UV Absorption Spectrum of the ClO Dimer (Cl2O2) between 200 and 420 nm

UV Absorption Spectrum of the ClO Dimer (Cl2O2) between 200 and 420 nm

(Parte 3 de 5)

The isosbestic wavelength in the limit of zero concentration change was determined in the source 1 experiments to be 312.9 ( 0.16 where the quoted uncertainty is the standard deviation of seven measurements. The isosbestic wavelength determined using source 2 was consistent with the source 1 results although the precision in nine measurements was lower, (0.69 nm. In addition to the isosbestic points observed at 312.9 and 408.5 nm, a pseudoisosbestic point was observed near 271 nm at low

Cl2O conversion. In an earlier study, DeMore and Tschuikow-

Roux13 reported an isosbestic point at 271 nm in their Cl2/Cl2O photolysis experiments. In our experiments, the behavior of the absorbance near 271 nm was very sensitive to Cl2O2 loss and the breakdown of the reaction stoichiometry. This sensitivity made the isosbesticwavelengthdeterminationless accurate, and therefore it was not utilized in our spectral analysis. However, as shown later, the Cl2O2 cross section determined in this work is consistent with an isosbestic point near 271 nm.

On the basis of the observed isosbestic behavior the Cl2O2 absorptioncrosssectionsat 312.9and 408.5nm were determined

includes estimated systematic uncertainties in the Cl2O and Cl2 crosssectionsat thesewavelengthsand temperatures.Combining the Cl2O2 absorption cross section determined at the isosbestic points with the wavelength dependence of the Cl2O2 spectrum enables the determination of the peak absorption cross section as well as the absorption cross sections at other wavelengths (see section 3.3). 3.2. CT-TD Results. CT-TD experiments were performed

(∼0.2 Torr), [O3], photolysis photon flux, total flow rate, flow velocity, and collection time were similar in each experiment and only the collection trap temperature was changed. For the concentrations used, Cl2 condensation was shown to be near our detection limit, for TC >148 K. The absorption spectra and composition of the gas-phase sample as a function of TC and desorption temperature,TD, are summarized in Figure 5. Similar results were obtained using the 351 nm excimer laser as the photolysis light source although the production of Cl2O2 was significantly less. We have used the ratio of absorbances at 330 and 244.5 nm, A(330 nm)/A(244.5 nm), to evaluate the fractionation of the sample as a function of collection and desorption temperature. Smaller values of the ratio correspond to higher fractionation and purer gas-phase samples of Cl2O2. In the limit of a pure Cl2O2 sample, the ratio corresponds to the ratio of Cl2O2 absorptioncross sectionsat these wavelengths. As shownin Figure5 for TC ) 148 K, the ratiois nearlyconstant

Figure 5. Results obtained in the cold trap thermal desorption (CTTD) experiments. Frame a: Sequence of absorption spectra measured during thermal desorption of a sample collected at a trap temperature,

TC, of 148 K. The spectra color code is given in frame c. Frame b: The UV absorption spectra shown in frame a normalized at the peak of the

Cl2O2 spectrum, 244.5 nm, showing the change in the Cl2O2/Cl2 fractionationof the sample as the trap temperatureincreased.The heavy black curve is the Cl2O2 spectrum obtained in this work. Frame c: The ratio, A(330 nm)/A(244.5 nm), as a function of the desorption

K( 0), 153 K (]), and 155 K(O). The error limits shown are 2σ (95% confidence level) values from the absorption measurements. The larger size symbols indicate the desorption temperature where the maximum

Cl2O2 absorption signal was observed. The dashed line and shaded 2σ uncertainty range is the ratio from the Cl2O2 spectrum obtained in the PLP-DA experiments in this work.

13718 J. Phys. Chem. A, Vol. 113, No. 49, 2009 Papanastasiou et al.

for TD <140 K but decreasesrapidlyat higher TD and approaches an asymptotic value of 0.037. The ratio also shows a systematic decrease with increasing TC. Measurements made with TC >151 K yielded a ratio of 0.027 for all values of TD; that is, the sample composition did not change with increasing TD. The absolute

Cl2O2 absorptionsignalincreasedwith increasingTD and reached maximum values for desorption temperatures in the range

≈ 183KC l2O2 and Cl2 were completely removed from the trap and the measured spectrum returned to the initial background

value ((0.001 absorbance unit). The approach to an asymptotic ratio with increasing TD and TC is consistent with a fractional distillation of the sample that reduces the amount of Cl2 in the trapped sample at the higher TC values. The absorption spectra measured throughout the desorption process were analyzed within the precision of the measurement to be due to contribu- tions from only two absorbing species, Cl2 and Cl2O2, for all conditions and temperatures shown in Figure 5. The lowest values of the ratio obtained using the CT-TD method, 0.027, correspond to an absorption spectrum that is in excellent agreement with the Cl2O2 spectrum obtained in our PLP-DA experiments, as shown in Figure 5.

The temperature dependence of the Cl2O2 spectrum was investigated over the range 201-218 K using Cl2O2 samples prepared using the CT-TD method with TC ) 151 K. A high trap collection temperature was used to minimize the Cl2 contribution to the absorption spectrum. The Cl2O2 spectrum showed only small, <2%, changes over this very narrow

temperature range. The weak temperature dependence of the

Cl2O2 spectrum, particularly between 200 and 300 nm, implies that direct comparisons of the Cl2O2 spectra measured in this work with those reported in other studies but at slightly different temperatures can be made quantitatively.

Finally, a weak Cl2O3 absorption signal (A < 0.01 at 265 nm) was observed when TD approached 200 K; Cl2O3 is less volatile than Cl2O2. In addition, near TD ≈ 215 K an unidentified low volatility condensate desorbed from the trap and condensed in the absorption cell. Absorption spectra recorded while using higher absorptioncell temperatureseliminatedthe condensation, but we were unable to characterize the gas-phase absorption spectrum of the compound due to the very weak absorption signal. The formation of Cl2O3 or species other than Cl2 and

Cl2O2 when using the CT-TD method was not reported in previous studies using this method.10,14 Test measurements performed using broad band photolysis, 320-800 nm, of the

Cl2/O3 mixture led to significant production of OClO and Cl2O3. The amount of OClO and Cl2O3 produced was sensitive to the initial O3 concentration in the reactor. It is most likely that photolysis of O3 in the Chappuis band (400-700 nm) leads to the formation of O atoms and secondary chemistry leading to OClO and Cl2O3, although, additional studies are needed to identify the reaction mechanism. Optically filtering the pho- tolysis wavelengths to 320-430 nm and minimizing the initial

O3 concentration reduced the production of OClO below our detection limit in most experiments.

3.3. Cl2O2 UV Absorption Spectrum Analysis. The Cl2O2 spectrum was determined using knowledge of the gas-phase chemistry and reaction stoichiometry combined with the isosbestic wavelengths observed in the PLP-DA experiments. The wavelength dependence of the Cl2O2 spectrum in the 200-280 nm region, where the contribution from Cl2 absorption is negligible, was obtained using results from the PLP-DA experimentsand sources1-3 and the CT-TD experimentsusing source 3. The Cl2O2 spectrum in this wavelength region was also determined via the PLP-DA method using the previously reported location of the Cl2O2 absorption peak at 244.5 nm. The pulsed laser photolysisof Cl2/O3 mixturesat 351 nm, source 3, yielded low Cl2O2 concentrations, but reliable measurements of the Cl2O2 spectrum in the 200-280 nm range were obtained. Both experimental methods and the three Cl2O2 sources used yielded Cl2O2 spectra that were in excellent agreement, within 2%, in this wavelength region.

The spectral subtraction of Cl2O in each photolysis step of the PLP-DA experiments yields the change in Cl2O concentration. The loss of Cl2O is related to the formation of Cl2O2 and

Cl2 via the reaction stoichiometry (eqs I and II) described earlier from which the cross sections for Cl2O2 in the wavelengthrange 200-280 nm were determined. Figure 6 shows a summary of the results obtainedin the PLP-DA experimentsfor both sources 1 and 2. The experimental data follow the linear relationship of eq V at low Cl2O conversionand negative curvatureat higher

Cl2O conversion due to the loss of Cl2O2 (lower residual absorption), which is greater in the 248 nm photolysis experi- ments. Several experiments shown in Figure 6 were performed using multiple laser pulses per step and are indistinguishable from the results obtained using less photolysis per step. The initial slope, i.e., at low Cl2O conversion, obtained from a leastsquares polynomial fit to the data yielded a Cl2O2 absorption

Figure 6. Data used in the determination of the Cl2O2 absorption cross section at 244.5 nm from the pulsed laser photolysis diode array absorption(PLP-DA)experimentsusing the 351 nm photolysisof Cl2O/

Cl2 mixtures (open symbols) and the 248 nm photolysis of Cl2O (solid symbols). The symbols represent individual experiments that were performed over a range of experimental conditions. The lines were obtained from a global least-squares fit to all the data. The initial slope uncertainty is the 2σ precision of the fit.

UV Absorption Spectrum of the ClO Dimer J. Phys. Chem. A, Vol. 113, No. 49, 2009 13719

cross section at 244.5 nm of (7.58 ( 0.3) × 10-18 cm2 molecule-1 where the quoted error is the 2σ uncertainty of the fit. The absorption cross sections at other wavelengths between 200 and 280 nm were obtained relative to the peak value using the wavelength dependence of the Cl2O2 spectrum obtained in the PLP-DA and CT-TD experiments. The temporal profiles for Cl2O2,C l2O, and Cl2 and the observed reaction mass balance are described later.

A similar absolute cross section determination method to that shown in Figure 6, eq V, was used to obtain a peak cross section for Cl2, which is well-known. The peak Cl2 absorption cross section obtained using PLP-DA data, source 1, agrees with the

currently recommended value of 2.68 × 10-19 cm2 molecule-16 at 200 K with a standard deviation in seven determinations of 7%. The good agreement achieved for the peak cross section of Cl2 confirmsthe self-consistencyof the reactionstoichiometry used in the spectral analysis and that the Cl2 absorption in our measured absorption spectra was accounted for accurately.

The determination of the Cl2O2 absorption spectrum at wavelengths >280 nm required the quantitative subtraction of absorption due to Cl2. The Cl2O2 absorption spectrum at wavelengths >280 nm was determined using the cross sections

at the isosbestic wavelengths combined with the cross section values obtained in the 200-280 nm region. Residual spectra were obtained for each photolysis step by subtracting Cl2O and Cl2 from the measured absorption spectrum. A typical set of residual spectra, normalized at 244.5 nm, the peak of the Cl2O2 spectrum, is shown in Figure 7. The agreement for the wavelengthdependenceof the residualabsorptionspectra,Cl2O2, at each photolysis step is excellent. The percent deviation of the residual spectra from the average, Figure 7, is less than 2% between 200 and 300 nm, 5% between 300 and 350 nm, and increases at longer wavelengths. The consistency of the absorption spectra for each photolysis step shows that the spectra measured in each photolysis step can be interpreted, within the precision of the measurements, by absorption from Cl2O and

Cl2 and another species that we assign to Cl2O2. The residual spectrain Figure7 show a precisionthat is typicalfor the various

Cl2O2 sources and experimental conditions used in this study. The residual spectra obtained in the PLP-DA experiments and the spectrum obtained from method 2, for spectra with the ratio A(330 nm)/A(244.5nm) ) 0.027,are also in excellentagreement as shown in Figure 5. The formation and stability of absorbing species other than

Cl2O2, such as isomers of Cl2O2 (chloryl chloride, ClClOO) or

Cl2O3, were evaluated by allowing a Cl2O2/Cl2O/Cl2 mixture to stand unperturbed in the absorption cell at 200 K for up to dependence of the residual absorption spectrum remained constant. We conclude from these measurements that other chlorine species did not make a significant contribution to the measured absorption spectrum in this study. 3.3.1. Mass Balance.The partitioningand total chlorinemass balance (total ) Cl2O2 + Cl2O + Cl2) for the PLP-DA spectra shown in Figure 3 is given in Figure 8. This data set is representative of the profiles obtained using sources 1 and 2. A simulation of the reaction system that included reactions 1–3, 5-1, and pseudo-first-order rate coefficients for the loss of mined Cl2O2,C l2O, and Cl2 temporalprofiles are consistentwith the behavior predicted by the reaction mechanism. Good agreement between the experimental data and simulated temporal profiles was obtained by allowing the photolysis laser fluence to vary (5% from the measuredvalue. It is worth noting that the concentrations of OClO and Cl2O3 calculated in the simulation are <1 × 1012 molecules cm-3 and therefore make negligible contributions to the measured UV absorption spectra.

Figure 7. Cl2O2 UV absorption spectra obtained from the analysis of UV absorption spectra measured following the 248 nm pulsed laser photolysis of Cl2O at 201 K and 725 Torr (He) (see text for details).

The data points at 271, 312.9, and 408.5 nm (b) are the Cl2O2 absorption cross section values determined at the observed isosbestic points (see text). The lower frame shows the residuals of the spectra from the average. The residuals show deviations that are less than 2% between 200 and 300 nm, 5% between 300 and 350 nm, and increase toward longer wavelengths.

Figure 8. Temporal profiles of Cl2O2 (b), Cl2O( 2), Cl2 (9), and the sum ([) obtained in a pulsed laser photolysis diode array absorption experimentat 201 K and 725 Torr (He). The lines shown were obtained from a simulation of the reaction system (see text for details).

13720 J. Phys. Chem. A, Vol. 113, No. 49, 2009 Papanastasiou et al.

The chlorine mass balance is very good, constant to within 1%, in the early stages of the experiment. However, following the maximum in Cl2O2 concentrationa small systematicloss of total chlorine was observed in nearly all experiments with the loss at the end of the experiment being ∼8%. The loss of total chlorine is reproduced reasonably well in the simulation when

Cl2O2 and/orCl2O losses,eitherhomogeneousor heterogeneous, that do not lead to the formation of gas-phase Cl2 are included. Several experiments were performed to examine the chlorine gas-phase loss in more detail. At the completion of a PLP-DA experiment the reaction cell was pumped out, sealed off, and warmed to room temperature. Cl2 and a small amount of OClO were observed to gradually build up as the cell warmed. The total amount of Cl2 accounted for the majority of the observed decrease in chlorine mass balance. We conclude that thermally unstable nonvolatile chlorine containing species were produced, either homogeneously or heterogeneously, in the later stages of the PLP-DA experiments. However, the UV absorption spectra measured in the later stages of an experiment yielded

Cl2O2 absorption spectra that were identical to spectra recorded in the early stages. Therefore, the formation of unidentified compounds did not significantly influence the measured UV absorption spectra or the determination of the Cl2O2 absolute absorption cross sections in our experiments.

3.3.2. Cl2O2 Spectrum. The Cl2O2 absorption spectrum obtained in this study over the wavelength range 200-420 nm is shown in Figure 9 and listed in 0.5 nm intervals in the

Supporting Information. The various experimental methods and chemical sources used in this study yielded Cl2O2 absorption spectra that were in very good agreement. The Cl2O2 absorption spectrum reported here was obtained from measurements near

200 K. The temperature dependence of the Cl2O2 absorption spectrum over the range 200-218 K was found to be small.

The Cl2O2 UV absorption spectrum in the wavelength range 200-420 nm consists of several overlapping electronic bands.

(Parte 3 de 5)