J. Phys. Chem. 1995, 99, 3137-3143
3137
O(lD) Quantum Yields of Ozone Photolysis in the UV from 300 nm to Its Threshold and at 355 nm W. Armerding, F. J. Comes,* and B. Schulke Johann Wolfgang Goethe Un'iversitat, Znstitut f i r Physikalische und Theoretische Chemie, Marie-Curie Strasse I I , 0-60439 Frankfurtmain, Germany Received: March 23, 1994; In Final Form: December 1, 1994@
In this paper, we present measurements of O(lD) quantum yields of ozone photolysis, 4(A),in the wavelength region between 300 and 330 nm and at 355 nm. The data are obtained from an experiment which is based on the reaction of O(lD) atoms with water and the subsequent detection of the generated OH by laserinduced fluorescence (LIF). Due to the performance of this method, providing a high level of accuracy and precision, it was possible to answer the question concerning the wavelength dependence of the O(lD) quantum yield in the threshold region. In particular, the value of @(A) at 302.5 nm was determined to be unity, the existence of a saddle point at 315 nm was verified, and the wavelength of the threshold was determined to be between 331 and 333 nm. In addition, measurements performed at 355 nm have shown a negligible contribution to the O(lD) quantum yield. Thus, it can be deduced that a participation of the spin-forbidden process with the products O('D) and 02(32,-) is unlikely.
1. Introduction The absorption of light by ozone molecules in the UV, the visible, and the near-IR leads to their dissociation into oxygen atoms and m o l e c ~ l e s . ~ - These '~ dissociation products necessarily carry the excess photon energy of the decomposition process, leading to electronic excitation if performed in the U V . Among the products, oxygen atoms in their excited ('D) state are of special interest in atmospheric chemistry as they are very reactive, reactive in a sense that either energy transfer or a chemical reaction occurs upon nearly every collision. The most important collision partners of O(lD) in the troposphere are N2, 02,and H20. The first two candidates initiate an energy-transfer process after which the resulting O(3P)atoms react with oxygen molecules, thus re-forming the ozone molecules. The collision with water vapor molecules, however, leads to the formation of OH radicals, thus forming the very important oxidizing particle in the troposphere.' It is therefore of main interest in tropospheric chemistry to know how efficient the O(lD) production can be. Under tropospheric conditions, only ozone is a precursor molecule for O('D) because other precursors will not give O(lD) dissociation products at the wavelengths which penetrate to the lower atmosphere. The observation of the O(lD) channel as a result of the UV absorption of ozone is therefore of permanent interest. These measurements are not without experimental difficulties. Conservation of electron spin makes two processes optically allowed. Since the ground electronic state of ozone is a singlet, either two singlet or two triplet particles are expected to be the products. Thus, from thermodynamical considerations, the longwavelength limit for the O(lD) production from ozone at 0 K is calculated to be 310.2 nm.2 If, however, a nondlowed transition into a state forming O(lD) and triplet 0 2 as products can occur, the long-wavelength limit could change to 410.6 nm.2 Such a possibility should be of great importance for tropospheric chemistry, because the intensity of the sunlight near the earth's surface strongly increases with wavelength. The determination of the quantum yield for O(lD) production from ozone remains an important subject in tropospheric @Abstractpublished in Advance ACS Abstracts, February 1, 1995.
0022-365419512099-3137$09.00/0
chemistry. This quantum yield is not only a question of quantum energy but it will also depend on temperature because the internal energ? of the 0 3 precursor molecule enters the energy balance.
2. Some Problems in O(lD) Quantum Yield Determinations The known dissociation channels of tropospheric importance in ozone photolysis are
+ IW- o(,P> + 0, -I- hv - O('D) + 02('A)
0,
(la) (1b)
Among these channels, (lb) is a precursor pathway for OH production (eq 2) in case the O(lD) atom reacts with water vapor: O('D)
+ H,O - 2 0 H
(2)
Another possibility is that forming O(lD) atoms is a spinforbidden process, and it shall further be proven whether it exists and what it might contribute to tropospheric OH production. As O(lD) is the only particle of interest in the presented context, we will seek to determine its quantum yield from all possible ozone photolysis processes. The quantum yield, 4, is determined as the ratio of O(lD) atoms produced per number of photons absorbed. 4 will depend on the spectroscopic properties of ozone as well as on its thermal energy content. Therefore, 4 is a function of wavelength and temperature: 4 = 4(A,q. The wavelength range of interest is limited at long wavelengths by the absorption property of the molecule and at short wavelengths by the sunlight spectfum which penetrates into the troposphere. This is, roughly speaking, the range between 290 and 330 nm. Beyond 330 nm, the ozone absorption cross section drops rapidly, and a possible O('D) formation is then of tropospheric interest only if it extends fairly deep into the long-wavelengthregion with a sufficient quantum yield. It is, however, of spectroscopic interest even if the quantum yields are low. 0 1995 American Chemical Society
3138 J. Phys. Chem., Vol. 99,No. IO, 1995 Measurements of the quantum yield function are difficult because of the rapid decrease of the ozone absorption cross section and the parallel decrease of the O(lD) quantum yield. Under otherwise constant conditions, the expected O('D) production rate will drop by more than 4 orders of magnitude in going from 290 to 330 nm. In this same interval, the sunlight intensity is increasing rapidly with wavelength so that the determination of O(lD) quantum yields is important even in the long-wavelength range. A number of experimental and theoretical results have been published with regard to the above p r ~ b l e m . ~ -Experimental '~ results are of essential importance since at present the change of +(%) with wavelength cannot be calculated ab initio with sufficient precision. But the published data show remarkable differences considering the accuracy and precision required for model calculations of atmospheric chemistry. These discrepancies will be reflected in the calculated OH formation ratio and the related photochemical processes. In particular, the open questions are concerned with the problem of whether +(%)reaches unity at shorter wavelengths, with the shape of the @(%) curve in the fall-off region, with the problem of the determination of the exact position of the threshold, and with the related question of a potential contribution of the spin-forbidden photodissociation channel below 41 1 nm. Summarizing the present knowledge on the ozone photolysis, the value of the O(lD) quantum yield, &I is) nearly , 0.9 between 250 and 300 nm.7.8 It is possible that the value of 1 is reached at a weak maximum near 303 nm. Thus, the question arises whether the dissociation of ozone into O(lD) and 02(lAg) at 303 nm is the sole process or whether the formation of the O(3P)/ O Z ( ~ Z ~photofragment -) pair has to be taken into account in addition to the O(lD)/02(lAg) formation due to the crossing of the potential energy s ~ r f a c e s . ~ - ~ ~ , ~ ~ Discrepancies exist in the discussion of the wavelength dependence of Q(%) in the threshold region. This is especially true when concerned with a possible long-wavelengthtail, which is observed by Brock and Watson?4 whereas others find a steeper decrease of 4 with q5 = 0.0 at 320 nm for 298 K.17,21,23 The behavior of +(%)in the fall-off region is of considerable interest even for its long-wavelength dependence due to the strong increase of the transmitted sunlight beyond 310 nm in the troposphere. Besides a contribution from ozone hot bands, a possible participation of the spin-forbidden dissociation channel could shift the threshold for O(lD) formation toward lower photon energies. Since the earlier measurements could be performed only for wavelengths no longer than 325 nm?4,26it was not possible to investigate the position of the threshold of the O(lD) production with sufficient precision. In particular, it was not possible to answer the question whether a contribution of the spin-forbidden O(1D)/02(3Zg-)reaction channel has to be taken into account. In this paper, we present measurements of #(A) in the wavelength region between 300 and 330 nm and at 355 nm. The data are obtained by means of an experiment which is based on the reaction of O(lD) atoms with water and the subsequent detection of the generated OH by laser-induced f l u ~ r e s c e n c e . ~ ~ Since the method of measurement has been substantially improved-thus minimizing the amount of uncertainty-it has been possible to investigate the problem of the threshold and thus to answer the questions concerning the wavelength dependence of the O(lD) quantum yield even at longer wavelengths. The measurement at 355 nm were performed to prove O(lD) formation in a spin-forbidden channel.
Armerding et al.
3. Measurement of O(lD) Quantum Yields between 300 and 330 nm by Chemiluminescence The direct determination of 4 following eq 1 has to be based on an analysis of the dissociation products utilizing the Ot3P)/ O(lD) ratio as observed. This requires determination of the concentration of oxygen atoms in different electronic states, which is difficult to perform in any case. The overall pressure in the reaction chamber must be held very low since the metastable oxygen atom is very sensitive toward collision^,^-^ leading to rapid losses. The absorption cross section of ozone, u,as well as the quantum yield, 4, decrease rapidly at longer wavelengths. Thus, the product 4u decreases as already discussed by more than 4 orders of magnitude within the region of interest.1,24,26,30,31 Consequently, measurements of @(,I) performed by direct determination of the concentration of the metastable O(lD) atoms are not promising. Since the cross section of the O(lD) atom in reactive collisions is very high,3-6 one can use this property to determine the concentration of the O(lD) atoms by means of a well-suited chemical reaction. Well-suited means that the reaction performs a transformation of the O(lD) atoms into detectable products and that the product analysis can be made accurately and precisely. A major problem in the chemical determination of O(lD) is reaction channels which are competitive to the chosen O(lD) reaction. In this case, it is necessary that all competitive channels be known and taken into account or that the experimental conditions be arranged in a way that the competitive reactions are negligible. In addition, as already mentioned in the Introduction, the quantum yield is temperature dependent. Since chemical reactions show in general a considerable temperature dependence, this fact has to additionally be taken into account. The high level of difficulty to fulfill the above requirements can be seen from the considerable number of respective publications. A broad spectrum of different photolysis light sources and tracers of O('D) has been used in several experiment^,^^-^^,^^-^^ showing considerably differing results under the aspect of the required accuracy and precision. Moortgat and Warneck (1975),21Arnold et al. (1977)?2 Brock and Watson ( 1980p4 and Philen et al. ( 1979)25made use of the reaction between N20 and O(lD). They have observed the chemiluminescence of the final product of this multistep reaction system, which is electronically excited NO2. Trolier and Wiesenfeld argued in 1988 that vibrationally excited NO as a product of the reaction O(lD) N20 2N0 might lead to artifacts in the determination of #(%),in particular at wavelengths longer than 315 nm.26 They therefore proposed to measure the quantum yield of the O(lD) production more directly using the excitation of CO2 by collisions with O(lD) and to detect the subsequent IR fluorescence of the vibrationally excited COZ.*~ An empirical approach for the description of the temperature dependence of the O(lD) quantum yield within the spectral region between 300 and 330 nm was published by Moortgat et al.,17J* and a theoretical approach was published by AdlerGolden et al.27 Both models differ considerably in the longer wavelength regions above 315 nm. A detailed knowledge of the shape of 4 in the fall-off region, in particular inside the "tail" for a given temperature, e.g., 298 K, should offer the possibility to decide which of the models fits the @(A) curve best.
+
-
4. Determination of O(lD) Quantum Yields by OH LIF Measurements Fundamental Measurements. The method presented in this paper is based on the highly efficient reaction (2) between O('D)
J. Phys. Chem., Vol. 99, No. IO, 1995 3139
O(lD) Quantum Yields of Ozone Photolysis atoms and H20 molecules, generating OH. The product OH is determined by means of laser-induced fluorescence (LIF). The experimental setup is arranged as a pump and probe experiment where the ozone is photolyzed by a pump pulse (VI). The OH radicals formed by the reaction of O(lD) with water vapor are excited by a probe pulse ( ~ 2 ) . The subsequent OH fluorescence (v3) is detected. The wavelength of the photolysis pulse is changed for the detection of q5 = &I while ),that of the probe pulse remains at a constant wavelength position for all measurements. The frequency of the observed fluorescence radiation (v3) is in general different from that of the exciting radiation. The complete formation and detection scheme is given as follows:
0,
+ hv,- O(’D) + O,(’A) O(’D) + H,O - 2 0 H OH + hv, - OH* OH* - OH + hv,
(1b)
(3) (4)
L F experiments are in general very sensitive and selective compared to chemical methods. Thus, the determination of O(lD) quantum yields by OH LIF measurements should lead to a higher confidence in the q5(01D) and J ( 0 3 ) data, respectively. But due to the required high accuracy and precision, it is necessary to compensate for the unavoidable systematics and random noise very carefully. In the following, we will discuss the reaction scheme (Le., the influence of consecutive, competitive, and secondary reactions as well as the temperature and pressure dependence of the applied chemical processes and the change of the involved concentrations in time), the detection of the fluorescence light, and the absolute calibration of the measured #(A) values. It should be noticed here that all data presented in this paper were determined as relative values in the first step and made absolute ones by calibration to a known standard in the second step. A crucial point of the experiment is the reproducibility of the boundary conditions. It is obvious that changes in the experimental conditions, e.g., the partial pressures of the reactants or a possible wavelength-dependent variation of the (measured) intensity of the photolysis pulse, have to be controlled carefully. This holds-with a view on the optical detection device of the fluorescence light-in particular for the geometric invariance of the reaction volume, i.e., the overlapping region of the photolysis pulse, the probe pulse, and the fluxes of the reactants 0 3 and H20. O(’D) Detection Scheme. The determination of the O(lD) concentration by measurements of the OH produced following reactions 1-4 provides a number of advantages. Reaction 2 has no activation energy, and it should thus not be dependent on temperat~re.~.,~-~’ Uncertainties in the temperature determination will not affect the precision of the measurements. On the other hand, this property should be ideal for the determination of the temperature dependence of the ozone photolysis. A competitive reaction, that is, the fast reaction of O(lD) with 03,is well-known, and no products are formed which will affect the measurement. Furthermore, reaction 2 is complete and occurs on a single collision, and the OH detection can be made fast enough so that the influence of consecutive reactions is negligible. The OH yield from the reaction scheme (1) and ( 2 ) is known to be 1.90 2~ 0.06,5$36 and byproducts are not observed.
As reaction 2 proceeds according to pseudo-first-order kinetics, the OH concentration increases exponentially so that the observable LIF intensity can be optimized by a set of the delay time between the photolysis and the probe pulse. This delay is made variable in order to match a possible variation in the O3/H2O mixture which was found to be necessary at longer wavelengths. The problems arising from secondary reactions are negligible if the observations are performed at low pressures and short observation times. If the OH products are probed long before any products from consecutive reactions will appear, the method is discriminative. This is realized in the presented experiment by utilizing short pump and probe pulses (7 ns) as well as by utilizing only a short delay (27 ns) between them and a pressure in the reaction zone (measured via the lifetime of the excited OH molecules) reduced to the submillibar regime (PH~o= po3 2 0.5 mbar). A reduction in the O3 pressure of less than 10% of its original value during one measurement resulted in a very small shift of the population to higher rotational quantum numbers which is neglected in the following discussion. There is influence on the measured signal from collisions in the excited state. Under the reported experimental conditions, the lifetime of the excited OH radicals in the A2Zg- state (v’ = 0) is about 800 ns.4O Thus, rotational relaxation can cause a measurable change of the excited OH population of about 6%. But since the partial pressures are kept constant during a complete q5@) measurement, this contribution is also constant, and it is considered in the calibration. Furthermore, the Q transitions at two positions (J = 3, J = 7) are probed for the measurements as an independent control (see below), and the same values of the O(lD) quantum yield have been measured for both rotational transitions. It thus becomes clear that the uncertainty from the rotational relaxation on the q5 values can be kept negligible at the applied pressures. The influence of varying temperatures during the measurements is negligible since most of the measurements have been performed at 298 K. The maximum uncertainty of this value is f l K. Corresponding uncertainties of the value of O(lD) can be taken into account by the total error bars. LIF Determination of OH. The presented method makes use of a selected excitation. The Ql(3) transition39 and, as a control, the &(7) transition are excited in the A2Zg-, v’ = 0 X211, v” = 0 transition. Under these experimental conditions, the observed fluorescence signal is a direct measure of the number of generated OH molecules. Potential sources of errors which have to be discussed carefully are interfering contributions to the fluorescence signal due to probe laser generation of OH, stray light, and background noise. The problem of laser-generated OH has been reported in a number of publications concerning tropospheric hydroxyl measurernents$l and the same problem affects the laser-optical detection of OH in the 03/H20 mixture of the present experiment. The artificially produced amount of OH results in a higherand measured-OH concentration. However, the contribution from laser-generated OH can be calculated from the partial pressures of the educts and the laser parameters on the basis of the known rate constants, or it can be eliminated by means of additional reference measurements as shown below. In addition, the detection procedure of the LIF signal has to be designed carefully since the fluorescence is very weak, particularly at the longer wavelength end of the spectral region under investigation. It is thus necessary to suppress the stray light effectively by a suitable setup of the experiment (baffles, Wood’s horns, etc.). The effect of the remaining stray light is reduced by reference measurements performed in addition to
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3140 J. Phys. Chem., Vol. 99, No. IO, 1995
Armerding et al.
TABLE 1: Experimental Parameters of the Different Wavelength Subregion@ wavelength subregion 300-310 nm 305-318 nm 313-325 nm dye soln SRh 101 Fa3 (1:l) SRh 101 SRh 101 DCM (1:l) solvent methanol methanol methanol pulse energy photolysis laser, yJlpulse 10 50 100 mixing ratio HzO103,mbar 0.510.2 0.710.5 0.710.7 calibration data ( P 3 0 5 ~= 0.98 (P31Jnm = 0.22 ( P 3 2 h m = 0.12
+
+
322-330 nm SRh 101 DCM (3:l)
+
methanol
130 111 (P3zsnm = 0.06
( P 3 0 8 ~= 0.79
SRh 101 = Sulfrhodamine 101,RhB = Rhodamine B; the probe pulse energy was 70 yJlpulse for all measurements (dye solution of the probe laser: SRh 101 in methanol).
the OH fluorescence measurement. Hence, the composite fluorescence signal observed in the measurement of $(A) contains four different contributions: (1) the background signal (both lasers shut down), ( 2 ) the stray light from the photolysis laser (only photolysis laser operates), (3) the signal from the probe laser (only probe laser operates), and (4) the full fluorescence signal (both lasers operate). Each signal out of the four can be determined with an accuracy of 10.3%. The above four measurements allow for correction of the measured signal for background contributions due to lasergenerated OH, stray light, and general background. Stray light is an important issue in this experiment because of the probe laser wavelength, and therefore, the fluorescence radiation is within the tuning range of the photolysis laser. Furthermore, every value of 4(A)is the result of an averaging process based on a series of measurements at the wavelength position A. It was thus possible under the reported experimental conditions to detect a less than 2% contribution of the OH fluorescence to the averaged total signal. Calibration of $(A) at Selected Wavelength Positions. In principle, the absolute values of 4(A) can be evaluated from the presented experiment. Knowing the absolute quantum flux of the photolysis light source and the concentration of OS,it is possible to determine the number of dissociated 0 3 molecules. The respective number of O(lD) atoms and thus the absolute values of @ can (, beI) derived by the determination of the total fluorescence radiation emitted by the excited OH molecules. This procedure is rather complicated. Because the fluorescence light is emitted into all directions, only a certain part can be detected, and the ratio of this part of the total amount of fluorescence radiation is difficult to determine. This problem is general to all LIF measurements, and consequently, as already mentioned above, all data presented in this paper were determined as relative values in the first step and made absolute by calibration to a known standard in the second step. This basic idea is to determine the fluorescence signalcorrected for the above sources of error-at a well-selected wavelength for which a sufficiently exact value of #(A) is known. Under the condition that all experimental parameters of measurements performed at different wavelength positions are constant or that eventual changes which can appear when, e.g., different dye solutions are used are well-known, it is possible to evaluate the complete quantum yield curve @(A) from the fluorescence signals. All measurements were performed at two different OH lines-the Ql(3) and the Q2(7) line-as discussed above. The calibration has been made twice for the presented measurements. As a primary calibration standard, the value &A) = 0.79 at A = 308 nm was used, which was determined by Greenblatt and W i e ~ e n f e l d .This ~ ~ reference has been chosen for the present investigation since these measurements have been made very carefully at two wavelength positions (266 and 308 nm), and the uncertainty of @ was stated to be no larger than
2.5%. This calibration was compared to a second one based on an independent determination of #(A) at 302.5 nm for which the O(lD) quantum yield of 1.0 was given by Arnold et aLZ2 and other authors.17-19,21-24Both producers led to identical results within the error limits. Extention of the Calibration over the Complete Region of Interest. The extension of the measurements over the full wavelength scale requires a modification of the reported method of measurement. Difficulties are caused by the properties of the ozone itself and by problems due to the experimental arrangement. As mentioned above, a decrease of the fluorescence signal of about 4 orders of magnitude has to be expected within the selected spectral range between 300 and 330 nm due to the strong decrease of the ozone absorption and the O(lD) quantum yield. In order to compensate for this very strong change in the observable fluorescence signal, the laser output energy and the 0 3 and H20 partial pressures were changed, and different dye solutions were used in order to optimize the laser output power. As a consequence, the wavelength dependence of the output energy of the photolysis laser and the spectral sensitivity of the photodiode have to be known as functions of the wavelength, and in addition, the change in pressure has to be considered. In the reported measurements, a variation of the laser output power of the order of 13 (10-130 pJlpulse) and a variation of the partial pressures of the order of 5 (0.2-1.0 mbar) were utilized. The photodiode applied for the measurement of the intensity of the photolysis laser was calibrated by a comparison with a pyroelectric radiation detector (Laser Precision Corp., pyroelectric detector RJP 735: uncertainty for 200-1200 nm, -0.5% to +2%). These changes in the experimental conditions made it necessary to divide the complete wavelength region under study into overlapping subregions. Each of the subregions overlapped with the neighboring ones for more than 2 nm. The experimental conditions used for the different subregions are summarized in Table 1. The measurements of $(A) performed twice in the overlapping regions allow us to extrapolate the absolute calibration of the measured values. Measurements at 355 nm. For the measurements performed at 355 nm, a modified laser arrangement had been used since the absorption cross section of ozone decreases by more than a factor of 50 from 330 to 355 nm. In order to increase the number of photons up to a sufficiently high level, the third harmonic of a Nd-YAG laser was utilized as the photolysis source. Thus, the output energy of the photolysis light source at 355 nm is increased by a factor of about 1000 compared to the energy of the frequency-doubled dye laser at smaller wavelengths (330 nm). The (primary) intensity of this photolysis radiation can be used at its full amount since the stray light is caused only by the probe laser under the reported experimental conditions.
J. Phys. Chem., Vol. 99, No. 10, 1995 3141
O(lD) Quantum Yields of Ozone Photolysis
5. Experimental Section The applied mechanism requires the use of two light sources, a tunable one for the ozone photolysis ( V I ) and one (fixed wavelength position) for the OH excitation ( ~ 2 ) . The setup used for the measurements consists thus of the photochemical reactor including the ozone generation device, the photolysis laser, the OH-detection laser, and the electronics for data acquisition and processing. The 0 3 required for ozone photolysis is generated following the standard procedure of Clough and T r ~ s h . ~The * water vapor required for the O(lD) detection is added to the ozone directly before entering the photoreactor. In order to ensure that the 0 3 and the H20 concentrations remain constant during the LIF experiment and that no photolysis products or consecutive products can accumulate in the reaction cell, the experiment has been carried out under weak flow conditions. The photolysis laser in the wavelength region between 300 and 330 nm is a frequency-doubled, narrow-banded pulsed dye laser (Quantel TDL 4). The output energy is about 120 pJ/ laser pulse for a pulse duration of about 7 ns and a spectral resolution of 0.5 cm-'. In order to measure the O(lD) formation in the longer wavelength region, a very powerful photolysis laser has to be utilized since the absorption cross section of 0 3 at 355 nm of 1 x cm2/molecule (298 K) is very small.' In this experiment, the third harmonic of a Nd-YAG laser at 355 nm is used for ozone photolysis. The output energy of this light source is 150 &/pulse at a pulse duration of 10 ns. For the detection of the OH radicals, a frequency-doubled dye laser (Molectron DL 18 P) is in use which is tuned to the resonance absorption lines of the OH near 308 nm. As this laser is tunable, this opportunity is used for a second measurement as a control. Thus, the waelength of the probe laser is switched between two line positions, Q1(3) and Q2(7), respectively. The output energy is 7 pJ/pulse at a pulse duration of about 7 ns and a spectral width of 0.5 cm-'. The photolysis and the probe laser are pumped synchronously by the same pump laser, but the pump beam for the probe laser is transmitted over a delay line of 9 m, resulting in a 27-ns delay for which the maximum OH production is expected. The use of an optical delay line is advantageous because it is jitter free. A jitter could cause additional noise because the OH concentration is time dependent. The pump laser is a frequencydoubled Nd-YAG laser with a pulse energy of 250 mJ and a repetition rate of 10 Hz (Quanta-Ray DCR la). A UV sensitive photomultiplier (Thorn Emi side-on photomultiplier Type 9781 B) is used for the detection of the OH fluorescence and a boxcar integrator (Stanford Research Systems, Boxcar SR 250) for data acquisition in order to improve the signal-to-noise ratio. The resulting electronically filtered signal is digitized by means of an AD converter (12 Bit) and stored by a laboratory computer (AT) for further processing. 6. Noise Considerations
The procedure discussed in section 4 has demonstrated that the measured OH fluorescence light is a direct measure of the O(lD) quantum efficiency. The influence of the involved reactions on the systematics-consecutive, competitive, or secondary-can be neglected. The same argument holds for time-dependent changes in the concentrations of the involved species and in addition for the influence of the temperature and the pressure on the results. Systematic variations in the experimental conditions (partial pressure of 0 3 and H20; photolysis energy) are necessary in order to optimize the measurements in the different wavelength regions. These variations can be made with sufficient precision due to the double check in the overlapping regions. Thus, the
TABLE 2: Values of O(lD) Quantum Yields of Ozone Photolysis between 300 and 330 nm (298 K) I/nm 4 error bar d/nm 4 error bar f8% 314 300 0.94 0.21 f8% 1.00
302.5 305 306 307
0.98 0.93 0.83
308
0.79
309 310 311 312 313
0.68 0.48 0.40 0.30 0.23
33% f5% f4% f5% 62.5% f5% f5%
f6% f6% f5%
315 316 317 318 319 320 322.5 325 327.5 330
0.22 0.20 0.20 0.15 0.13 0.12 0.09 0.06 0.033 0.012
fll% 115% 4r 14% 515% f19% 2~17% f34% 639% f56% f72%
problem of error discussion is reduced to the following questions: What is the uncertainty of a single +(A) determination due to random noise? What is is the influence of error propagation on the absolute calibration of the #(A) values? What is under these conditions the overall uncertainty (accuracy and precision) of a single $(A) value? In order to minimize fluctuations resulting from pressure, temperature, or fluxes, these parameters have been controlled very carefully. For each determination of a single 4(A)value at a certain wavelength position A, 5000 measurements have been performed representing each of the 4 measurement contributions listed in section 4. A further control is given by measuring at two wavelength positions (Ql(3), Q2(7)). The precision of a 4(A)measurement for a given A including the influences of random changes in the experimental conditions and of photon statistics can be thus calculated statistically using Gauss's law. The accuracy of the @(A) values obtained by OH LIF measurements is determined by the accuracy of the external standard as well as by the uncertainties of the measurements in the overlapping regions which are growing at longer wavelengths. Both contributions have been considered for using the laws of error propagation. The uncertainties in the measured 4(A) values have been calculated following the above discussion. The results are summarized in Table 2. It should be kept in mind that the presented error bars include both the systematics and the random noise and are thus a measure of the precision of the presented value and of the respective accuracy as well.
7. Results and Discussion The results of the O(lD) quantum yield determination from ozone for the wavelength region between 300 and 330 nm are numerically summarized in Table 2, and graphically in Figure 1. The quantum field curve of the O('D) formation from ozone photolysis can be characterized as follows: in the wavelength region below the threshold at 3 10 nm, 4 increases from a value of 0.94 at 300 nm to a value of 1.0 at 302.5 nm. This absolute maximum is continued up to 305 nm. Starting from 305 nm to longer wavelengths, 4 decreases rapidly to a value of 0.6 at 310 nm and then shows a saddle point at 315 nm. From 317 nm, 4 decreases continuously again and obtains values of 0.033 and 0.012 at 327.5 and 330 nm, respectively. The threshold for O(lD) formation extracted from this quantum yield vs wavelength curve is 331 nm. This threshold was calculated by linear extrapolation since the decrease of the measured values at wavelengths longer than 320 nm is nearly linear. However, using an exponential extrapolation-which better fits a thermal distribution-a threshold of 333 nm can be calculated with a value of 4 5 0.008. Both extrapolations are possible within the error limits.
h e r d i n g et al.
3142 J. Phys. Chem., Vol. 99, No. 10, 1995 0,Chv
-
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330
wavelength / rim
Figure 1. Wavelength dependence of the O(lD) from ozone photolysis in the Hartley band obtained by different authors. The determination of the threshold is in agreement with the measurements performed at 355 nm. Despite the compensation of the very weak 03 absorption cross section by the powerful photolysis laser (see section 5 ) , no useful LIF signal due to OH could be measured within the error limits. The absolute calibration of these data is difficult, as the measurements are not directly fitted to the other ones since the photolysis laser oscillates on a fixed frequency and cannot be tuned to other wavelengths suitable for a continuous calibration. In addition, the output of the OH fluorescence signal depends critically on the overlap of both the photolysis and the probe beam, and the geometry is changed considerably compared to the dye laser measurements. As a consequence, the 355-nm value is affected by systematic errors which are difficult to estimate for the given experimental conditions. The calculated value of the quantum yield 4 5 0.006 is thus only an approximate one. But it demonstrates unequivocally that the threshold for the O('D) production from ozone photolysis has to be found at shorter wavelengths, i.e., between 331 and 333 nm. The measured wavelength dependence of 4 confirms the presumption which has been used by Arnold et al?z as well as by Brock et al.24in their determination of 4 that the formation of O(lD) atoms by ozone photolysis around 303 nm is complete; that is, a quantum yield of 1.0 is found. As mentioned above, this result has been utilized as a well-suited second reference for the calibration procedure. At wavelengths shorter than 300 nm, 4 decreases to a value of 0.94. It is interesting that this result was predicted in the measurements of Arnold et alez2These data are in contrast to the results of an experiment performed by Valentini et al.42 These authors found that the O( 'D) quantum yield is below 1.O at all wavelengths, and at wavelengths shorter than 305 nm, it should always be about 0.94 due to the A splitting of the ~ ( ' B z ) state and the relationship of the A components to different photofragmentation pairs. However, the measurements presented here confim the recent results of Brock and Watson24 as well as those of Trolier and Wiesenfeld at longer wavelengths.26 These authors have published that the decrease of the O(lD) quantum yield is flatter in the longer wavelength region, showing a saddle point at about 315 nm. The saddle point in the down slope of the O(lD) quantum yield at 315 nm which has been determined by the L F method can be interpreted as being caused by the additional absorption from populated vibronic states of 03.z7~28 A model calculation performed by Adler-Golden et al.,27 taking into account the thermal vibrational excitation of 03,confirms the experimental result that a saddle point exists at about 316 nm.
TABLE 3: Comparison of the Results Obtained by Different Authors authors tail threshold ratio (this worklearlier papers) this work yes 331 nm 1.02 BrockJWatsonU yes 325 nm 318 nm 1.088 Moortgat et aL2' no
Linear extrapolation of the data points at the five longest wavelengths results in a threshold at 331 nm as mentioned above. An exponential extrapolation shifts this value to the red and results in a threshold at 333 nm, where the respective value of &(A) is below 0.008. This extrapolation is consistent with the results of Jones and who found no formation of O(lD) atoms at 334 nm. Following the above results, the threshold for the formation of O('D) atoms from ozone has to be located between 331 and 333 nm (for 298 K) and not at 318 nm as published by several authors in earlier This red shift of the threshold for O(lD) formation in ozone photolysis as found here and in other recent publication^^^^^^ can be explained by a thermal population of vibrational levels of oz~ne.~.~~ The nonformation of O(lD) in the ozone photolysis at 355 nm (4 5 0.006) is in agreement with calculations performed by Banichevich.44 Following Banichevich, it is more likely that an excited dissociative triplet state will dissociate into O(3P)/ 02(3&g-), O(3P)/02(1Ag),and O(3P)/Oz('Eg-) photofragmentation pairs than into the O(1D)/02(3Zg:g-) photofragmentation pair.44 This has been confirmed in a recent publication in which the O2(lAg) quantum yield was mea~ured.4~ These values coincide excellently with the present data in the fall-off region, deviating only in the long wavelength part where obviously an additional channel opens to form O2('Ag) together with O(3P). The maximum quantum yield of 1 which we observed at 302.5 nm (see Table 2) can well be within their error limits.
8. Application to Tropospheric Photochemistry The wavelength dependence of the O('D) quantum yield (298 K) was carefully determined up to its threshold. These measurements have shown an increased O(lD) production rate compared to earlier publications. 17,18,21,22 The differences between the present results and those of earlier ones ca be expressed by the photolysis rate
a(A)is the absorption cross section of
03, Z(A) the normalized solar flux at ground taken from the literature,' and I$(,?) the respective 0 3 quantum yield. A0 is the threshold for O(lD) production. The values of 4(A)and A0 depend on the publication on which the calculations are based. Table 3 shows a comparison of the results presented in this work (1, = 330) with those from two other pioneering m e a ~ u r e m e n t s . ~The ~ . ~work ~ of Moortgat et al. (1977)22 is typical for measurements without tail and saddle points (20= 318 nm). The values of Brock and Watson (1980)24which show a maximum quantum efficiency of &(A) 5 1.04 between 300 and 304 nm have been divided by 1.04 for this comparison. The ratios shown in the fourth column of Table 3 are those obtained by dividing the J(O3) values from the present results by those from the respective publications. Within the error bars, the results of this work and the results of Brock and Watsonz4 are identical, but a difference of 8% exists, taking the earlier results of Moortgat et al. which do not contain the contribution from the tail region. The most important message from the present measurements is the exclusion of a possible contribution
J. Phys. Chem., Vol. 99, No. IO, 1995 3143
O(lD) Quantum Yields of Ozone Photolysis of excited triplet 0 3 states to 4(1), the identification of the longwavelength threshold, and a confirmation of earlier data of Brock and Watson,24thus reducing existing uncertainties.
9. Conclusion The determination of the O(lD) quantum yield of the 03 photolysis in the UV is of great importance for atmospheric chemistry since O3 photolysis in the UV leading to O(lD) and its subsequent reaction with water vapor determine the primary source rate of the OH radical. The measurements presented here have overcome the existing uncertainties concerning the wavelength region around 303 nm as well as the threshold region at wavelengths longer than 325 nm. Measurements performed at 355 nm have shown a negligible value of the O(lD) quantum yield. Thus, it can be deduced that a contribution of the spinforbidden process with the products O(lD) and 02(3Z,-) is unlikely. In addition, these experimental results can be used as a sufficiently precise input into atmospheric model calculations to determine the primary rate of OH formation.
Acknowledgment. This work was financially supported by the DFG. We gratefully acknowledge this grant. References and Notes (1) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry, 1st ed.; John Wiley & Sons: New York, Chichester, Brisbane, Toronto, Singapore, 1986. (2) Heicklen, J. Atmospheric Chemistry, 1st ed.; Academic: New York, San Francisco, London, 1976. (3) Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J.; Watson, R. T. J . Phys. Chem. Ref. Data 1980, 9 (2), 295. (4) Baulch, D. L.; Cox, R. A.; Crutzen, J.; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J.; Watson, R. T. J . Phys. Chem. Ref. Data 1982, I 1 (Z), 327. ( 5 ) Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A,; Troe, J.; Watson, R. T. J . Phys. Chem. Ref. Data 1984, 13 (4), 1259. (6) Atkinson, R.; Baulch, D. L.; Cox, R. A,; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J. J . Phys. Chem. Ref. Data 1989, I 8 (2), 881. (7) Steinfeld, J. I.; Adler-Goldmann, S. M.; Gallagher, J. W. J. Phys. Chem. Ref. Data 1987, 16 (4), 911. (8) Wayne, R. P. Ahn. Environflnt. Ed. 1987, 2 (8), 1683. (9) Banichevich, A.; Peyerimhoff, S. D. Chem. Phys. 1993, 174, 93. (10) Banichevich, A,; Peyerimhoff, S. D. Grein, F. Chem. Phys. 1993, 178, 155. (1 1) Sinha, A.; Imre, D.; Goble, J. H., Jr.; Kinsey, J. L. J . Chem. Phys. 1986, 4 , 6108. (12) Imre, D.; Kinsey, J. L.; Field, R. W.; Katayama, D. H. J . Phys. Chem. 1982, 86, 2564. (13) Imre, D.; Kinsey, J. L.; Sinha, A.; Krenos, J. J . Phys. Chem. 1984, 88, 3956. (14) Thomas, R. J.; Barth, C. A.; Rusch, D. W.; Sanders, R. W. J . Geophys. Res. 1984, 89, 9569.
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