J. Phys. Chem. 1982,86,696-699
696
Not taken into account in the present discussion are the possible effects of avoided crossings of potential curves. As rotational motion is required to couple states of Q = 0 and R = 1symmetry, the major perturbations would be associated with the crossings of states possessing the same symmetry. Such phenomena have been invoked to explain the wavelength dependence of atomic species formed in the photodissociation of T1Br18 and might be expected in the present case where the manifold of atomic electronically excited states is relatively dense. For example crossing of the O+ state correlating with T1* + I (corresponding with the D continuum shoulder) by the O+ state associated with the A3110+ XIZo+ bands above the threshold for formation of T1+ I but below that for TI+ I- could explain the minor pathway to ground-state thallium throughout the 238-290-nm region. The efficiency of such a process could in principle be calculated at various excitation energies, but the apparent weak wavelength dependence of the minor pathway (Figure 3) and poor characterization of the highly repulsive region of the 3110+state would make this difficult. The states presented in Figure 5 do account for the major absorption features of the T1I spectrum in the region studied. Finally, the current experimental results demonstrate that photodissociation of TlI in the middle ultraviolet does offer the opportunity for significant energy storage in
-
+
(18)M.S. de Vries, N. J. A. van Veen, T. Baller, and A. E. de Vries, Chem. Phys. Lett., 75,27 (1980).
optically metastable atomic thallium. It should be possible to construct a relatively high repetition rate thallium atom photodissociation laser operating at 1280 nm by optical excitation anywhere in the region 240-290 nm where the yield of T1* is well in excess of the threshold of @* = 0.67 required for the stimulated emission T1 62P31? 6'P1/2. Because of the unfavorable degeneracy ratio in thallium where J = 312 for the excited state and J = 112 for the ground state, the thallium laser will not match the efficiency of the iodine atom photodissociation laser in which the state degeneracies are reversed and @ = 0.33 represents the threshold for stimulated emission. The optical pumping of TI1 and subsequent production of T1* could be used in frequency upconversion processes involving stimulated Raman processes.lg The yield of T1* following T1I photolysis at 248 nm is significantly higher than in the photolysis of TlCl at the same wavelength9and much higher than measured20for TlBr photolysis at 266 nm. Further studies of thallium halide regeneration in sealed systems will indicate whether such processes are, in fact, practical.
-
Acknowledgment. This work was supported by the National Science Foundation (CHE-80-04125). This work was carried out in the Cornel1 Facility for Laser Spectroscopy. (19)R. L.Carman and W. H. Lowdermilk, Phys. Rev. Lett., 33,190 (1974). (20)J. C. White and G . A. Zdasiuk, J. Chem. Phys., 69, 2256 (1978).
Temperature Dependence of the Ozone Absorption Coefficient in the Hartley Continuumt D. C. Astholr, A. E. Croce,' and J. Troe' Instnut fur Physikalische Chsmie der Universltat GGttingen, D 3400 GGttingen, West Germany (Received: June 29, 1981: In Final Form: August 12, 198 1)
The absorption coefficient of O3 has been measured in the spectral range 210 IX I320 nm at 300 IT I1050 K in shock waves. The temperature dependence of the spectrum is represented by an empirically modified Sulzer-Wieland expression.
Introduction The absorption coefficient of ozone in the Hartley band, because of its importance for atmospheric photochemistry, has been measured with great precision by several authors.l-1° The band is characterized by a broad continuum, with a maximum absorption around 255 nm, and a number of much weaker diffuse structures superimposed on this continuum which extend into the range of the Huggins bands at 300-350 nm. The temperature dependence of the spectrum has been measured at 180-320 K by Vigroux? Simons et al.," and others. Most of the small variations in the absorption coefficient observed in this range were attributed to changes in the shape of the superimposed structures, in particular to a lowering of the minima at lower temperatures. Little information was 'Dedicated to S. H. Bauer on the occasion of his 70th birthday. leave from Instituto de Investigaciones Fisicoquimicas TeBricas y Aplicadas, Universidad Nacional de La Plata, Argentina.
* On
0022-3654/82/2086-0696$0 1.25/0
available on the temperature dependence of the underlying Hartley continuum itself." We have, therefore, undertaken a study of the temperature dependence of the absorption coefficient of the Hartley continuum from 300 to about 1000 K. Over this large range, the temperature (1)E. C. Y. Inn and Y. Tanaka, J. Opt. SOC. Am., 43, 870 (1953). (2)E. Vigroux, Ann. Geophys., 25,169 (1969),and earlier work cited herein. (3)A. G. Hearn, Roc. Phys. SOC., 79,932 (1961). (4)W. B. De More and 0. F. Raper, J. Phys. Chem., 68,412 (1964). (5)M. Griggs, J. Chem. Phys., 49, 857 (1968). (6)For summaries and discussions of earlier measurements of the ozone absorption coefficient in the Hartley band see, e.g., ref 7-9. (7) M. Ackermann in 'Mesospheric Models and Related Experiments", G. Fiocco, Ed., Reidel Publishing Co., Dordrecht, 1971. (8) M. Nicolet, "Etude des r6actions chimiques de I'ozone dans la stratosph8ren, Institut Royal MBGorologique de Belgique, 1978. (9)D. L.Baulch, R. A. Cox, R. F. Hampson, J. A. Kerr, J. Troe, and R. T. Watson, J. Phys. Chem. Ref. Data, 9,295 (1980). (10)W. M. Jones and N. Davidson, J. Am. Chem. Soc., 84,2868(1969). (11)J. W.Simons, R. J. Paw, H. A. Webster, and E. J. Bair, J. Chem. Phys., 59,1203 (1973).
0 1982 American Chemlcal Society
T Dependence of
the Ozone Absorption Coefficient
effects on the continuum become clearly visible and can he separated from the temperature effects on the diffuse superimposed bands. A study of the temperature dependence of the Hartley continuum over the largest accessible temperature range is important also for another reason. During the recombination of 0 atoms with Oz,'2-L9as well as in other experiments where excited 0,molecules are produced,2O remarkable changes of the 0, absorption spectrum in the Hartley continuum have been reported. These have been attributed to the spectra either of vibrationally or of electronically excited 0, species."" Recent experiments by Kleindienst et al.17-L9appear to favor unequivocally the origin from vibrationally excited electronic ground-state species. The detailed analysis of time-resolved ohservations of such "hot spectra" could provide some insight into the stepwise collisional stabilization of 03*species formed during the recombination. Such studies are of particular interest in relation to unimolecular rate theory as applied to the 0, =+ 0 + Ozsystemz7and to trajectory studies of 03*energy transfer." A necessary condition for such an analysis is the understanding of the spectra of the various highly excited 03* states. Thermal excitation studies can help to analyze these absorption spectra of excited states. Here, well-defined populations of excited states are produced. Unfortunately, for small polyatomic molecules, only small average excitation energies are accessible in thermal excitation experiments leaving a large gap up to the energies of states near to the reaction threshold. Nevertheless, at least trends in the spectrum become visible, and also quantum-chemical calculations can he controlled (see, e.g., T dependence of the HOPspectrum=). The situation is more favorable for polyatomic molecules with many oscillators, where the accessible thermal average excitation energies range up to values above reaction threshold enFor these molecules the use of hot electronic e~gies.~O.~l absorption spectra has become quite common for the direct observation of unimolecular isomerization,31-34unimolec-
(12) V. D. Baiamonte, D. R.Snelling, and E. J. Bair, J. Chem. Phys., 44, 673 (1966). (13) C. J. Hochanandel, J. A. Ghorrnley, and J. W. Boyle, J. Chem. Phys., 48, 2416 (1968). (14) J. F. Riley and R. W. Cahill, J . Chem. Phys., 52, 3297 (1970). (15) P. L. T. Bevan and G. R. A. Johnson, J. Chem. Soe., Faraday Trans. 1.69, 216 (1973). (16) C. W. van Rasenberg and D. W. Trainor, J. Chem. Phys., 61,2442 (1974); 63,5348 (1973). (17) T.Kleindienst. J. R. Locker, and E. J. Bair, J. Photochem., 12, 67 (1980). (18) T.Kleindienat and E. J. Bsir, Chem. Phys. Lett., 49,338 (1977). (19) T.Kleindienst, J. B. Burkholder, and E. J. Bair, Chem. Phys. Lett., 70. 117 (1980). (20) I. C. McLla.de and W. D. MeGrath, Chem. Phys. Lett., 72, 432 (1980); 73,413 (1980); S. M. Alder-Golden and J. I. Steinfeld, ibid., 76, 479 (1980). (21) R.J. Celotta, S. R. Mielcuuek, and C. E. Knyatt, Chem. Phys. Lett.. 24,428 (1974). (22) N. Swanson and R. J. Celotta, Phys. Re". Lett., 35,783 (1975). (23) P. J. Hay, T. H. Dunning,and W. A. Goddard, Chem. Phya. Lett., 23, 457 (1973). (24) P. J. Hay and T. H. Dunning, J. Chem. Phys., 67, 2290 (1977). (25) R. T.Pack, J. Chem. Phys., 65.4765 (1976). (26) E. J. Heller, J . Chem. Phyr., 68, 3891 (1978). (27) J. Troe, J . Phys. Chem., 83, 114 (1979). (28) A. J. Stace and J. N. Murrell, J. Chem. Phys., 68, 3028 (1978). (29) S. R. Langhoff and R. L. Jaffe, J. Chem. Phys., 71,1475 (1979). (30) D. C. Astholz, L. Brouwer, and J. Troe,Ber. Bunsenges. Phys. Chem., 85,559 (1981). (31) H. Hippler, K. Luther, and J. Troe,Foradoy Discuss. Chem. Soc., 67, 173 (1979).
The Journal of phvsical Chemistry, Voi. 86, No. 5, 1982 6B7
*
-~
1 H
200 p s
t
Flgure 1. Absorption of Os behind incaent and reflected shmk wave = 490 K. = 730 K. [OJ/[Ar] = 0.33 x io3. [Ar], = 1.8 x mol/L, [ArI5 = 3.0 X mol/L. absorption behind reflected wave = 50%).
(re
r,
ular bond fmsion,= and energy transfer processes of highly excited specie^.^,^^ Experimental Technique In our experiments, the UV absorption spectrum of ozone has been investigated behind incident and reflected shock waves. Pure ozone was produced in a commercial ozonizer and adsorbed a t -78 "C in a silica gel packed Then, mixtures of ozone in Ar a t concentrations of 3OC-2500 ppm were prepared in 100-L mixing vessels made of Pyrex glass. The concentration of this mixture was controlled by ahsorption measurements in a Cary 17D spectrometer directly before each experiment. A value of the absorption coefficient a t 254 nm of 4254 nm, 25 "C) = 2954 L mol-' cm-' from ref 5 was used as reference value. This value was verified, together with a measurement of the wavelength dependence of the absorption coefficient, in separate experiments in a Cary 17D spectrometer with simultaneous pressure control by a Setra capacitance manometer. The 0,-AI mixtures were introduced into a conventional stainless steel shock tube which has been described ea1lier.3~ After introduction of the mixture into the shock tube, the 0, concentration again was controlled through quartz windows close to the reflecting end plate of the shock tube. There was always a noticeable loss of 0, from the mixture by wall adsorption. For absorption measurements in the shock tube, a Xe high-pressure arc lamp (Varian VIX 150 W) was used as the light source. The analysis light was blocked until directly before each experiment in order to prevent any photolysis of O3before the experiment, even of gas samples which are moved away from the windows by the incident shock waves. After traversal of the shock tube, the light beam was passed through a gating monochromator (Oriel, Model 7240) with a spectral hand width of typically 3 nm (fwhm). Light intensities were monitored by a photomultiplier and an oscilloscope. Since the absorption coefficient of 0, had to be measured with great precision, in order to make visible even (32) H. Hippler, K. Luther, J. Roe, and R. W&h, J. Chem. Phys., 68, 323 (1976). (33) H. Hippler, K. Luther, and J. Roe, to be published. (34) D. Dudek, K. Ghnzer, and J. k, Ber. Bunsenges. Phys. Chem., R. .?. , 77fi . .-,7RR .. .11974) ,. ...,. (35) H. Hippler, V. Sehubert, J. Troe, and H. J. Wendelken, Chem. Phys. Lett., 84, 253 (1981). (36) Yu. A. Kudriavtsev and V. S. Letokhov, Chem. Phys., 50, 353 ll9ROI ~.--.,. (37) H. Hippler, J. Roe,and H. J. Wendelken, Chem. Phys. Lett., 84, 257 (1981). (38) P. N. Clough and B. A. Thrush, Chem. Ind., 19, 1971 (1966). (39) D. C. Astholz, J. Troe,and W. Wieters. J. Chem. Phys., 70,5107 (1979).
698
The Journal of Physical Chemistry, Vol. 86, No.
5, 1982
small changes with temperature, great care had to be taken to analyze also the smallest details of the oscillograms recorded in the shock wave experiments. Figure 1shows a typical oscillogram demonstrating the problems encountered. The oscilloscope is triggered before the arrival of the incident shock wave such that the first part of the trace corresponds to the room temperature absorption of the introduced 03-Ar mixture. After passage of the incident shock wave, there is a step in the absorption profile with, in some cases, a marked Schlieren signal. After this step one observes a relatively long further increase of the absorption, before the reflected shock arrives. After the corresponding second step of the absorption signal, ozone decomposes completely and no absorption signal remains. Behind the reflected shock wave we never observed a further increase of the absorption signal like that behind the incident wave. Depending on the ozone concentration, the decay of O3 behind the reflected wave (or, at sufficiently high temperatures, behind the incident wave) was either first order (at low [03]/[Ar]) or accelerated (at high [03]/[hl). At high [ 0 3 ] / [ the h ] apparent , first-order rate constant of the decay during the reaction increased markedly by more than a factor of two. The interpretation of these observations is the following: during the introduction of the 03-Ar mixture into the shock tube and the time until the shock wave arrives, there is considerable adsorption of O3 at the walls as shown by the continuous decrease of O3 concentration monitored via the O3 absorption. In the gas flow behind the incident shock wave, some part of the ozone adsorbed at the wall of the shock tube is scrapped from the walls and windows and reappears in the gas phase. A similar behavior was observed earlier in our laboratory in high-pressure shock waves, where aluminum dust particles originating from the diaphragm bursting stick firmly to the wall of the shock tube, and nevertheless can be scrapped from the wall in the flow behind the incident wave. The absorption rise behind the incident wave in Figure 1,therefore, is attributed to a change in O3 concentration by wall desorption. No such effect was observed behind the reflected waves where the gas is at rest. A contribution to the change of absorption by vibrational relaxation of O3 in Ar can be ruled out. From measurements of the O3 relaxation in He and Ar at room temperature and the measured temperature coefficient for He40 one estimates, in comparison to the relaxation of other triatomic molecule^,^^ that behind the incident wave the relaxation is over after at most a few microseconds. The preceding interpretation of Figure 1 suggests the following evaluation of the absorption signals: The zero absorption level is given by the level at the end of the O3 decomposition; the ratio t 2 / t 1 of absorption coefficients at temperature T2behind the incident wave and room temperature follows from the absorption step directly behind the incident wave, before scrapping off from the walls sets in; the ratio eg/e2 of absorption coefficients at temperature 16 behind the reflected wave and T2follows from the absorption step directly before and behind the reflected wave. For both measurements, of course, the relevant p2/p1 and p 6 / p 2 ratios of the shock are also taken into account. In the present work, we did not further evaluate the O3 dissociation rates. The rate constants in the initial period of reaction corresponded well to earlier experiments such as ref 42. The acceleration of the decomposition is due (40)J. Moy, C.-R. Mao, and R. J. Gordon, J. Chern. Phys., 72,4216 (1980). (41)B. Stevens, "Collisional Activation in Gases", Pergamon Press, Oxford, 1967.
Astholz et al.
TABLE I: Absorption Coefficients of 0,"
E / ( Lmol-' cm-') 300 K
hinm 210 215 220 225 230 235 240 24 5 250 254 255 260 265 270 27 5 280 285 290 295 300 305 310 315 3 20 325 a
500K
156 280 530 830 1200 1680 2150 2510 2810 2954 2900 2770 2580 2190 1660 1150 7 20 417 217 112 54 30 15.5 9.5
720K-
900K
20 1 343 468 786 1090 1370 2100 2200 2335 2415
162
367 1540 2740 2623
392 1030 1310 1570 1870 2060 2037 2050 1780 1760 1400 1260 946 642
2650 2240 2220 1850 1125 1040 750 517
2290 1090 553 173
361 274 212 130 87 73
238 158 126 79 49
47 29
See eq 1. X/nm
2M
10'
2LO
260
,
280
300
I
I
320
1i
,-
E
10'
'
l
46000
_ -L !--
LLOOO
12000
LOOOO
38WO
36000
34000
32000
-
v / cm-'
Flgure 2. Absorption coefficients of 0,: (0)T = 300 K, (0)T = 500 K, (A)T = 720 K, (0) T = 900 K, (full lines) numerical representation of eq 2-4.
-
to the fast consecutive reaction 0 + O3 202 and to a temperature rise of the gas by the exothermicity of this reaction which is not negligible for the relatively high O3 concentrations of the present experiments. Results As described above we measured 0, decadic absorption coefficients E
=
1
-log
[o&
(;)
( x in cm, [O,] in mol/L), often realizing the same temperatures behind incident and reflected shock waves. Our results are given in Table I and demonstrated in Figures 2 and 3. The absorption continuum broadens considerably with increasing temperature with an increase of e in the (42)H.Endo, K.Glhzer, and J. Troe, J . Phys. Chern., 83,2083(1979).
T Dependence of the Ozone Absorption Coefficient
The Journal of Physlcal Chemlstty, Vol. 86, No. 5, 1982 699 I
enom[ tanh
(
e-[
-tanh
( 2)(3;;;)] v - Uno
(3) and tb describes an additional broadening at lower wavelengths in the form
0
)
P 1500'
300
I
1
I
500
700
900
T/
L !
uno
1100
red wing and a decrease at the center of the band. The spectrum becomes increasingly unsymmetric; at the short wavelength wing of the band no increase of e with temperature is observed. Our room temperature values of c agree with those from ref 5. The present values of e at high temperatures have all been measured relative to the room temperature values. Our results on the temperature dependence of E at 297.5 and 248.5 nm also well agree with previous results by Jones and Davidson'O who measured the T dependence of c in shock waves a t these two wavelengths only. For these two wavelengths we have derived
t(T)/t(300 K) = 1 - 3.9 X 10-4(T- 300 K)/K
+ 2.8 X 10-3(T- 300 K)/K
at 248.5 and 297.5 nm, respectively, and over the temperature range 300-1000 K,whereas
e(T)/e(300 K) = 1 - 3.5 X 10-4(T- 300 K)/K c(T)/e(300 K) = 1
+ 4.0 X 10-3(T - 300 K)/K
was obtained by Jones and Davidson. At temperatures below 300 K,where other measurements of the temperature dependence have been the character of the temperature dependence changes. From 300 to 180 K not only a slight narrowing, but also a slight decrease of the absorption at the center of the continuum near 250 nm is observed which does not directly correlate with the behavior of above 300 K. We represent the decrease of e with T for the center of the continuum at 254 nm in Figure 3. Points from incident and reflected waves coincide satisfactorily. Apart from the tabular and graphical representation of t in Table I and Figures 2 and 3, we give in the following a numerical representation of e(v,T). We base this representation on the simple SulzeI-Wieland but we keep in mind that this approach has been designed for diatomic molecules, and does not, in general, give satisfactory results for polyatomic molecules. Several types of empirical modifications have been applied (see, e.g., ref 30, 43, and 44). For the present case of a fairly unsymmetric absorption profile we use a two-term expression e(v,T)
=
ea(v,T)
+ cb(v,T)
= 38800 cm-'
€born =
K
Flgve 3. Absorptbn coefficients of O3at 254 nm. (Experiments: (0) incident shock waves; (0)reflected shock waves; (full line) numerical representation of eq 2-4.)
e(T)/e(300 K) = 1
The corresponding eight fitting parameter are egom = 3105 L mol-' cm-l Oae = 900 K
(2)
where e, is given by the simple Sulzer-Wieland equation (43) P. S h e r and K. Wieland, Helu. Phys. Acta, 211, 663 (1962). (44) L. Brouwer and J. Troe, Chern. Phys. Lett., 82, 1 (1981).
Vbo
Avao = 2900 cm-'
1625 L mol-' Cm-'
= 43000 Cm-'
AVbo
Obe
= 660 K
= 3300 Cm-'
Equations 2-4 with the eight fitting parameters give a good representation of the measured spectra at 220 IX I320 nm in the given temperature range. At the lowest wavelengths, the measurements apparently deviate from the representation, but they also show a larger scatter. One might be tempted to interpret the two terms e, and t b by contributions from two electronic transitions. However, such a conclusion could only be made by an accurate quantum-chemical calculation. Also the significanceof the parameters 6,, and Avao for the main component of the spectrum, in terms of main steepness of the upper potential surface and the corresponding vibration of the ground state, has to wait a more detailed quantum-chemical treatment similar as done for H02.29 In any case we would like to emphasize strongly that eq 2-4 for the present case of a polyatomic molecule are completely empirical expressions derived from a nontheoretical model. They are chosen only as a representation of the experimental data, and any use outside the given temperature range should be done with great precautions. In particular, an extension to temperatures below 300 K should include additional effects from the diffuse bands which were discussed in ref 10. It should be pointed out that the present spectra show a small temperature dependence of the integrated absorption strength (decrease by a factor of 1.03 at 720 K, and by a factor of 1.21 at 900 K).We have observed such temperature effects (decreases or increases) also for other molecules (ref 30). Although we have not found any evidence for experimental errors in the present work, and our results agree with ref 10, the temperature effect on the integrated absorption is small and remains at the limit of experimental detectability.
Conclusions At temperatures above 300 K,the Hartley continuum broadens considerably. The present experiments give a numerical representation of the temperature-dependent absorption coefficients by an empirically modified Sulzer-Wieland equation. The present data and their extrapolation to higher excitation energies are of use in the interpretation of transient UV spectra such as observed in the early stages of the 0 + O2 O3 recombination reaction. Acknowledgment. Discussions with P. J. Crutzen and E. J. Bair, assistance by L. Brouwer, and financial support of this work by the Deutsche Forschungsgemeinschaft are gratefully acknowledged. A. E. Croce also thanks the Consejo Nacional de Investigaciones Cientificas Y Tgcnicas de la R. Argentina for a foreign exchange fellowship.
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