J . Phys. Chem. 1987, 91, 2134-2738
2734
Summary and Conclusions The fluorescence anisotropy is very high and constant with excitation wavelength in the 265-nm band of uncharged thymine, uracil, and UMP in aqueous solution at r c ” temperature. Thus, no evidence for an n r * state is observable, probably due to its weakness in absorption. However, at physiological pH (pH 7.3) the small amount of fluorescent anion (pK, = 9.9) causes a
spurious drop in the anisotropy when exciting at the long wavelength edge. To observe the true anisotropy of the uncharged species alone requires a lower pH (pH 4-6) for thymine. In room temperature water the high anisotropy for these compounds also rules out the possibility that emission is by a lower lying transition, too weak to be seen in absorption. (The absence of excitation wavelength dependence cannot alone rule out this possibility if the weak emitting transition moment was parallel to the strongly absorbing one.) However, a significantly forbidden transition would exhibit a much longer natural radiative lifetime and thus a much longer fluorescence lifetime, leading to significantly lower anisotropies due to rotational d i f f ~ s i o n . ~
(24) Callis, P. R. Photochem. Photobiol. 1986, 44:3, 315. (25) Chargaff, E., Davidson, J. N., Eds. The Nucleic Acids; Academic: New York, 1955.
Acknowledgment. We thank Dr. Leigh Clark for making the 1-methyluracil work available to us prior to publication. This work was supported by the National Institutes of Health (GM 31824) and the National Science Foundation (ISP8011449).
coming below the strong 260-nm band, even when the M N scheme was used.24 We therefore feel that the theoretical basis for expecting multiple mr* states within the 260-nm band of uracils is weak at present.
Absorption Cross Sections for OClO as a Function of Temperature in the Wavelength Range 240-480 nm Andreas Wahner, Geoffrey S . Tyndall, and A. R. Ravishankara* Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80303, and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309 (Received: October 20, 1986; In Final Form: January 23, 1987)
The absorption spectrum of OClO has been measured by using a diode array spectrometer with a resolution of 0.25 nm at 204,296, and 378 K in the wavelength range of 240-480 nm. Absolute absorption cross sections were determined by measuring OClO concentration using two independent methods. The general features of the spectrum at 296 K agree with previous observations. The individual bands of OClO get sharper and the peak cross sections increase as the temperature is lowered from 378 to 204 K.
Introduction
Symmetric chlorine dioxide, OCIO, is believed to play an important role in the stratospheric chlorine chemistry when the mixing ratio of total chlorine exceeds approximately 10 ppb’,2a condition commonly referred to as the high chlorine scenario. OClO is expected to be formed via reactions3 such as
+ CIO C10 + BrO C10
-
+ C1 OClO + Br OClO
Recently, the role of OClO has taken on added importance because of the hypothesized involvement of C10, and BrO, compounds in the annual spring depression of O3 column abundance in the Antarctic ~tratosphere.~The most likely method for detection of OClO in the stratosphere involves long-path near-UV absorption, since OClO has a very structured intense absorption in this region5 and would be identifiable amidst the continuous absorption of other atmospheric species. To this end, accurate UV absorption cross sections are needed at temperatures prevalent in the stratosphere. Also, the cross sections are needed to calculate the rate of photolytic destruction of OClO in the stratosphere and for laboratory studies of reactions involving OClO where it is monitored via its UV absorption. There have been many studies on the near-UV spectroscopy of OClO which were aimed mainly at elucidating its structure.+” However there are three investigations in which the absorption cross sections were measured at room temperature. Coon et al.’ measured the cross sections of a few bands near the band origin at 476 nm. In a kinetics study Clyne and CoxonI2obtained a cross section of I . 1.5 X I O-I7 cm2 at the 35 1.5 nm maximum and Basco
and Dogra13 found a similar value (1.19 X lo-’’ cm2). Birks et al.I4measured the absorption spectrum between 300 and 464 nm but did not report the absolute cross section. There are no systematic studies of the absorption cross sections as a function of temperature. In the present study, the absolute cross sections of OClO were measured in the wavelength region 240-480 nm at 204, 296, and 378 K. Experimental Section OClO was prepared by flowing a mixture of Cl, and N, at atmospheric pressure through a U-tube packed with NaCIO, and ( I ) Prather, M. J.; McElroy, M. B.; Wofsy, S. C. Nature (London) 1984, 312, 227. (2) Cicerone, R. J.; Walters, S.; Liu, S. C. J . Geophys. Res. 1983,88, 3647. (3) Yung, Y. L.; Pinto, J. P.; Watson, R. T.; Sander, S. P. J . Atmos. Sci. 1980, 37, 339. (4) McElroy, M. B.; Salawitch, R. J.; Wofsy, S. C.; Logan, J. A. Nature (London) 1986, 321, 759. ( 5 ) Watson, R. T. J . Phys. Chem. Rex Data 1977, 6, 871. (Caution: Figures are mislabeled in this reference. The correct OClO spectrum is on p 908.) (6) Coon, J. B.; Ortiz, E. J . Mol. Spectrosc. 1957, I , 81. (7) Coon, J. B.; DeWames, R. E.; Loyd, C. M. J . Mol. Spectrosc. 1962, 8, 285. (8) Richardson, A. W.; Redding, R. W.; Brand, J. C. D. J . Mol. Specrrosc. 1969, 29, 93. (9) Brand, J. C. D.; Redding, R. W.; Richardson, A. W. J. Mol. Spectrosc. 1970, 34, 399. (10) Sakurai, K.; Clark, J.; Broida, H. P. J . Chem. Phys. 1971, 54, 1217. (11) Curl, Jr., R. F.; Abe, K.; Bissinger, J.; Bennett, C.; Tittel, F. K. J . Mol. Spectrosc. 1973, 48, 72. (12) Clyne, M . A. A.; Coxon, J. A. Proc. R . SOC.London, Ser. A 1968, A303, 207. (13) Basco, N.; Dogra, S. K. Proc. R. Soc. London, Ser. A 1971, A323, 29.
*Address correspondence to this author at NOAA/ERL, R/E/AL2, 325 Broadway Boulder, CO 80303.
0022-365418712091-2734$01.50/0
(14) Birks, J. W.; Shoemaker, B.: Leck, T. J . ; Borders, R. A,: Hart, L. J.
J . Chem. Phys. 1977, 66, 4591.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2135
Absorption Cross Sections for OClO
Fiat Focal Field SpecrrOmeIsr
I TEMPERATURE COIFFROUTR, R U S H GAS
PUMP
FLUSH GAS
THERMOCOUPLE
-i
TLa I
t Xe LAMP
t APERTURE
PRESSURE
5 0 MICRON SLIT
1
COMPUTER(
RESERVOlR
N2
Figure 1. A schematic diagram of the experimental setup used to measure the absorption cross section of OClO as a function of temperature.
glass beads and maintained at room t e m p e r a t ~ r e . 'The ~ effluent from the U-tube was diluted with N 2 to produce a mixture of approximately 3% OClO in N2 and collected in a blackened 12-L Pyrex storage bulb. The OC10/N2 mixtures were always used within a few hours of their preparation. N o evidence was found for gradual decomposition of OClO in these mixtures. For preliminary experiments and relative cross section measurements, the OC10/N2 mixtures were used without any purification. For experiments where absolute cross sections were determined OClO was purified in a separate gas handling manifold as follows. OClO from an OC10/N2 mixture was first condensed in a trap cooled by liquid N2. Most of the N 2 was pumped out. The trap was warmed to 195 K and the remaining N2,all of the C12, and small amounts of OClO were pumped out. Then OClO was distilled over to a trap cooled to liquid N 2 temperature leaving behind a very small amount of an unidentified dark orange solid and a translucent solid (presumably H20). Upon purification, a known pressure of OClO was metered into the darkened 12-L bulb and diluted in U H P N 2 to produce a 1% mixture of OC10. It was determined that the purity of OClO eluting from the NaC102 trap was approximately 90%, the remainder being mostly C12 and H20. A diagram of the apparatus used to measure the absorption spectra and the cross sections of OClO is shown in Figure 1. It consisted of an OClO preparation section, a temperature controlled absorption cell through which gases were flowed, a dc xenon arc lamp, and a diode array spectrometer. The preparation and handling of OClO was described above. The absorption cell could be maintained at a known constant temperature by circulating a fluid through its outer jacket from a reservoir whose temperature was controlled. The temperatures of the fluid at the entrance and the exit of the outer jacket were measured with thermocouples. The difference in temperature between the two ends was never greater than 2 OC and the average of these two temperatures was taken to be the cell temperature. The cell was equipped with quartz windows that restricted the light path through the 50cm-long cell to the constant temperature region (see Figure 1). When the cell was cooled, dry nitrogen gas was sprayed on the windows to prevent condensation. Light from the xenon lamp was passed through the absorption cell and focused on the entrance slit of the spectrometer. Before entering the cell, the light beam was passed through a variable aperture to control the light flux into the cell and to ensure that no part of the detected beam was reflected by the walls of the
cell. A lens was placed at an appropriate distance from the 50-pm-wide entrance slit so as to uniformly illuminate the 1200 grooves/" holographic grating (blazed at 300 nm). The dispersed beam exited the spectrometer and was detected by a cooled array of 1024 diodes placed in the flat focal field of the 0.28-m crossed Czerny-Turner spectrometer. The diode array was run by a commercial controller. The data accumulated in the controller were transferred to a personal computer in which all data manipulations were carried out. The dispersion of the grating used was such that a range of 70 nm was detected by the array at a given orientation of the grating. The 70-nm range translated to approximately 15 diodes per nanometer. The resolution of our spectrometer for the present study was 0.25 nm (full width at half-maximum), as determined by recording the profiles of the 253.65, 312.57, and 404.66 nm atomic mercury lines from a low pressure lamp. For each orientation of the grating, the system was recalibrated for wavelength assignment. Wavelength calibrations were carried out by using mercury and cadmium atomic emission lines. For each 70-nm segment, at least two well-separated emission lines were recorded and wavelengths between the atomic line positions were obtained by interpolation. The wavelength scales are all accurate to better than 0.1 nm (one diode). To measure a spectrum, the diode array pattern noise (which is a composite of dark current and other electrical noise) was first recorded. Then Io, the intensity of the light transmitted through the empty cell, was recorded. A variable aperture in front of the spectrometer allowed the intensity of the beam reaching the detector to be varied to a reading which was approximately 90% of the full scale at the most intense wavelength. (This Io was constant to within f0.2% for hours, indicating that both the lamp and the spectrometer system were stable.) Then a sample whose spectrum was to be recorded was flowed at a known constant rate and I , the transmitted intensity, was measured. Each of the light intensity measurements involved summing 100 17-ms exposures. At X C 300 nm, due to a very sharp drop in the xenon lamp intensity, the exposure times were up to 200 ms long. Upon completion of these measurements, the pattern noise was subtracted from both I, and I and the ratio I / I o calculated for each diode, from which the absorbance at each diode, i.e. wavelength, was calculated. All experiments were carried out in a slow flow mode to avoid excessive photolysis of the gas in the absorption cell. The flow
2736 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 TABLE I: List of Reactions Used in Simulating OClO Titration Experiments reaction k(298 K V OClO + NO --* NO2 + CIO 3.4 x 10-13 C10 NO C1 + NO2 1.7 X lo-” CI + OClO CIO + CIO 5.9 x 10-11 CI + NOz 3 CINOz 1.5 X CI + NO 3 ClNO 9.0 x 10-32 1.8 x 10-31 CIO + NO2 3 CION02 CI + ClNO NO + C1, 2.3 X IO-”
-
+
+
“All data were taken from DeMore et For termolecular reactions M is taken to be nitrogen. Units for bimolecular reactions are cm3 molecule-’ s-’. Units for termoiecular reactions are cm6 s-1.
rate through the cell was approximately 6 cm sui so that the entire volume of the cell was replenished with a new mixture in approximately 7 s. Absolute cross section measurements were carried out in the wavelength region of 350-430 nm. The concentration of OClO in the absorption cell was determined by two independent methods, one based on pressure measurement, and the other on a chemical conversion. Emphasis was placed on OClO concentration determinations since OClO is a labile species. In the first method, the OClO concentration was calculated by manometrically preparing a 1% mixture of purified OClO in N2 and diluting a known flow of this mixture with a known flow of N2 (usually 1:lO). The OClO absorbance was then related to the cross section via the Beer-Lambert law by using the calculated concentration. The second method of determining the cross section, which did not depend on a measurement of the OClO pressure, was to add up to 5 Torr of NO to the mixture of OClO and 40 Torr of N, shortly before it entered the absorption cell, and measure the concentration of NO2 product. N O reacts with OClO to produce C10 and NO,. C10 reacts further with N O to generate another NO,; therefore, for each molecule of OClO removed two molecules of NO2 were produced. NO OClO NO2 C10 C1 NO2 C10 + NO
-
+
+
-+
+
+
net
-
2N0
+ OClO
2N02
+ C1
If C1 atoms produced in the above sequence reacted with OC10, C1+ OClO 2C10, it would still lead to formation of 2 molecules of NO2 and hence the above stoichiometry would hold. However, if C1 reacted with NOz, the yield of NO, p,roduced per OClO lost would be less than two. Therefore, it 1s important to intercept C1 atoms before they react with NO2. C1 atoms were converted to ClNO by using moderate pressures of N 2 (-40 Torr) and fairly large pressures of N O (5 Torr). Computer simulations, using the reactions and rate coefficients listed in Table I, showed that the NO2 yield would be greater than 1.90 for a pressure greater than 30 Torr and [NO]/[OClO] > 100. Therefore, in all titration experiments, 40 Torr of N2and a N O partial pressure of 5.0 Torr were used, such that the [NO]/[OC10] ratio ranged from 200 to 1600, and that the calculated yield of NO2 was very close to 2.00. One series of experiments was carried out where the [NO]/[OCIO] ratio was varied from 50 to 200 at a fixed [OCIO] of 8 X lOI4 ~ m - The ~ . concentration of NOz produced changed by less than 2% over this range. Therefore, we are quite confident that the stoichiometry factor in our experiments was close to 2.00. There are sufficient uncertainties in the values of the rate coefficients listed in Table I that we prefer to rely on our experimental check and make no corrections to the measured cross sections. The NO2concentration was measured from the differential absorption between the minimum at 412.0 f 0.3 nm and the strong peak at 413.5 nm. The cross section for this line was measured to be (2.12 0.01) X cm2, in excellent agreement with a recent measurement of (2.15 f 0.05) X cm2.I5
*
(15) Schneider, W.; Tyndall, G. S.; Burrows, J . P.; Moortgat, G . K. J Photochem, in press.
Wahner et al. 0.5
Q0
0.4
0
\ -c
n
0.3
0
w
0.2
0.1
0.0 0
I
I
I
I
2
4
6
8
t
10
1,
OClO 1 0 1 4 cm-3 Figure 2. A plot of In (I,,/I) measured for the peak of the a(10) band of OClO as a function of OClO concentration at 296 K. The OClO concentration was determined from pressure and flow rates measurements. The length of the absorption path was 43.5 cm. As mentioned earlier, for a given position of the grating, we could observe a wavelength range of 70 nm. The complete spectrum in the wavelength range of 240 to 480 nm was obtained by combining four 70-nm-wide overlapping segments. Since the absolute cross section of the OClO absorption in the wavelength region of 350-430 nm was measured, the cross sections at other wavelengths were normalized to those of the peaks in the 350430-nm region. At each temperature, a composite spectrum covering the range 240-480 nm was obtained. The segments were chosen such that there were at least two OClO bands in each overlap region. These segments were superimposed by using a least-squares fitting technique to optimize the overlap between adjacent sections. The final spectrum was adjusted to give zero cross section at the flat base line near 460 nm. The maximum correction due to these two operations corresponded to not more than 0.5% absorbance units.
Results and Discussion As mentioned earlier, the absolute cross sections were measured by two methods at 296 K: (1) using a manometrically prepared OClO mixture and (2) by reaction of OClO with NO and measuring NO2. In all absolute cross section measurements, (1-8) X 1014 cm-3 of OClO was flowed in the cell. The cell pressure was maintained at -40 Torr by adding UHP N2. The observed absorbance at the peak of the a(10) band [using the notation of Coon and Ortiz: that a(i) refers to the transition A(2A2)(i,0,0) X(2Bi)(0,0,0)] as a function of OClO concentration is shown in Figure 2. It is clear that the Beer-Lambert law is obeyed. The cross section for this band is (1.23 f 0.04) X cm2, where the error represents 2 standard deviations in the measurement of the slope. Similar regression lines were calculated for bands a(4)-49). Of course, we could have gone through this procedure for every single diode in the array. But it is much easier just to convert each absorbance into a cross section once the cross sections for the peaks were known. In experiments where OClO was converted by N O into NO,, plots of OClO absorbance vs. differential NO2 absorbance were generated. Figure 3 shows one such plot for the a( 10) band at 296 K. Again, OClO absorbance varied linearly with NO, differential absorbance. From the measured NO, differential cross section, the absolute cross section of OClO at the peak of the a( 10) line is calculated to be (1.18 0.06) X cm2. The agreement between the two methods is excellent (4% difference) and the average of these two values, 1.20 X lo-’’ cm2, is recommended.
-
The Journal of Physical Chemistry, Vol. 91, No. 1 1 , 1987 2737
Absorption Cross Sections for OClO
TABLE II: Absorption Cross Sections for Peaks of P (0) to P (26) Bands of WlO at 204, 296, and 378 K u (378 f 2 K),” u (204 f 2 K),* u (296 f 1 K),” peak wavelength! nm 1 0 - l ~ cm2 10-l~cm2 cm2 40) 41) 42) 43) 44) 45) 46) 47) 48) 49) 410) 4 1 1) 412) 413) 4 14) 415) 4 16) 417) 418) 4 19) 420) 421) 4-22) 423) 424) 425) 426)
475.53 f 0.50 461.15 f 0.40 446.41 f 0.05 432.81 f 0.05 420.58 f 0.36 408.83 f 0.1 1 397.76 f 0.08 387.37 f 0.08 377.44 f 0.08 368.30 f 0.08 359.73 f 0.08 351.30 f 0.08 343.44 f 0.08 336.08 f 0.08 329.22 f 0.08 322.78 f 0.08 317.21 f 0.08 311.53 f 0.08 305.99 f 0.12 300.87 f 0.23 296.42 f 0.31 291.77 f 0.22 287.80 f 0.10 283.51 f 0.13 279.64 f 0.08 275.74 f 0.10 272.93 f 0.10
0.013 0.017 0.069 0.166 0.304 0.479 0.670 0.844 0.992 1.136 1.219 1.275 1.230 1.139 0.974 0.791 0.618 0.435 0.312 0.219 0.160 0.1 14 0.086 0.072 0.060 0.046 0.033
0.017 0.094 0.220 0.393 0.578 0.821 1.046 1.212 1.365 1.454 1.531 1.507 1.441 1.243 1.009 0.771 0.542 0.393 0.256 0.190 0.138 0.105 0.089 0.073 0.059 0.053
0.016 0.057 0.134 0.250 0.378 0.547 0.698 0.808 0.920 0.984 0.989 0.938 0.864 0.746 0.628 0.516 0.390 0.291 0.216 0.167 0.130 0.105 0.090 0.079
u (296 f 1 K):
cm2
0.301 f 0.018 0.476 f 0.020 0.674 f 0.023 0.851 f 0.027 0.988 f 0.029 1.138 f 0.031 1.203 f 0.034
a Wavelength at peak of the band at (296 f 1 K). *Cross section calculated by normalizing to all peaks listed in the last column. Estimated error f 4 X IOmi9 cm2. cAverageof cross section values measured by titration and manometric methods for OClO concentration determinations.
0.5
on cell walls by varying the linear flow rate of the gas through
a4
0
a 0
0.3
0.2
0.1
0.0
a
0.5
1
1.5
2
Figure 3. A plot of In (Io/l)for the a(l0) band of OClO against In (Io/l) for the NO2differential absorption between 412.0 and 413.5 nm at 296 K. The length of the absorption path was 43.5 cm.
It is estimated that the total error in this value is -7% (20) which includes estimated systematic errors in OClO concentration measurements, NO2 differential cross section, length of the cell, and the temperature. Table I1 lists the measured cross sections for all the a(i) bands (the strongest progression in the OClO spectrum). At 204 and 378 K, the cross sections were derived after the bulb concentration had been determined by absorbance measurements at 296 K. Our experiments at 296 K clearly indicated that the mixtures were stable and the results obtained agreed well with those obtained by the titration method. Therefore, we felt that it is unnecessary to carry out titrations. Experiments were carried out to check for possible thermal decomposition and loss
the cell; none was found. One set of experiments was carried out with a 375-nm long-pass filter placed in front of the xenon lamp. The cross sections for bands a t wavelengths longer than 375 nm were measured and found to be the same as those measured in the absence of the filter. Another experiment was carried out where a known concentration of OClO was filled into the absorption cell and the loss rate of OClO was monitored as a function of time with the cell illuminated. The loss rate was first order s-l. This rate in OClO with a rate coefficient of 2.5 X coefficient, which is a sum of wall loss and photodissociation loss rate constants, indicates that less than 1% of OClO could have decomposed during the course of the cross section measurements. The calculated cross sections obtained at 204 and 378 K are also listed in Table 11. The error given in Table 11 reflects the uncertainties in defining the base line during superposition of the spectra and applies therefore to all wavelengths. The wavelengths given are those of the maximum cross section at 296 K. At the other temperatures the maxima were slightly shifted due to changes in the band structure, the largest effects being for the a(16) and a(17) bands (approximately 0.5-nm shift for going from 204 to 378 K). Figure 4 shows the entire spectrum of OClO at the three temperatures. Integrated band intensities were also calculated over approximately 10-nm windows (corresponding to one of the a(i) bands). The integrated areas remained constant between 204 and 296 K for all bands (within 5%), and between 296 and 378 K for bands below 420 nm. Above 420 nm at 378 K the integrated cross sections were approximately 30% larger due to the increased importance of hot bands. These hot bands were observed only at 378 K. The absorbance of OClO was found to increase linearly with OClO concentration up to an optical density of one. At higher concentrations, the optical density was lower than that predicted from low concentration pints, with the discrepancy getting worse at higher concentrations, Le., it exhibited non-Beer-Lambert law behavior. Such behavior is typical of systems where saturation in absorption is taking place. We checked our spectrometer using neutral density filters and found that it responded linearly to optical densities (OD) up to 6. (At higher ODs, the light level was too low to get consistent results.) Therefore, we are sure that this
2738 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 1.5
1
1 378K
I
rn
296K
I
1.o
0.5
0.0
260
320
380
440
WAVELENGTH, nm Figure 4, Plots of absorption cross sections of OClO as a function of wavelength at 378, 296, and 204 K. Note that the peak cross sections
increase., the widths of bands decrease, and the valleys get closer to the base. line as the temperature is decreased. Small shifts in the wavelengths of the peak positions observed with changes in temperature are not apparent in this figure. phenomenon was observed only due to OC10. One reasonable explanation is that individual rotational lines which are unresolved by our instrument lead to very high optical densities at specific wavelengths, giving an apparent reduction in cross section. Individual rotation line widths have been measured at long wavelengths at high r e s ~ l u t i o n . ' ~ However, ~'~ we do not know what the line widths of individual rotational lines would be at shorter wavelengths since the predissociation lifetime, and hence the line width, does change with energy.l* Another possibility is that OClO dimerizes and the dimer does not absorb in this wavelength region. However, this possibility can be discounted, since measurements made in a long-path (659 cm) cell showed curvature at the same optical density, corresponding in this case to much lower (a factor of 15) OClO concentration. In any case, we did not use optical densities greater than 0.5 in our measurements. The peak cross sections obtained by us are about 10% higher than those measured by Clyne and Coxon,'* and 7% higher than those of Basco and Dogra.I3 These two previous studies relied on pressure measurement for OClO concentration calibration and hence the cross sections. Furthermore, if high optical densities were used they could have had slight non-Beer-Lambert behavior (16) Demore, W. B.; Margitan, J. J.; Molina, M. J.; Watson, R. T.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. 'Chemical Kinetics and Photochemical Data for use in Stratospheric Modeling'', Jet Propulsion Lab, CA, 1985 Publication No. 85-37, (17) McDonald, P. A.; Innes, K. K. Chem. Phys. Lerr. 1978, 59, 562. (18) Michielsen, S.;Merer, A. J.; Rice, S. A.; Novak, F. A.; Freed, K. F.; Hamada, Y.J . Chem. Phys. 1981, 74, 3089.
Wahner et al. resulting in lower cross sections. Considering the sparsity of data in papers by Clyne and Coxon, we feel that their values are in good agreement with our measurements. Our cross sections are also in reasonably good agreement with those obtained for a few bands by Coon et al.' Our experiments were aimed at determining cross sections (as opposed to just band positions) and a great deal of care was taken to make accurate measurements. We have used two methods, one chemical and one manometric, and they agree extremely well with each other. Therefore, we believe that our numerical values should be preferred over others. It should be emphasized that, in regions where rotational structure is resolvable, cross sections will ultimately be resolution dependent, particularly at lower temperatures. Since the rotational line width is of the order of 0.3 cm-l, at least a factor of 10 higher resolution will be required before the cross section will be resolution dependent. Our relative peak heights agree quite well with those of previous investigators at room temperature as well as the measurements of Richardson et aL8 at 193 K. Birks et al.14 obtained a spectrum of OClO at 298 K which they normalized to the value of Clyne and Coxon. Their spectrum shows much larger differential cross sections. It is because they seem to have much smaller values for cross sections at the valleys. The most obvious effect of temperature on the spectrum is the sharpening of the bands and the apparent reduction in intensity of the underlying continuum between 300 and 400 nm as the temperature is reduced. An explanation for this observation is that the continuum is due to many unresolved high J rotational lines, which are not populated as the temperature is reduced. To test this hypothesis, a simple rigid symmetric top rotor model was used to predict the shapes of the vibrational bands at the three temperatures studies and compare them with the measured profiles. Rotational constants for the ground and excited states were taken from Richardson et a1.8 The effects due to coupling of spin to the rotation, the deviation from the symmetric top assumption, and centrifugal distortion were not included. Both 3sCl and 37Cl isotopes were included. The model reproduced the measured full width at half-maximum for isolated bands to within 10%at both 296 and 204 K. However, it was also found that if the "Cl and 37Clbands were separated by more than 10 cm-l the sharpening of the individual bands was masked. Obviously, the observed bands are a complex overlap of rotational lines from several vibronic transitions, and a simple quantitative explanation is not possible. However, measured integrated band cross sections at 296 K were found to be within 5% of those at 204 K suggesting that the changes in the spectrum are in fact simply due to a redistribution of the population in the ground state. The integrated cross sections at 378 K for A < 420 nm were the same as those for 204 and 296 K. As mentioned earlier the longer wavelength integrated cross sections are increased by the appearance of hot bands. The room temperature spectrum does not seem to contain many bands originating in excited vibrational levels of the ground state, and integrated cross sections did not change when the temperature was lowered. Since the stratospheric solar flux in this wavelength region is not structured, and the stratosphere is optically thin for OClO absorption, the atmospheric photolysis rate will not change with temperature. On the other hand, the sharpening of the bands at lower temperatures suggests that the optical detection techniques for measuring OClO will have a better chance of succeeding, since the differential absorption cross sections in the 400-nm region are up to 35% higher at stratospheric temperatures than at room temperatures.
-
Acknowledgment. We thank Drs. S. Solomon and G. Mount for providing the impetus for these measurements, and Dr. A. Schmeltekopf for valuable help in setting up the diode array spectrometer system. Supplementary Material Available: A complete listing of the cross sections in the 245-475-11111region in 0.07-nm increments measured at 204, 296, and 378 K (9 pages). Ordering information is given on any current masthead page.
