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 2 de 5)

where the photolysis thresholds, λT, were calculated using standard heats of formation. Cl2O absorbs strongly at 248 nm with a cross section of 1.7 × 10-18 cm2 molecule-1 at 298 K and has a photolysis quantum yield of unity. The formation of Cl + ClO is the major product channel and the quantum yield for the formation of Cl atoms has been reported to be >0.9 at

248 nm. The initialCl2O concentrationwas variedover the range (4.7-7.7) × 1015 molecules cm-3 and the calculated initial ClO radical concentrations were in the range (5.6-10.8) × 1013

Cl atoms formed in the photolysis of Cl2O react primarily with Cl2O following the initial laser pulses of an experiment to form Cl2 and a ClO radical where ∆[Cl2O] is defined as [Cl2O]initial - [Cl2O]final and

These stoichiometric relationships are used in the interpretation of the measured UV absorption spectra and determination of

Cl2O2 absorptioncross sectionvaluesdescribedlater.The further photolysis of the reaction mixture leads to a gradual but predictable breakdown of the reaction stoichiometry due to the secondary loss of Cl2O2 via photolysis where the quantum yield for photolysis is unity and by its reaction with Cl atoms conditions. The significance of Cl2O2 secondary loss increases as the Cl2O concentration decreases over the course of an experiment. As a result, the Cl2O2 concentration is expected to increase initially and go through a maximum. The Cl2O photolysis experiments were used to determine the wavelength dependence of the Cl2O2 spectrum and its absolute absorption cross sections.

A second source that was extensively used was the 351 nm pulsed laser photolysis of mixtures of Cl2/Cl2O. In this method, ClO radical formation was initiated by the photolysis of Cl2

ClO + ClO f Cl2 + O2 (5a) f ClOO + Cl (5b) f OClO + Cl (5c)

13714 J. Phys. Chem. A, Vol. 113, No. 49, 2009 Papanastasiou et al.

where the quantum yield for Cl atom production is 2. The initial

Cl2 concentration was in the range (3-8.3) × 1015 molecules cm-3 and the initial Cl atom concentrationswere (6-17) × 1012 atom cm-3 for the range of photolysis laser fluences used. The

Cl2 concentration was kept <9 × 1015 molecules cm-3 in order to minimize its initial absorption between 300 and 370 nm. Less than 5% of the initial Cl atom concentration was from Cl2O photolysis at 351 nm. Cl atoms react with Cl2O to produce an equimolar amount of ClO. An advantage of this ClO source was that Cl2O2 photolysis was significantly reduced due to its smaller absorption cross section at 351 nm compared to 248 nm, the difference in cross section being about a factor of 100.

Therefore, higher Cl2O2 concentrations were obtained from 351 nm photolysisof Cl2/Cl2O than from 248 nm photolysisof Cl2O. This source was used to determine the wavelength dependence of the Cl2O2 spectrum and absorption cross section values. Another source used to produce ClO radicals was the reaction of Cl atoms with O3

Cl2. This was the only source used in the CT-TD experiments.

In the PLP-DA experiments, the use of high O3 concentrations interfered with the UV absorption measurements while using low initial O3 concentrations yielded low Cl2O2 concentrations. Therefore, results obtained using this source were primarily limited to determining the wavelength dependence of the Cl2O2 absorption spectrum in the wavelength range between 200 and

280 nm. 2.4. Pulsed Laser Photolysis Diode Array Absorption (PLP-DA), Method 1. In this method, the ClO sources described earlier were used to produce Cl2O2 in the reactor/ absorption cell under static conditions at temperatures in the range 200-228 K. Experiments were primarily performed at high pressure, ∼700 Torr (He), although a few test measurements were performed at lower pressures. The experimental apparatus shown in Figure 2 consists of a temperature-regulated reactor/absorption cell that is optically coupled to the pulsed laser photolysis sources and diode array spectrometer. The photolysis reactor/absorptioncell was a jacketed Pyrex tube 105 cm long with an internal diameter of 2.5 cm. The ends of the reactor were sealed with O-ring joint connectors that held two quartz windows separated by an evacuated region, which enabled the entire absorption path to be contained within the temperature-regulatedregion and preventedcondensationon the outside windows. The temperature of the reactor was regulated by circulating methanol from a cooled reservoir through its jacket. The temperature of the reactor was measured at the fluid inlet and outlets and was stable to 0.5 K with a gradient of ∼2 K for a cell temperature of 200 K. PLP-DA experiments were performed using the following procedure. First, a lamp reference spectrum, I0, was recorded for both D2 lamps while flowingHe bath gas throughthe reactor. The reactor was evacuated and filled with reactants while

continuously monitoring the contents of the cell by UV absorption. The sample was diluted with He bath gas to a total pressure of >700 Torr (ambient pressure ) 623 Torr). The reactant concentrations in the cell were monitored by UV absorption to ensure a well-mixed and homogeneous sample prior to starting photolysis. A spectrum of the sample, I, was recorded for each lamp, and then the photolysis beam steering mirror (RM) was inserted. The sample was then exposed to the photolysis light source. The number of laser pulses used varied depending on the type and conditions of the experiment being performed. The mirror (RM) was then removed, and spectra were recorded for the change in compositionof the reactor. This sequence of photolysis and spectrum measurements was then repeated. At the completion of the photolysis experiment, typically after 10-30 photolysis steps, the reactor was flushed out and reference spectra, I0, recorded again. The total duration of a typical experiment was ∼15 min, although experiments of shorter and longer durations were also performed. The lamp spectra, I0, measured before and after the sequence of photolysis steps were compared and typically agreed to better than 0.001 absorbance units at all wavelengths. Reference spectra of Cl2 and OClO were measured immediately following a photolysis experiment at the same temperature and pressure. 2.5. Cold Trap Thermal Desorption (CT-TD), Method 2.

The cold trap thermal desorption (CT-TD) apparatus is shown in Figure 2 and was constructed entirely of quartz. The general approach used to study the UV absorption spectrum of Cl2O2 with this apparatus follows the methodologydeveloped by Pope et al.10 to collect bulk samples of Cl2O2 and measure its gasphase absorption spectra during thermal desorption. The three main regions of the apparatus consist of a gas-phase photolysis reactor, a cold trap, and an absorption cell.

The three regions of the apparatuswere temperatureregulated independently, and the continuity of the temperature-regulated regions helped minimize Cl2O2 thermal decomposition during flow through the apparatus. The photolysis reactor was a jacketed 25 cm long cylinder (3 cm i.d.) that was temperature controlledby circulatingmethanol from a temperature-regulated reservoir through its outer jacket. The temperature of the gas within the reactor was typically 208 K as monitored with a thermocoupleinsertedin the gas flow (withdrawnduring sample preparation). The ends of the reactor were sealed with O-ring joints with quartz windows as described earlier. The cold trap region was a 15 cm long jacketed tube (1.5 cm i.d.) that was temperature-controlled by flowing precooled N2 gas through its outer jacket. The N2 gas was cooled by passing it through a copper coil submersed in a liquid N2 bath. The temperature of the cold trap was regulated between 120 and 155 K by varying the total N2 flow rate. The temperature of the sample gas flow through the trap region was monitored with a thermocouple located near the center of the trap. Although the temperature of the trap was stable to within 1 K during an experiment, there were significant gradients along its length as measured with a thermocouple inserted along the axis of the gas flow. At 120 K, the difference between the sample gas temperature at the entrance and exit of the trap region was ∼20 K. From here on we will refer to the lowest temperature as the effective temperature of the trap. For sample collection the trap temperature was held constant and independent experiments were performed at various temperatures between 138 and 155 K during our study. The absorption cell had an optical path length of 92 cm, and its temperature was either 200 or 218 K.

The CT-TD experiments were performed by first recording and Cl2 (∼0.2 Torr)were then addedto the flow and the sample exposed to the photolysissource while monitoringthe formation of Cl2O2 and loss of O3 and Cl2 in the UV absorption cell. The residence time of the gases in the reactor was ∼6 s. When the absorption signal stabilized, the trap temperature was reduced and a sample collected for ∼90 min. The photolysis source and reactant gas flows were then stopped, the trap temperature was

Cl + O3 f ClO + O2 (4) UV Absorption Spectrum of the ClO Dimer J. Phys. Chem. A, Vol. 113, No. 49, 2009 13715 reduced to ∼120 K, and the apparatus was flushed with bath gas. In several experiments the sample was held at 120 K while flushing for ∼30 min although this did not significantly affect the absorption results. With the trap at 120 K, there was no detectable absorption in the absorption cell and I0 was recorded. The trap was then warmed at a rate of ∼0.2Ks -1 while recording UV absorption spectra of the desorbed sample as it flowed slowly through the absorption cell. During warming, the total bath gas flow was reduced to ∼12 STP cm3 s-1 which resulted in a ∼50 s residence time for the sample in the absorption cell. In these experiments the D2 lamp intensity was attenuated to minimize sample photolysis during its residence in the absorption cell.

2.6. Materials. He (UHP, 9.9%), N2 (UHP, 9.9%), O2 (UHP, 9.9%), and air (Zero grade) were used as supplied.

Cl2 (UHP grade) mixtures in He or N2 bath gases were used from commercial sources, 0.2% Cl2/He (UHP) and 5% Cl2/N2 (UHP), or prepared manometrically in a 12 L Pyrex bulb from

Cl2 (UHP, 9.97%), 5.1% Cl2/He. Ozone was generated by flowing O2 through an ozonizer and trapped on silica gel at 195 K. Ozone was added to the gas flow by passing a small flow of bath gas through the trap.

Dichlorine oxide (Cl2O) was prepared by oxidizing Cl2 with

HgO using two methods. Briefly, Cl2 was condensed at 7 K in a Pyrex reactor containing dried HgO (heated overnight at

400 K) in a Cl2:HgO mole ratio of ∼1:2. The mixture was warmed to 195 K and allowed to stand for 24 h. In the second method, a flow of Cl2 gas passed through a reactor containing glass beads coated with dry HgO. The gas stream exiting the reactor contained Cl2O and Cl2 and was condensed at 195 K. The Cl2O samples were purified by trap-to-trap distillation to remove the Cl2 impurity. The Cl2O purity, >9%, was measured by UV absorption.6 Cl2O was added to the apparatus by passing a small flow of bath gas over the liquid sample held in a Pyrex reservoir at 195 K. Reference UV absorption spectra for Cl2O and Cl2 were recorded using the diode array spectrometer. The measured Cl2 spectrum was in excellent agreement with currently recommended values at all temperatures used in this study. For Cl2O, the temperature dependence of the UV/vis absorption spectrum was not available and was measured in a separate set of experiments. The details of these measurements are given elsewhere.12 Chlorine dioxide (OClO) was prepared online by slowly flowing a 5% Cl2/N2 mixture through a 25 cm long Pyrex tube containing NaClO2 crystals. The residence time of Cl2 in the reactor was >60 s, and conversion of Cl2 to OClO was >95% as determinedby UV absorption.NaClO2 crystalswere changed frequently to maintain a high OClO production efficiency. Gas flow rates were measured using calibrated electronic mass flow meters. Pressures were measured using a 1000 Torr capacitance manometer.

3. Results and Discussion

Results from the pulsed laser photolysis diode array absorption (PLP-DA) and cold trap thermal desorption (CT-TD) experimentsare presented separately below. Analysis of the UV absorption spectra and determination of the wavelength depen- dence of the Cl2O2 UV absorption spectrum and its absolute absorption cross sections follow, and an evaluation of potential errors in the Cl2O2 spectrum determination is given. Finally, a brief comparison of the present results with previous studies of the Cl2O2 UV absorption spectrum is presented. 3.1. PLP-DA Results. Three photochemical sources were used to produce gas-phase Cl2O2 in the reactor/absorption cell under static conditions at reduced temperature. The two Cl2O2 sourcesused in the majorityof the experimentsutilizedthe same gas-phasechemistrybut were initiateddifferently.Source 1 used the 248 nm photolysis of Cl2O and source 2 used the 351 nm photolysis of a Cl2/Cl2O mixture to produce ClO radicals. In the third photochemical source, ClO radicals were produced in the Cl + O3 reaction following 351 nm photolysis of Cl2 in a Cl2/O3 mixture. Although experimental conditions were usually optimized for maximum production of Cl2O2, measurements were performed using a range of experimental conditions including variations in temperature, total pressure, photolysis laser fluence, and initial reactant concentrations. Figure 3 shows an example of the UV absorption spectra measured in a 248 nm Cl2O photolysis, source 1, experiment performed at 201 K and 725 Torr (He). Prior to photolysis the spectrum is due to Cl2O absorption. The sequence of spectra shown in Figure 3 was recorded following stepwise photolysis of the reactor/absorptioncell contents where each spectrum was recorded ∼10 s after photolysis. In the initial photolysis steps the gas-phase photochemistry leads to the loss of Cl2O and production of Cl2 and Cl2O2. This is qualitatively observed in the sequence of spectra by (1) an increase in absorbance in the

330-380 nm region due to the formation of Cl2 and Cl2O2, (2)

Figure 3. Reference Cl2O and Cl2 spectra (frame a) and absorption spectra measured following the pulsed laser photolysis of an initially pure Cl2O sample at 248 nm (frame b) (see text for details) at 201 K and 725 Torr (He). In frame b the spectrum of the initial Cl2O sample is shown in black. The additional spectra shown in the figure were obtained following the sequential photolysis of the sample (a static experiment) and show the evolution of the combined absorbance due to Cl2O, Cl2, and Cl2O2. Spectra were recorded approximately 10 s after each photolysis step (see text for details). The sequence of absorption spectra from the beginning to the end of the experiment is the same as the order of absorbance at 350 nm from low to high values.

The initial Cl2O concentration was 4.7 × 1015 molecules cm-3 and the photolysis laser fluence was ∼7m Jc m-2 pulse-1. The inset shows an expanded view of the isosbestic point observed near 313 nm. An isosbestic point is also observed near 409 nm (see text for details).

13716 J. Phys. Chem. A, Vol. 113, No. 49, 2009 Papanastasiou et al.

a decrease in absorbance at ∼280 nm, primarily due to the loss of Cl2O, and (3) an increase in absorbance near 250 nm and a shift in the absorbance maximum toward shorter wavelengths due to the formation of Cl2O2 and loss of Cl2O. An analysis of the spectra yields the temporal profiles of Cl2O2,C l2O, and Cl2 that are presented later.

Figure 4 shows an example of the UV absorption spectra obtained using source 2, 351 nm pulsed laser photolysis of a initial Cl2O/Cl2 mixture, at 201 K and a total pressure of 766 Torr (He). The spectra shown are representative of those recorded over the range of experimental conditions used in this study. As observed with source 1, the sequence of UV spectra shows a loss of Cl2O combinedwith formationof Cl2. The Cl2O2 concentration goes through a maximum, although higher Cl2O2 concentrations were achieved using this source due to the much lower extent of Cl2O2 photolysis at 351 nm compared to 248 nm.

PLP-DA experiments performed at temperatures >210 K showed OClO formation that increased with increasing tem- perature. OClO leads to the formation of Cl2O3 via and evidencefor the formationof Cl2O3 was also observedunder some conditions. Test experiments were also performed at 200

K and low pressure, ∼10 Torr (He). At low pressure the bimolecular ClO + ClO reaction increases relative to the termolecular reaction and leads to the increased formation of

OClO via reaction 5c and correspondingly to lower Cl2O2 production. The formation of OClO and Cl2O3 complicates the determination of Cl2O2 absolute absorption cross sections due to uncertaintiesin the secondarychemistryand spectralanalysis.

Therefore, only experiments performed at temperatures <210 K and high pressure were included in the final spectral analysis to determine the Cl2O2 UV absorption spectrum. PLP-DA experiments were usually performed with the shortest delay possible between photolysis steps to minimize possible losses of Cl2O2. Experiments performed using longer delays, up to 60 s, between photolysis steps, however, yielded very similar results, which indicate that no significant gas-phase chemistry occurred during the time required to measure the UV absorption spectra. Experiments were also performed in which the extent of photolysis per step was altered by varying either the number of photolysis laser pulses or the photolysis laser fluence. The absorption spectra measured using sources 1-3 yielded complementary results and provide the basis for the determination of the wavelength dependence of the Cl2O2 UV absorption spectrum and its absolute absorption cross sections described below.

The sequence of UV absorption spectra given in Figures 3 and 4 clearly shows the presence of isosbestic wavelengths near 313 and 409 nm. At these wavelengths, the absorbance remains nearly constant although the concentrations of Cl2O2,C l2O, and

Cl2 change significantly between photolysis steps. The reaction stoichiometry given in eqs I and II defines a relationship between the absorption cross sections for Cl2O2,C l2O, and Cl2 at the isosbestic wavelength where λiso is the isosbestic wavelength and σCl O(λiso,T) and σCl (λiso,T) are the absorption cross sections for Cl2O and Cl2, respectively,at λiso and temperatureT. This relationshipprovides a methodto directlydeterminethe Cl2O2 absorptioncrosssection at the isosbestic wavelengths from the Cl2 and Cl2O absorption cross sections at the same wavelength. Although sources 1 and

2 initiate the Cl2O2 formation chemistry differently, the reaction stoichiometry for these two sources is the same. Therefore, the isosbestic points are expected to be the same for both sources as was observed.Note that the observationof an isosbesticpoint at 408.5 nm provides direct evidence that Cl2O2 absorbs in this wavelength region.

For perspective, we note that isosbestic points would have been predicted, for the Cl2O2/Cl2O/Cl2 reaction system even if

Cl2O2 did not absorb between 200 and 450 nm. However, in that case the isosbestic points would have fallen at ∼322 and

∼400 nm (for experiments performed at 200 K). The isosbestic wavelengths observed here are significantly shifted from these values indicating that Cl2O2 absorbs at these wavelengths. Upon close examination, the isosbestic point observed near

313 nm is not strictlyconstantbut shifts graduallytoward longer wavelengths during the course of an experiment as shown in the insetsin Figures3 and 4. The shift towardlongerwavelength is consistent with the breakdown of the reaction stoichiometry due to the loss of Cl2O2 via secondary chemistry, photolysis, and reaction with Cl atoms. The breakdown in stoichiometry is systematic and predictable with the shift toward longer wave- length resulting from the fact that the Cl2O2 absorption cross

Figure 4. Absorption spectra measured following the pulsed laser photolysis of a Cl2O/Cl2 mixture at 351 nm (see text for details) at 201 K and 766 Torr (He). The spectrum of the initial sample is shown in black. The additional spectra shown in the figure were obtained following the sequential photolysis of the sample (a static experiment) and show the evolution of the combined absorbance due to Cl2O, Cl2, and Cl2O2. Spectra were recorded approximately 10 s after each photolysis step (see text for details). The sequence of absorption spectra from the beginning to the end of the experiment is the same as the order of absorbance at 350 nm from low to high values. The initial mJ cm-2 pulse-1. The inset shows an expanded view of the isosbestic point observed near 313 nm. An isosbestic point is also observed near 409 nm (see text for details). The observed isosbestic points are consistent with the results obtained in the 248 nm pulsed photolysis of

Cl2O shown in Figure 3.

UV Absorption Spectrum of the ClO Dimer J. Phys. Chem. A, Vol. 113, No. 49, 2009 13717 section at this wavelength is greater than that of Cl2. A more detailed analysis of the systematic dependence of the isosbestic point shift and the dependence on the Cl2O2 absorption cross section is given in the Supporting Information. A systematic shift in the 408.5 nm isosbestic point was not observed within the measurementuncertaintybecause the Cl2O2 absorptioncross section is similar in magnitude to those of Cl2O and Cl2 at this wavelength.

The observed isosbestic wavelengths were found to be independent of temperature (200-228 K), the initial concentra- tions of Cl2 and Cl2O, and addition of O2 (10 Torr) to the reaction mixture. Experiments performed using band-pass (309 nm, fwhm ) 10 nm) and cutoff optical filters (WG-320 (λ >320 nm) and WG-375 (λ >375 nm) to isolate the isosbestic wavelength regions yielded identical results. The 408.5 nm isosbestic point was also examined in an experiment with a higher Cl2O2 concentration (a factor of ∼4) than that shown in Figures 3 and 4. In this measurement absorption at wavelengths

<300 nm was optically thick. However, the signal in the long wavelength region was greater and thereby enabled a more accurate absorption measurement. The isosbestic wavelength was identical to that observed in the lower concentration experiments,and no significantshift in the isosbesticwavelength with extent of reaction was observed which is consistent with aC l2O2 cross section that is similar in magnitude to that of Cl2 and Cl2O at 408.5 nm.

(Parte 2 de 5)

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