand, Myers, Silver, and Kaufman’s study of the quenching of NO2 fluorescence6 showed an increased quenching cross section for those collision partners with a dipole moment and “molecular complexity.” In both of these systems the quenching cross sections were of the order of the gas kinetic cross sections or slightly less. This situation may be contrasted with the general immunity to fluorescence quenching of many carbonyl and aromatic vapors using the relatively simple collision partners under consideration. The present study extends these quenching studies to the singlet, and in less detail to the triplet, systems of 502. It is found empirically that a linear relation exists between the probability of fluorescenge quenching per gas kinetic collision ( 1 / Z g ~ )and the polarizability of the collision partner for most partners. The data for 1 2 , NOZ, and that for OH measured and collected by Tanaka, et al.,’ are satisfactorily presented in a similar way. For OH and IZthe quenching mechanisms are probably collision-induced predissociation, while for NO2 and SO2 the processes of collision-induced internal conversion and/or intersystem crossing are likely channels.
The apparatus is the same as previously described.8 All of the commercially available gases were checked for purity by use of a Consolidated Engineering Model 21-401 mass spectrometer and were found to be at least 99% pure, except D20 (Stohler Isotope) which was not analyzed, and Dz (Union Carbide) which was ca. 6% HD. Because the gases do not vary over orders of magnitudes in their quenching efficiencies (except for NO quenching the phosphorescence), even impurity levels of a few per cent were tolerable. The thermocouple and Magnevac gauges were calibrated when necessary by the known-volume-ratio expansion method and a Wallace and Tierman 0-50-mm circular diaphragm gauge was used as a reference. A typical experiment consisted of condensing 200 p of SO2 in the right-angle emission cell cold finger with liquid NZand adding the selected pressure of quencher gas followed by allowing the SOZ to vaporize and refill the cell. For those foreign gases condensed by liquid N2, a technique which measured their partial pressures after the experiment by known-volume expansion was employed. The emission intensities were measured a t 3400 ( I t ) and 4152 (Ip),as described,8 using in most cases four exciting wavelengths, namely 2750, 2850, 2960, and 3020 The data were plotted in the usual Stern-Volmer form, ( I E O / I E ) X vs. quencher pressure, where I E O is an emission intensity of 200 p of SO2 and I E is the same in the presence of a quencher using an exciting wave-
A A.
(1) Chemistry Department, Youngstown State University, Youngstown, Ohio 44503. ( 2 ) (a) A. B . Callear, “Photochemistry and Reaction Kinetics,” P. G . Ashmore, F. 9. Dainton, and T. M. Sugden, Ed., Cambridge University Press, Cambridge, 1967, p 133: (b) A. B. Callear, A p p l . Opt., 2, 145 (1965). (3) (a) J. I. Steinfeld and W. Klemperer, J. Chem. Phys., 42, 3475 (1965): (b) J. I. Steinfeld, ibid., 44, 2740 (1966). (4) R . L.Brown and W. Klemperer, ibid., 41, 3072 (1964). (5) B. RSssleF Z . Phys., 96, 251 (1935). (6) G. H. Myers, D. M. Silver, and F. Kaufman, J. Chern. Phys., 44, 718 (1966). (7) M.Kaneko, Y. Mori, and I. Tanaka, ibid., 48, 4468 (1968). (8) H.D. Mettee, ibid., 49, 1784 (1968). Volume YS, Number 4 April 1968
H. D. METTEE
1072 Table I: SO2 Emission Quenching D a t a Compared with Selected Other Data Collision partner
lT,
2
LY.
x
1024,
cmab
IAel,O
eV
1/zQF
SHe 4He Ne Ar Kr Xe
2.16 2.16 2.54 3.60 4.12 4.80
0.204 0.204 0.392 1.63 2.47 4.01
...
...
0.079 0.14 0.21 0.26 ...
0.0041 0.0040 0.0053 0.0060
0.018 0.056 0.20 0.45 0.98
Hz
2.83 2.83
0.79 0.79
0.12 0.13
0.0063 0.0057
0.054 0.071
3.86 3.69 3.78 3.44 4.94
1.76 1.60 1.70+ 1.95 2.65
0.28 0.27 0.59 0.35 0.63(1.l)“
0.0074 0.010 1.73 0.0082 0.0085
0.22 0.27 0.73
CHI CHaCl CF4
4.27
2.60 4.56 4.0”
0.34
0.0068
HzO
5.36 5.36 6.08
DO
Nn 0 2
NO
co coz
D20 SF0 ”3
so2 NO? In
... 4.70
...
4.34 4.20 .,.
...
... 0.53
...
1.40+ 1.40 6.0+ 2.26 3.72 3.1 9.3+
1.2 0.68 .
I
.
1 .o
I
...
...
0.04
0.085
0.08
0.69
... ...
*..
.,.
... .,.
...
0.06
0.26 0.26
7.7 7.7
0.79
0.10 0.13 0.20
0.32 0.54
...
2.3 0.94
4.9 2.0 1.8 4.4 2.0
...
0.27
...
0.16
...
16.9 16.9 13.1 8.1 6.9
0.078
2.0
4.8
...
...
0.0089
0.39
8.1
...
0.48
0.028 0.016
...
1.5
0.35
...
0.90 1.07
0.027
...
0 072 0.105 0.32 0.49 0.79 1.51
...
... 4.6
...
3.4
2.6
3.0 3.0 2.0
...
...
...
0.0
0.24
,..
...
...
a Most collision diameters are root-mean square values tabulated in ref i. Others from various sources. Polarizabilities are taken directly from ref j , except for noble gases and those marked (+). The latter were estimated from an empirical plot of polarizability vs. polarization, whose latter values are given in ref j . )(: NO2 value used was that of N2O. Noble gas values are given in ref k. c Involves the assumption Reference 3. 6 Reference 6. that all those ‘SO2 molecules which do not fluoresce reach the 3S0 level, the parent level for phosphorescence. Reference 7. The gas kinetic collision diameter of OH* used was 4.0 A. The gas kinetic collision cross sections were calculated using the = [ ~ ( u E U M ) ] ~ , 8 The absolute value of the electronic energy difference between ‘SO(vibrationally relaxed) and accepted expression (u&2 Calculated from data of Greenough and Duncan,’$ the closest eleotronic level of the quencher of the same multiplicity if choice was possible. using the present collision diameters. W. Kauzmann, “Kinetic Theory of Gases,” W. A. Benjamin Co., New York, N. Y., 1966, p 209. i Landolt-Bornstein, “Zahlenwerte und Funktionen,” Vol. I, Part 3, Springer-Verlag, Berlin, 1951, p 510. W. Edgell, “Argon, Helium and Rare Gases,” Vol. I , Ed., G. A. Cook, Interscience, New York, N. Y., 1961, p 151.
+
length A. The 90% confidence limits of the slopes showed error limits of ca. &15% for fluorescence quenching and ca. &25% for phosphorescence quenching. The quenching rate constants, k,, were divided by the calculated gas kinetic collision number to estimate the probability of quenching per collision. This number is effectively the ratio of the quenching cross section to the gas kinetic cross sections.
Results The basic characteristics of the SOz vapor absorption and emission spectra have been outlined elsewhere.8 The measured emission intensities at 3400 and 4152 A were taken to correspond to the total fluorescence and phosphorescence intensities, respectively. (About 20% of the intensity at 3400 k was contributed as fluorescence to the intensity measured a t 4152 A, so that the phosphorescence intensity was corrected accordingly.) Using a fixed exciting wavelength, fluorescence emission measurements were also made a t 3200 and 3650
A.
The Journal of Physical Chemistry
For the cases of pure SO2 added in small pressure increments at low pressures (5-50 p ) and of Nz quenching experiments, the fractional changes in the emission intensities measured a t these additional wavelengths were the same as those measured a t 3400 A, for a given pressure change. Thus both the foreign gas and self-quenching of fluorescence data were demonstrated to be independent of the emission wavelength selected. For the relatively high pressures of SO2 used here (200 p ) the quenching rate constants were also found to be independent of exciting wavelength. Two experiments using 50 p of 502 with Ar and He showed slightly steeper Stern-Volmer plots for the longer wavelengths of excitation. When proper account of the lifetime differencess was taken, however, the same quenching constant was found for all exciting wavelength^.^ (9) No exactly analogous experiment was performed for the phosphorescence quenching. It was observed that the relative band intensities showed no dramatic pressure dependence, which is anticipated if the parent levels are vibrationally equilibrated as assigned.
FOREIGN GAS QUENCHING OF SO2 VAPOREMISSION
1073
Discussion The following mechanism is a useful framework for discussion. So
+ hvn
In
'Sn
0.5
"0 I
(b)
I
-
I
3so =
s
--
+ s = 2s
k,['S0][S]
I
I
I
I
I _
-
0 H20
SFe 0
"4
-
0 Cog C" 4 0
HE$7H2
3s0
I
I
0.5 (I/Za)
I
cog / 0'
2.0
(C)
I
'NO
6 N2
,
I
-
(0-02) I
I
1
(7)
I n this mechanism any bimolecular process involving
S might equally well involve a quencher molecule M , and in the following these steps will be denoted by the corresponding rate constant primed. Fluorescence Quenching and Vibrational Relaxation. The present author's earlier work8 indicated that steps l a and l b are unimportant. The recent experiments of Rao, Collier, and Calvertlo suggest that the relative importance of these unimolecular steps kl:k l , : kn, is 0.20:0.73:0.07. In neither ca,se are the data compelling, and for the purpose of the present work the question may be put aside since these pathways are unimportant a t the pressures used here, compared to steps 2a and 2b. The standard steady-state treatment of fluorescence quenching gives the following expression. (ha'
(lfo'lf)
= +
+ k2b')
+ (kza + kZb)[S] [MI = k, and +
kl
(8)
Combining ( k z a -/- k2b) (k2a' k2b') = k ,' and recognizing that at the pressure of SO2 used the bimolecular term dominates the denominator, it is is easily seen that IC,' may be estimated from the slope of the Stern-Volmer plot. The value of k, was taken to be the gas kinetic collision number, k, = 2.63 X 10-lo cm3 molecules-1 sec-I, as previously established.8J1 The two low SO2 pressure experiments would be expected to show the observed -exciting wavelength dependence of the slope since kl, the reciprocal fluorescence lifetime, likely decreases with increasing exciting wavelength.8
The fact that k,' is independent of exciting wavelength, and the values themselves are fairly close to the gas kinetic values, precluding extensive vibrational relaxation (if it occurs a t all), suggests that the quenching cross section of 'SO2 is independent of its vibrational energy content. This may be compared to the case of I2 (Table I) where, even accounting for differences in the fluorescent lifetime12for v' = 15 or 25, it appears more probable to quench those excited states with more vibrational energy. Turnerla reported this fact some years ago and Steinfeldab has since documented it extensively. Since the results indicated there was no observable dependence of the quenching constant k,' on the fluorescence emission wavelength (for Nz and for pure SO2 and presumably the other partners), it is reasonable to conclude that little or no vibrational relaxation is occurring in 'SO2 prior to emission. This is in accord (10) N. Rao, 8 . Collier, and J. G. Calvert, J. Amer. Chem. Soc., submitted for publication. (11) K. F. Greenough and A. B. F. Duncan, i b i d . , 83, 555 (1961). (12) Ref 3 and Chutjian, J. K. Link, and L. Brewer, J. Chem. Phys.. 46, 2666 (1967).are not in complete accord on their actual values of the radiative lifetimes of 11 (B a t various levels. These differences, however, are obscured by the pronounced exciting wavelength dependence of quencher efflciencies. (13) L. A. Turner, Phys. Rev., 41, 627 (1932). Volume Y3, Number 4 April 1089
H. D. METTEE
1074 with the facts that the emission always begins a t the exciting wavelength, independent of pressure, and that the k,’ values are close to the gas kinetic values. That some amount of vibrational relaxation is occurring with a fairly large cross section is suggested by the apparent requirement for parent levels other than those thought to be produced optically to explain the “resonance” bands.8 On the other hand, Myers, Silver, and Kaufmana observe for NOz that IC’, is apparently larger for fluorescent emission wavelengths near the exciting “line” than for those measured at longer wavelengths. That is, for the same amount of added quencher the shorter fluorescence wavelengths are apparently preferentially quenched. Using 4358-A excitation, the difference amounts to a factor of 3 in going from an emission wavelength of 4680 to 6330 A using either Ar or NO2 itself. This “red shift” in the fluorescence intensity distribution with increased pressure plainly indicates vibrational relaxation is occurring prior to emi~si0n.l~ It is interesting that the cross sections for fluorescent quenching of both NOz and SOz are quite similar and yet vibrational relaxation appears to be much more probable for NOz. However, it should be borne in mind that the intensity wavelength distribution of fluorescence of SO2is relatively independent of exciting
.
I
l
l
io2
Figure 3. SKB plot (a) and polarizability plot (b) of the experimental quenching probability of various collision partners with 1) (B*IIo,+) (0’ = 25).3”
5.0 Figure 2. SKB plot (a) and polarizability plot (b) of the experiments: quenching probability of various collision partners with SO%[(A’Bl)]. The Journal of Physical Chemistry
wavelength (excepting that portion on the short wavelength shoulder). It is therefore still possible that appreciable vibrational relaxation in ’SO2 prior to emission might occur unnoticed. The earlier evidence suggests that it does occur but only slightly. Fluorescence Quenching Parameter. The values of ( ~ / Z Q Flisted ) in Table I show the quenching efficiency of various collision partners for both SO2 and NO2 range over about one order of magnitude, except possibly for “chemically reactive’’ species such as HzO or D2O. It so happens that the products of the square root of the reduced mass of the collision complex and the polarizability of the collision partner extend over about two orders of magnitude. In these cases a Steinfeld-Klemperer-Brown3~~(SKB) type plot “overpredicts” the quenching efficiency, normalized to He, or shows large scatter if normalized to a more suitable base. Various terms were selected to relate these quenching efficiencies and were found unsuccessful for SO2 and other systems, Among these were the ionization po(14) This effect has been noted by 8 . E. Schwartz, private communication, whose additional flnding of a shorter fluorescence lifetime of NO2 with shorter exciting wavelengths parallels that suggested for 802.
1075
FOREIGN GAS QUENCHING OF SO2 VAPOREMISSION
OH FLUORESCENCE (A)
l " " l " " l ' " ' ~ " ' ' " " ' ~
/
0.0
/
/
-1.0
4.0 [
0.0 I '
I
1.0 I
I
1
1
0
0
N
'.
1.0
2.0
Figure 4. SKB plot (a) and polarizability plot (b) of the experimental quenching probability of various collision partners with NO2 (A 2B1).6
Figure 5. SKB plot (a) and polarizability plot (b) of the experimental quenching probability of various collision partners with OH (A W ) .
tential of the quencher,lj the number of electrons in the quencher molecule, and the spin-orbit parameter for the heaviest atom in the quencher molecule. The first of these failed because the ionization potential was not variable enough and the spin-orbit term was far too variable. The fourth, the reduced mass term, was tried alone and showed much scatter. Because of the great attention focused on the "energy defect" (normally the residual energy converted into translation after a near-resonant transfer) in collisional relaxation phenomena,2 a plot of 1 A6 I-l, the reciprocal absolute electronic energy difference between the relaxed 'SO2 and the nearest electronic level of the collision partner (of the same multiplicity if available), was attempted. Figure l a shows such a plot to be reasonably successful for 'SO2 although it is somewhat less successful for Iz, with a few anomalous points in each case which may be rationalized. Unfortunately such was not the case for NO2 (Figure IC). Figures 2-5 compare the SKB plots with those based solely on polarizability. The points on the SKB plots may be shifted to the left or right, depending on the normalizing factor F, which was selected in each case to correctly predict Of these systems' however' the scatter is 4He' In still appreciable (even on a log-log grid), and the
experimental data do not generally show as wide a divergence as the predicted values. It may be noteworthy that for the polarizability plots those anomalously efficient quenchers are reactive, have dipoles, or have lower-lying electronic levels. (Unfortunately, some adequately predicted cases have similar characteristics.) Phosphorescence Quenching. For the pure SO2 system, the limit of the inverse phosphorescence yield a t high pressure is
If all of those lS02 which do not fluoresce become %,, so that kza = ha = 0, then the Stern-Volmer slope is (k&). Using Duncan and Caton's value16 of the phosphorescent lifetime a t low SO2 pressure as cms molecule-' sec-I. 7c7 may be estimated as 7 X This corresponds t o about 35 gas kinetic collisions, as a minimum, which are required to self-quench 3S0. Naturally if precursors to 3S0 other than those lS, molecules) are lost by other which fluoresce (and (15) A. B. Callear, Chem. SOC.Ann. Rep., 61, 48 (1964). (16) R. B. Caton and A. B. F. Duncan, J. Amer. Chem. Soc., 90. 1945 (1968). Volume Y.9,Number 4 April 1969
H. D. METTEE
1076 channels k7 would be reduced accordingly. The results of most other w ~ r k e r s ~ ~ indicate J ~ J ' this is happening so that perhaps several hundred to several thousands of gas kinetic collisions are required to collisionally quench For the present purposes it is possible to estimate a limiting, high pressure slope for Ipo/Ip as
Ipo/Ip = constant
If the same assumption is again made, then I p o / I p= constant
+
k?t
-* Jc7'
[SI CM1
and values of k,' may be estimated. Converting these values in the usual way into a quenching probability per gas kinetic collision, one finds a minimum of 100 to 200 collisions required for most partners except NO to remove %,. Because of the varying correctness of the major assumption with each M and its questionable validity to begin with, further comments are deferred.
Conclusion The present collision quenching efficiencies for foreign gases with electronically excited 'SO2, and those meassured by others for I*, NOz, and OH, are found to be adequately predicted by use of only the polarizability
The Journal of Physical Chemistrv
of the quenching partner. It would thus appear that the dependence of quenching efficiency on the duration of the collision is unnecessary in these cases. If this relationship is physically meaningful then the collision quenching of fluorescence may involve an induced dipolar field of sufficient strength to enhance an intersystem crossing process. Steinfeld's formal expression for q ~ e n c h i n gwhich ,~~ is based on conventional scattering theory that leads to a reduced mass term in the final expression, does not specify a quenching mechanism.18 In spite of this the formalism may be to treat the collision quenching of fluorescence, at least as for Iz in the case of the breakdown of molecular symmetry caused by a van der Waals interaction. On this basis a polarizability relationship is not surprising but a satisfying quantum mechanical description of these collisions has yet to be achieved.
Acknowledgments. The author is extremely grateful to The Robert A. Welch Foundation for their support of this work and to Dr. W. Albert Noyes, Jr., for his continued interest. The author also thanks the many colleagues at the University of Texas with whom this work was discussed. Finally the cooperation of Drs. Rao, Collier, and Calvert for sending a preprint of their work is gratefully acknowledged. (17) 9. J. Strickler and D. B. Howell, J. Chem. Phys., 49, 1947 (1968). (18) It should be added that a reduced mass term may be correct, but that it might not always be detectable.
