Measurement of line strengths in the à 2A’ ← X˜ 2A” transition of HO2 and DO2

Accepted Manuscript

˜ 2 A’ ← X˜ 2 A” Transition of Measurement of Line Strengths in the A HO2 and DO2 Emmanuel Assaf , Oskar Asvany , Ondrej Votava , Sebastien Batut , Coralie Schoemaecker , Christa Fittschen ´ PII: DOI: Reference:

S0022-4073(17)30372-2 10.1016/j.jqsrt.2017.07.004 JQSRT 5771

To appear in:

Journal of Quantitative Spectroscopy & Radiative Transfer

Received date: Revised date: Accepted date:

16 May 2017 7 July 2017 7 July 2017

Please cite this article as: Emmanuel Assaf , Oskar Asvany , Ondrej Votava , Sebastien Batut , ´ ˜ 2 A’ ← X˜ 2 A” Coralie Schoemaecker , Christa Fittschen , Measurement of Line Strengths in the A Transition of HO2 and DO2 , Journal of Quantitative Spectroscopy & Radiative Transfer (2017), doi: 10.1016/j.jqsrt.2017.07.004

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Highlights

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 The à 2A’ ← ̃ 2A” transition of HO2 and DO2 radicals have been measured by cw-Cavity Ring Down Spectroscopy  Absolute absorption cross sections have been determined for both radicals

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Measurement of Line Strengths in the à 2A’ ← ̃ 2A”

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Transition of HO2 and DO2

Emmanuel Assaf1, Oskar Asvany2, Ondrej Votava3, Sébastien Batut1, Coralie Schoemaecker1, Christa Fittschen1

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Université Lille, CNRS, UMR 8522 - PC2A - Physicochimie des Processus de Combustion et de l’Atmosphère, F-59000 Lille, France I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

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J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague, The Czech Republic

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Submitted to

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Journal of Quantitative Spectroscopy and Radiative Transfer

Revised version

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Abstract The absorption spectrum of the à 2A’ ← ̃ 2A” 000-000 band of HO2 and DO2 radicals has been measured in the range 6941 – 7077 cm-1 with a resolution of around 0.005 cm-1. HO2 and DO2 radicals were generated from the reaction of Cl-atoms with CH3OH and CD3OD, respectively, whereby Clatoms were generated by pulsed photolysis of (COCl)2 at 248nm. One time-resolved absorption curve was measured by cw-Cavity Ring Down Spectroscopy (cw-CRDS) for each wavelength. The relative

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absorption coefficient for each wavelength was then obtained from the ring down time , extrapolated to t = 0 s with respect to the photolysis pulse, and the ring down time 0 before the photolysis pulse. Absolute absorption coefficients were quantified for several selected lines relative to a well characterized absorption line of HO2 at 6638.21 cm-1 by a second cw-CRDS absorption path. The maximum absorption cross section at 50 Torr Helium was for HO2 the transition N Ka Kc J = 11 0

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11 11.5 ← 11 1 11 11.5 with 7000.28 cm-1 = 2.12×10-19 cm2 and for DO2 the doublet transition N Ka Kc J = 18 1 18 18.5 ← 18 0 18 18.5 and 18 1 18 17.5 ← 18 0 18 17.5 with 7019.83cm-1 = 2.97×10-19 cm2.. The full spectrum has been very well reproduced by employing spectroscopic data from earlier works.

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Keywords

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Introduction

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HO2 radicals; DO2 radicals; near infrared spectroscopy; cw-CRDS

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HO2 radicals are key species in atmospheric chemistry1. They are formed during the oxidation of hydrocarbons and are involved in the formation of ozone. To understand their reactivity, a selective

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and sensitive detection is therefore necessary. DO2 does not play any role in atmospheric chemistry, but a selective detection is nevertheless highly desired in laboratory studies for carrying out mechanistic studies. Many laboratory studies on peroxy radicals have been carried out using UVabsorption spectroscopy which is a sensitive method, but highly unselective due to the very broad absorption features of these species2. Contrarily, diode laser absorption spectroscopy of vibrational transitions in the infrared region has been used since a long time for a very selective detection of both, HO23 and DO24. Millimeter wave spectroscopy of rotational transitions5-7 has been used to quantify HO2 in interstellar space8 or in the upper atmosphere9.

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ACCEPTED MANUSCRIPT The absorption spectra of HO2 and DO2 in the near infrared region have first been observed by Hunziker and Wendt10 and consist of vibrational overtone transitions as well as of a low-lying electronic transition, typical for peroxides. The overtone transition of HO2 near 6638 cm-1 has been recorded11-14 and already used for various laboratory studies15-17. The low-lying rovibronic à → ̃ 000-000 band has been recorded by Fink and Ramsay for both, HO2 18 and DO219 using FTIR emission spectroscopy. HO2 (and similarly DO2) is a bent triatomic molecule, and is a light asymmetric top rotor with Cs symmetry (with irreducible representations A' and A''). It is a radical with an unpaired

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electron, having thus a total electron spin S=1/2, and thus exhibits a doublet electronic structure in which the spin S couples to the rotational quantum number N to give the total angular momentum of J = N ± 1/2 (these two components are usually called F1 and F2). Accurate spectroscopic data for both radicals have been obtained in the work of Fink and Ramsay and will be used in the frame of this work for simulating our spectra. Some lines of this transition have also been characterized by Kanno

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et al.20,21 and have subsequently been used for studying the reactivity of HO222,23. The detection of DO2 in this wavelength region has been carried out by Estupiñán et al.24.

This work concentrates on the transition from the electronic ground state 2A'' into the 2A' excited state, 2A' ← 2A'', and in particular on its vibrational 000 - 000 band. Absolute absorption cross sections have been determined for both species, HO2 and DO2, and will allow selective quantification

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Experimental technique

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of both radicals while tuning the laser source within a small wavelength range.

A schematic view of the experimental set-up is shown in Figure 1.The experimental technique is a

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laser photolysis reactor coupled to three different detection techniques. It is an improved version of a setup that has been described in previous publications25-27. Compared to the earlier versions, the

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major change is the addition of a second continuous wave cavity ring down spectroscopy (cw-CRDS) absorption path to the photolysis cell, now allowing the simultaneous time-resolved detection of two

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species. The reaction cell has six arms and is made of stainless steel. Four short arms make a vertical plane containing a Laser Induced Fluorescence set-up (not used in this work) and two long arms (total length of 70 cm) are perpendicular to this plane and parallel to the table: the photolysis laser beam and the two cw-CRDS absorption paths propagate along these arms.

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Figure 1: Schematic view of the experimental set-up. APD: Avalanche Photo Diode, AOM: Acousto-Optic Modulator, M: Mirror, L: Lens

A delay generator (Princeton Research 9650) is used to synchronize the experiment. It triggers at the desired repetition rate (typically around 0.3 Hz) the data acquisition of both cw-CRDS systems

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(National Instruments PCI-6259). After a delay of typically 1 s the photolysis laser is triggered (Excimer Lambda Physik LPX 202i operating at 248nm), allowing for acquisition of one second of base line.

Cavity Ring Down Spectroscopy (cw-CRDS)

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A continuous laser source (Agilent mainframe 8164B with laser module 81600B) has been used on one cw-CRDS path for measuring the absorption spectrum. A DFB laser emitting around 6638 cm-1

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(Fitel FOL15DCWB-A81-W1509) is used on the other path for calibration of radical concentration (see further down). The laser beams are fibered from the exit of the respective laser (Agilent or DFB) and pass successively through an isolator and an acousto-optical modulator (AA opto-electronics MT110-

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IIR25-3FIO): the 1st order diffraction beam of the AOM is deflected into the CRDS cavity, thus allowing to switch off rapidly and completely the beam entering the CRDS cavity. The remaining 0th order

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beam is injected into a wavemeter (Burleigh). At the exit of the fiber, the beams pass a mode matching lens (f = 10 mm) and are deflected into the cavity. The cavity consists of 2 highly reflective

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concave (R = 1 m) mirrors (Layertec, R= 0.999968 for 6638 cm-1 and R = 0.999913 for 7000 cm-1) with the outer surfaces slightly wedged. In order to align the cavity in spite of the wedged surfaces, a HeNe laser can be coupled into the fiber at the exit of the AOM: the transmissibility of the mirrors at around 600 nm is high enough to visualize the beam at the exit of the cavity, allowing its rough alignment. The fine alignment is then achieved by optimizing ring-down time and signal intensity. One mirror is glued onto a piezoelectric transducer, which modulates periodically the cavity length. When the cavity length is in resonance with the wavelength of the laser emission, light intensity builds up in the cavity, and with this the light intensity transmitted through the mirrors increases as well. This transmitted intensity is monitored by an avalanche photodiode (Perkin Elmer C30662E), 5

ACCEPTED MANUSCRIPT and a homemade threshold circuit uses the signal intensity to turn off the AOM as soon as the intensity exceeds a user-defined limit. Once the AOM is off, no light enters the cavity anymore and a clean, exponential ring-down event is observed. Time resolved absorption traces are obtained by recording ring-down events at different delays with respect to the photolysis laser. Using cw-CRDS, the ring-down events occur more or less randomly, therefore it is necessary to record a timestamp for each ring-down event, indicating its delay relative to the photolysis laser. This is achieved as follows: once the acquisition system has been triggered by the delay generator, the signal of the

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avalanche photodiode is streamed continuously with a resolution of 800 ns to the PC, generally for a total of 2 s (1 s before the photolysis pulse and 1 s after the photolysis pulse). A numerical filter sets the signal of the photodiode to zero as long as the AOM is on, i.e. light enters the cavity and no ringdown event is occurring. After the 2 s, a homemade Labview program scans the entire data stream and recognizes the occurrence of a ring down event in the data stream by a non-zero signal. Once

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localized, each ring down event is fitted initially by linear regression over the first 10 µs in order to get a first estimation of the ring-down time. In a second fit, a more precise ring-down time is obtained by fitting the ring-down event to an exponential decay over 7 lifetimes such as obtained by the linear fit. The ring down time is then recorded, together with the delay relative to the photolysis laser. Ring down events do occur randomly with respect to the photolysis pulse, but accumulating

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ring-down events over several photolysis pulses leads to time resolved absorption profiles with well distributed data points. Depending on the mirror reflectivity in the two wavelength ranges, typical

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ring-down times for the empty cavity were 80 µs around 6638 cm-1 and between 20 and 40 µs in the range 6941 – 7077 cm-1. Ring-down times τ are converted to absorbance α by the following equation:

RL  1 1     c   t  0 

[1]

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 t  [ A]t   

with τ0 and τt the ring-down time before and after the photolysis pulse, respectively, σ the absorption cross section of the absorbing species, RL the ratio between cavity length (79 cm) and absorption

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length (in our case the overlap of photolysis laser and near IR laser: 37.3 cm), c the speed of light. For obtaining the absorption spectra of HO2 and DO2, one time resolved trace has been registered for each wavelength for each radical. Due to the random occurrence of ring-down events, their number per time unit can vary and in order to have kinetic decay with comparable quality for all traces, the number of photolysis pulses to be summed has been automatically varied: as soon as 70 ring-down events had occurred within 30 ms after the photolysis pulse, the Labview program automatically incremented the laser wavelength by 0.001 nm (≈0.005 cm-1). This was typically achieved after 3 – 5

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Experimental conditions The absorption spectra have been recorded at a total pressure of 20 Torr. The main flow consisted of 40 cm3min-1 Helium and 40 cm3min-1 O2 (both Alphagaz). Liquid CH3OH (CD3OD) was kept in a glass bottle at 12°C in a water bath. Helium was flown through this bottle and the total pressure in the

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bottle was stabilized at 800 Torr. 0.5 (0.8 for DO2) cm3min-1 of this mixture was added to the main flow, leading to [CH3OH/CD3OD] = 4.6/7.2×1014 cm-3 in the photolysis reactor. (COCl)2 was prepared as a diluted mixture in Helium (3.5%), and 0.5 cm3min-1 (0.8 for DO2) was added to the main flow. Absolute concentrations during the measurement of the spectrum were then [(COCl)2] = 1.35×1014 cm-3 for the measurement of the HO2 spectrum and 2.1×1014 cm-3 for the measurement of the DO2

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spectrum, [O2] = 3.3×1017 cm-3 (2.6 × 1017 for DO2), The photolysis energy was 70 mJ/cm2, leading to estimated initial Cl-atom concentrations of 7.3/11.2×1012 cm-3 for the HO2/DO2 measurements.

Results and discussion

HO2 radicals have been generated by the following reaction sequence:

(R1)

COCl → CO + Cl

(R2)

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(COCl)2 + h248nm → Cl + COCl + CO

(R3)

CH2OH + O2 → CH2O + HO2

(R4)

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Cl + CH3OH → HCl + CH2OH

DO2 radicals have been generated the same way, but using CD3OD instead of CH3OH. Under our

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conditions, (R3) and (R4) have (with rate constants28 of k3 = 5.5×10-11 cm3s-1 and k4 = 9.6×10-12 cm3s-1) pseudo-first order rates of k3’= 38.000 s-1, and k4’= 2.8×106 s-1, and are thus completed within few 10

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µs.

Measurement of HO2 spectrum A typical signal of the ring down time as a function of the delay to the photolysis pulse is shown in the left graph of Figure 2, while the right graph shows the corresponding absorption signal after application of equation [1]. At time 0 the photolysis laser generates Cl-atoms, which are converted immediately on the time scale of the experiment into HO2 radicals and hence the ring down time decreases due to absorption. The ring down time gets slowly back to the initial value (before the

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Figure 2: Typical signal of HO2 obtained at 7000.27 cm-1. Left graph: the ring down time  is plotted as a function of the delay with respect to the photolysis laser occurring at t=0. Right graph: Ring-down times have been converted to absorbance  by applying eq. [1]. [(COCl)2]= 1.4×1014 cm-3, [CH3OH] = 5×1014 cm-3, photolysis energy 70 mJ cm-2. The spectrum is obtained from such signal and a zoom of the HO2 spectrum over 3 cm-1 is shown in Figure 3. This portion of the spectrum has been extracted from 600 individual kinetic decays such as shown in Figure 2. The blue line is the ring down time 0 (left y-axis) obtained from the average of all

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ring down events having occurred before the photolysis pulse (shown as blue dots in the left graph of Figure 2), the red line (left y-axis) represents the ring down times at zero delay relative to the photolysis laser, t=0, obtained by applying an exponential fit of the time resolved decays between t =

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0.5 to 30 ms and a fixed decay rate of 55 s-1, shown as red line in the left graph of Figure 2. The extrapolation of this fit to t = 0s is used to calculate the spectrum by applying equation [1] and is

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shown as black line (right y-axis) in Figure 3. The full spectrum, shown in Figure 6, has been obtained

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from 28000 kinetic decays.

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ACCEPTED MANUSCRIPT Figure 3: A zoom of a small portion of the HO2 spectrum. Blue line (left y-axis): ring-down time 0, i.e. average of all ring down events having occurred before the photolysis pulse. Red line (left y-axis): ring down times t=0 from extrapolation of the time resolved decay to a zero delay relative to the photolysis laser. The black line (right y-axis): absorbance  obtained by applying eq [1]. Low concentrations of water are always present in our reactor, introduced to the cell by the carrier gas through permeation of ambient H2O through the Teflon tubes or through small leaks. Thus, several H2O absorption lines can be seen in the blue and red traces of Figure 3 and the H2O concentration can be calculated from the integrated line strength: in HITRAN one finds S=1.823×10-22

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cm for the line at 7025.05 cm-1, leading to [H2O] = 1.9×1014 cm-3 in our experiment. However, this is not perturbing the measurement of the HO2 spectrum, because these lines are present at the same intensity before and after the photolysis pulse and thus the corresponding absorbance  becomes 0, when applying Eq [1] (black line in Figure 3).

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The measurement of the full spectrum (Figure 6) took a total of 3 days. In order to control the stability of the radical concentration over the entire measurement period, the absorption feature at 7028.565 cm-1 has been measured twice a day (Figure 4). The absorption feature consists of five absorption lines as indicated by the results from the PGOPHER fit (shown a black bars in Figure 4) and all spectra have been fitted to Voigt profiles using Fityk software29. Total surface areas of 5.42, 5.66,

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5.65, 5.46, 5.94 and 5.73×10-9 cm-2 were obtained from these fits, indicating that the HO2 concentration has varied by less than 5% over the 3 days. From this result we conclude that

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calibrating a few selected lines over the entire spectrum will allow converting the relative absorbance

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 / cm-1

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into an absolute scale for the entire wavelength range.

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Figure 4: Selected HO2 lines, measured twice per day to verify the stability of the HO2 concentration over the measurement period. The integrated surface area agrees to better than 5%. Bars represent line positions simulated with PGOPHER30 using the spectroscopic data provided by Fink and Ramsay18.

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Calibration of the HO2 absorption spectrum In order to convert the absorbance  into absorption cross sections , the concentration of HO2 radicals needs to be determined. This has been achieved through quantification of the HO2 concentration on the well characterized absorption line11,12,31,32 at 6638.205 cm-1. For this purpose, one CRDS path was set to determine the HO2 radical concentration by measuring the kinetic decay on

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the well characterized line at 6638.205 cm-1 in the overtone transition while the other CRDS path was used to scan five selected lines in the à 2A’ ← ̃ 2A” 000-000 band. These experiments have been carried out at 50 Torr Helium total pressure.

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Table 1: Absorption cross section at 50 Torr Helium for 5 selected lines of HO2, obtained by simultaneous quantification of HO2 through the well characterized absorption line at 6638.205 cm-1 and the absorbance such as obtained during the measurement of the full spectrum (Figure 6) at 20 Torr.  / cm-1 at 50 Torr

 / cm2 at 50 Torr

6638.205

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2.72×10-19

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1.76×10-6

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1.37×10-6

7000.28

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2.12×10-19

1.68×10-6

7024.02

1.89×10-6

1.88×10-19

1.48×10-6

7024.08

1.30×10-6

1.29×10-19

1.04×10-6

2.36×10-20

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In Figure 5 are plotted the absorbance  from the full HO2 spectrum as a function of the absolute

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absorption cross section for the 5 lines such as summarized in Table 1. From the slope of the linear regression we deduce that the concentration of HO2 radicals during the measurement of the full spectrum was [HO2] = (7.87±0.04)×1012 cm-3. The given error is statistical only: for the full spectrum,

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additional errors need to be taken into account from the variability of HO2 concentration during the entire measurement period (5%) as well as the uncertainty of the absorption cross section of the line at 6638.205 cm-1, used for calibration. A small uncertainty on individual absorption cross sections might arise from differences in the broadening coefficients for different lines12. Indeed, our procedure (using absolute absorption cross sections obtained at our usual working pressure of 50 Torr Helium to calibrate an entire spectrum obtained at 20 Torr Helium) presumes identical broadening coefficients for all lines. A total uncertainty of 15% is estimated.

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Figure 5: Absorbance  measured during the full scan as a function of the absorption cross section , obtained from comparison with the simultaneously measured, well characterized absorption line at 6638.205 cm-1. Black dots: HO2, red dots: DO2 This concentration has been used to convert the absorbances  of the full spectrum into absorption cross sections, the full spectrum for HO2 is shown in Figure 6.

Using the accurate spectroscopic parameters for the 000 - 000 vibrational band of the electronic transition 2A' ← 2A'' (for details of the spectroscopic parameters please see Fink and Ramsey18) we

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simulated our absorption spectra using the program PGOPHER30. These simulations are compared to the measured data in Figure 6 for HO2. The simulated band is of C-type and therefore has the

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selection rules ΔKa = ±1 and ΔKc = 0. Prominent are thus the Q- and R-branches of the sub-band Ka'=0 ← Ka"=1 at about 7000 and 7020 cm-1, as well as the Q- and R-branches of the sub-band Ka'=1 ←

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Ka"=0 at about 7040 and 7060 cm-1, respectively.

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Figure 6: Measured (upper trace in black) and simulated (lower trace in red) spectra of HO2 for the 000-000 band of the 2A' ← 2A'' electronic transition. The simulation has been performed with PGOPHER30, based on the accurate molecular constant given by Fink and Ramsay18. The zoom in the range 7039-7045.2 cm-1 shows the strong C-type Q-branch transitions of the sub-band Ka'=1 ← Ka"=0, which show up in pairs belonging to the two spin manifolds F1 and F2, as well as a dense forest of weak lines originating from the A-type (ΔKa = 0, ΔKc = ±1) magnetic dipole transitions18. Fitting the contours of the simulated spectrum, general parameters like the internal temperature and

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the average linewidth, as well as the internal calibration of the commercial diode laser system can be checked. For the HO2 data set, the intensity distribution corresponds to fitted temperatures of about T=325 K. With this, the HO2 is somewhat colder during our measurements compared to the

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corresponding measurements of Fink and Ramsey18 (350K). A temperature close to room temperature can be expected, due to the fast collisional equilibration with He after the production of

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the investigated species. Our dataset has linewidths with an average FWHM of about 0.019cm-1 (= 600 MHz), which are dominated by Doppler broadening (for 300K, m=33 u and 7000 cm-1 we expect

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FWHM = 451 MHz) and pressure broadening (pressure in cell was 20 Torr), while the contribution of the Agilent laser (it is intrinsically narrow with a jitter of a few ~10 MHz) can be neglected. The comparison with the well-calibrated spectroscopic parameters18 also allows correcting for a slight frequency shift of our Agilent laser which was found to be about 0.003-0.005 cm-1 too low. While the vast majority of the line positions and intensities are very well reproduced by PGOPHER, the positions of some lines are slightly shifted or not predicted at all. An example is shown in Figure 7 with the deviations indicated by arrows. Non predicted transitions could principally originate from species present in the absorption cell other than HO2. However, given that the spectrum is obtained from the change in absorbance following the photolysis pulse, only photolysis products are possible 12

ACCEPTED MANUSCRIPT candidates, absorption of the precursors or other impurities will not show up in the absorption spectrum (see H2O lines in Figure 3). The most likely candidate in our system is formaldehyde, CH2O, the co-product to HO2 from the reaction (R4). Its absorption spectrum has recently been published33,34 and is reproduced as green line in Figure 7. It can be seen that there is no agreement between the CH2O spectrum and the non predicted absorption lines. Besides, the absorption cross sections for CH2O are roughly 100 times smaller than those for HO2, and given that both species are generated in equal amounts (CH2O is a stable molecule and could to a certain extend accumulate in

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the cell, but only the absorption due to “fresh” molecules would be detected, all absorption due to “old” molecules would also be present in the baseline), it can be considered that CH2O is not responsible for of the unidentified absorption lines. The most probable explanation of this deviation is thus a known perturbation in the upper 2A' state which is not captured by the standard spectroscopic constants and our corresponding simulation. More details about this perturbation can

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be found in Ref18.

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 / cm-1

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Figure 7: Zoom of the HO2 spectrum between 7000 and 7001.5 cm-1. Black line: measurement (left yaxis), red sticks: prediction by PGOPHER (in arbitrary units), green line: CH2O absorption spectrum (right y-axis), taken from Ruth et al.33,34. The disagreement between simulation and observation for the two rightmost red sticks is due to perturbations and has also been observed by Fink and Ramsey18 (see their Table1, lines Q1,2(21)1-0 ).

Pressure broadening The pressure broadening for HO2 has been determined for the line at 7000.28 cm-1. This is the strongest line and is hence susceptible to be used in laboratory studies for its quantification, as it will lead to the best sensitivity. The line consists of a convolution of two nearly perfectly overlapping transitions (a strong one at 7000.293 cm-1 and a roughly four times weaker one at 7000.284 cm-1). 13

ACCEPTED MANUSCRIPT The broadening was measured at four different pressures in helium: 21.4, 49.2, 73.8 and 100.1 Torr. The HO2 concentration has simultaneously been quantified on the second cw-CRDS path at 6638.205 cm-1. The line was first scanned with a resolution of 0.001 cm-1 for each pressure. The lines were then reproduced by a fit to a single Voigt profile. The following parameters can be used for practical purposes to retrieve peak absorption cross sections using a Voigt profile: line strength S = 5.0×10-5 cm and a broadening of γ = 4.0×10-5× p/Torr + 9.02×10-4 cm-1. The peak absorption cross sections  =

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2.19, 1.93, 1.75 and 1.59×10-19 cm2 are obtained for 20, 50, 75 and 100 Torr Helium, respectively.

Measurement of DO2 spectrum

The DO2 spectrum has been obtained by the same method as HO2, only CD3OD was used instead of

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CH3OH. As it turned out during the data analysis a good part of the observed absorption peaks were identical with HO2 peaks. This complication aroused from the fact that fast D/H exchange of the –OD in CD3OD with H2O, always present in low concentrations in the reaction cell and also on the walls of the Teflon tubing, leads to the partial conversion of CD3OD to CD3OH. Reaction with Cl atoms leads to the formation of CD2OH and subsequent reaction with O2 finally leads to formation of HO2 instead of

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the desired DO2. Therefore, the measured absorption spectrum was the sum of HO2 and DO2 and the pure DO2 spectrum was extracted by subtracting the HO2 spectrum. This is illustrated in Figure 8 by

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the zoom of two portions of the spectrum: the dashed blue line represents the initially measured spectrum such as obtained from the kinetic traces described above for HO2, the black line represents the pure HO2 spectrum such as obtained in this work and presented in the above paragraphs. The red

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line finally represents the pure DO2 spectrum, obtained after subtraction of the HO2 spectrum, multiplied by a factor such as to bring major HO2 transitions to zero, without getting a negative

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absorbance.

HO2 DO2 corrected for HO 2

 / cm

 / cm -1

1.010 - 6

1.010 - 6

DO2 + HO2

-1

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1.510 - 6

5.010 - 7

5.010 - 7

0 7037.0

0 7037.5

 / cm-1

7038.0

7059.0

7059.5

7060.0

 / cm-1

Figure 8: Dashed blue line: initially measured absorption spectrum of DO2, contaminated with HO2.

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ACCEPTED MANUSCRIPT Black line: Spectrum of pure HO2 as obtained in this work. Red line: DO2 spectrum obtained after subtraction of HO2 spectrum, multiplied by an appropriate factor. The multiplication factor changed over the measurement period of nearly four days, probably because the H2O initially adsorbed on the Teflon tubing was slowly transformed into HDO and D2O and thus the D/H exchange became less important with time. This is visualized in Figure 9: the left graph shows the regularly measured DO2 absorption line at 7022.98 cm-1, the right graph shows the maximum absorbance of that line as a function of the total measurement time. It can be seen that

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after a rapid change at the beginning of the measurement period, the concentration was rather stable after the first day. From the known absorption cross sections for HO2, the final, residual HO2 concentration has been calculated to (5.3±0.3)×1012 cm-3, to be compared with [DO2] = 7.0 × 1012 cm-3 (see below), i.e. only 57% of Cl-atoms were converted to DO2.

2.010 - 6

1.510 - 6

max / cm -1

 / cm -1

1.510 - 6 1.010 - 6 5.010 - 7

1.010 - 6

5.010 - 7

7022.98  / cm-1

7023.00

7023.02

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7022.96

0

0

20

40

60

80

100

120

t / hrs

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0 7022.94

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2.010 - 6

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Figure 9: Left graph: absorption line at 7022.98cm-1, measured twice a day during the measurement of the full spectra to check for the stability of the DO2 concentration. Right graph: maximum absorption at 7022.98 cm-1 for each individual spectrum as a function of the measurement time. Therefore, the entire spectrum was visualized in small portions and the multiplication factor was

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adapted accordingly. In good agreement with Figure 9, a higher multiplication factor for subtraction of the HO2 spectrum was needed for the first parts of the spectrum, i.e. a larger fraction of the initial Cl-atoms were converted to HO2 instead of DO2 on the first day. To take into account this fact, the

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DO2 absorbances of the total spectrum (Figure 10) have been corrected accordingly, supposing that the overall radical concentration, i.e. the initial Cl-atom concentration, was stable over the entire measurement period (just as has been found for the HO2 measurements).

Calibration of the DO2 absorption spectrum The calibration of the DO2 spectrum is less straightforward than described above for the HO2 spectrum. This is due to the difficulty of getting pure DO2, and hence the remaining fraction of HO2 needs to be determined simultaneously. As for the calibration of the HO2 spectrum, the Cl15

ACCEPTED MANUSCRIPT concentration was initially converted to HO2 by addition of CH3OH, and HO2 was quantified at 6638.205 cm-1. Once finished, the CH3OH flow was turned off and the main Helium flow was bypassed through a glass bottle containing D2O with the goal of replacing as much as possible of the adsorbed H2O by D2O. After around 30 minutes, CD3OD was added and nine absorption lines were scanned on one cw-CRDS path. Simultaneously, the remaining HO2 concentration was quantified on the second cw-CRDS path. The DO2 concentration was then calculated as the difference between the initial Cl-atom concentration and the remaining HO2 concentration. This way, 89% of the initial Cl-

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atoms were converted to DO2, which is more than the 57% during the measurement of the full spectrum (unfortunately, during the measurement of the full spectrum we were not aware of this problem, so we did not add any D2O). This is due to the fact that high concentrations of D2O accompanied the CD3OD, which was not the case during the measurement of the full spectrum. Also, the CD3OD flow was lower during the measurement of the entire spectrum, due to limited availability

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of CD3OD and the long time it took for measuring the full spectrum. The results are summarized in Table 2.

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Table 2: Absorption cross section at 50 Torr Helium for 9 selected lines of DO2, obtained by simultaneous quantification of the residual HO2 concentration through the well characterized absorption line at 6638.205 cm-1 and the absorbance such as obtained during the measurement of the full spectrum at 20 Torr.  / cm-1 at 50 Torr

 / cm2 at 50 Torr

6950.85

3.45×10-7

1.21×10-19

8.9×10-7

2.78×10-7

9.70×10-20

7.25×10-7

8.28×10-7

2.97×10-19

1.98×10-6

7.17×10-7

2.51×10-19

1.72×10-6

5.88×10-7

2.05×10-19

1.43×10-6

6.82×10-7

2.38×10-19

1.61×10-6

7020.24

2.97×10-7

1.04×10-19

7.57×10-7

7020.28

8.03×10-8

2.80×10-20

2.19×10-7

1.72×10-7

6.00×10-20

4.62×10-7

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 / cm-1

6989.52

7022.98 7034.13

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7034.18

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7019.83

7020.34

 / cm-1 full spectrum

The red dots in Figure 5 show the plot of the absorbance , obtained during the measurement of the entire spectrum, as a function of the absorption cross section. From the slope it can be deduced that the DO2 concentration during the measurement of the full spectrum was (6.9±0.1)×1012 cm-3, and this value has been used to convert the absorbances of the full spectrum into absolute absorption cross sections (Figure 10). The given error is statistical only, and same as for HO2 other sources of error have to be taken into account: variability of DO2 concentration during the entire measurement 16

ACCEPTED MANUSCRIPT period and uncertainty of the absorption cross section of the line at 6638.205 cm-1 used for calibration. A total uncertainty of 20% is estimated. Similar to the HO2 case, we simulated the DO2 spectrum with the program PGOPHER30, using the accurate spectroscopic parameters for the 000 - 000 vibrational band of the electronic transition 2A' ← 2A'', provided by Fink and Ramsey19. These simulations are compared to the measured data in

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Figure 12.

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Figure 10: Measured (upper trace in black) and simulated (lower trace in red) spectra of DO2 for the 000-000 band of the 2A' ← 2A'' electronic transition. The simulation has been performed with PGOPHER30 based on the accurate molecular parameters given by Fink and Ramsay19. The zoom in the range 7019.5-7030 cm-1 shows part of the Q-branch of the Ka'=1 ← Ka"=0 as well as Ka'=0 ← Ka"=1 sub-bands. The seen pairs belong to the two spin manifolds F1 and F2. As for HO2, general parameters like the internal temperature and the average linewidth, as well as the calibration of the used diode laser could be checked. For the DO2 data set, the intensity distribution corresponds to a fitted temperature of about T=300 K, a temperature close to room temperature as expected, and slightly below the temperature for HO2. The difference is within the experimental uncertainty, especially as only the central portion of the transition has been measured. The obtained line width and the small shift of laser are very similar to the HO2 case.

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ACCEPTED MANUSCRIPT Pressure broadening of DO2 The broadening with Helium of two absorption lines has been investigated: 7019.83 cm-1 (the most intense line) and 7022.98 cm-1, another intense line that has already been employed for kinetic measurements by Estupiñán et al.24. The broadening of the line at 7019.83 cm-1 has been investigated by two different methods: (a) the absorption line has been scanned at 4 different pressures and the broadening coefficient has been

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obtained by fitting the absorption line to a Voigt profile and (b) the peak absorption cross section has been measured at the same pressures by kinetic method11,35. Both methods lead to very consistent results:

(a) the lines have been scanned at 13.3, 51.9, 79.8 and 98.3 Torr with a resolution of 0.001 cm-1. A fit

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to a Voigt profile, using the Fityk software29, leads to the Lorentzian broadening parameter, plotted in Figure 11 as a function of the pressure. A linear regression leads to a broadening coefficient of γHWHM = 0.04382 cm-1atm-1.

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0.004

0.002

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HWHM / cm-1

0.006

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0.000 0.00

0.05

0.10

0.15

p / atm

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Figure 11: Lorentzian broadening for the DO2 absorption line at 7019.83 cm-1 obtained by fitting the absorption line to a Voigt profile using the Fityk software. Doppler linewidth (7.44×10 -3 cm-1) has been fixed for fitting (i.e. gwidth parameter in Fityk was set to 8.93×10-3). Linear regression leads to a broadening coefficient of γHWHM = 0.04382 cm-1atm-1. (b) Briefly, the kinetic method is based on determining the time resolved decays of the radicals. The decay function for radical-radical reactions can be expressed as:

 k  1 1    diff  2k t [ DO2 ]t [ DO2 ]0  [ DO2 ]0 

[2]

whereby k is the rate constant for the DO2 recombination and kdiff expresses other losses of radicals, mostly diffusion out of the photolysis volume. Plotting 1/[DO2]t = f(t) results in a slope of M = 18

ACCEPTED MANUSCRIPT (kdiff/[DO2]0 + 2k) and an intercept of I = 1/[DO2]0. Extrapolation of the slope to infinite [DO2]0 allows getting rid of the diffusion part and thus to determine the rate constant of the reaction. On the other hand, if the rate constant k is known, this method allows by plotting 1/t = f(t) (with  = [DO2] × , see [1]) to retrieve the absorption cross section from the slope M (now being 2k/). From measuring peak absorption cross sections at different pressures one can deduce the broadening coefficient by presuming a Voigt profile. For more details see11,35.

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In the case of DO2, two complications arise: (a) the rate constant of the DO2 self-reaction is not wellknown, only four determinations can be found for the rate constant DO2 + DO2 in the literature4,36-38, leading to values varying by more than a factor of three (4.4 – 14.3 × 10-13 cm-3); (b) A low concentration of HO2 was always present and the rate constant for the cross reaction HO2 + DO2 is not known at all and thus, it is not possible to estimate the loss of DO2 through reaction with HO2. However, the ratio of HO2 : DO2 was always small and similar for all pressures (≈ 6% HO2), therefore

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for our purpose (determining the pressure dependence of the peak absorption cross section) we do the approximation that the possible impact of HO2 on the DO2 decay is the same for all pressures. Figure 12 shows the DO2 and HO2 concentration time profiles at 51.9 Torr.

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210 1 3

1

10 1 3

0 0.00

[HO2] / cm -3

M

310 1 2

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[DO2] / cm -3

310 1 3

0.02

0.04

0.06

0.08

0.10

[(COCl)2] = 6.51014 cm-3 [(COCl)2] = 5.61014 cm-3

210 1 2

[(COCl)2] = 4.71014 cm-3 [(COCl)2] = 3.81014 cm-3 1

10 1 2

0 0.00

0.02

0.04

0.06

0.08

0.10

t/s

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t/s

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Figure 12: DO2 and HO2 concentration-time profiles obtained at 51.9 Torr for different (COCl)2 concentrations. [O2] = 1.2×1017 cm-3, [CD3OD] = 8.6×1014 cm-3, photolysis energy ≈ 60 mJ cm-2. Absorption coefficients  have been converted to absolute concentrations by using 7019.83 cm-1 = 2.97×10-19 cm2 for DO2 and 6638.21 cm-1 = 2.72×10-19 cm2 for HO2. Profiles with 4 different initial concentrations such as shown in Figure 12 have been measured for the 4 different pressures. DO2 absorption profiles have then been plotted according to [Eq 2] and the slope M has been plotted as a function of the intercept I, shown in Figure 13 for all four pressures. The peak absorption cross section at 50 Torr has been determined as explained above, and hence a rate constant can in principle be extracted from an extrapolation to I = 0, i.e. [DO2]0 = ∞. This leads to a rate constant of kDO2+DO2 = 6.2 × 10-13 cm3s-1, in good agreement with the values of Hamilton et al.38 19

ACCEPTED MANUSCRIPT (8.93 × 10-13 cm3s-1), Martin and Thrush3 (4.4×10-13 cm3s-1, measured between 6 and 15 Torr) and Sander et al.2,37 (4.9×10-13 cm3s-1), while the other value is more than a factor of 2 higher (1.43×10-12 cm3s-1)36. However, as explained above, a systematic error can arise from the presence of HO2 in the reaction cell (which might also have happened in the cited work, but was undetected), and therefore the absolute value of the rate constant should be taken with caution.

M / cm s -1

(1 / DO2) / cm

3.010 5

2.010 5

1.010 7

5.010 6

1.010 5

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1.510 7

4.010 5

13.3Torr

51.7 Torr 79.0 Torr 98.7Torr

0 0.000

0.005

0.010

0.015

1.0

10 5

2.0

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t/s

0.020

0 0

10 5

3.0

10 5

4.010 5

I / cm

M

Figure 13: Left graph: Plot of 1/ = f(t) for DO2 traces from Figure 14, obtained at 7019.83 cm-1. Right graph: The slope M as a function of intercept I for graphs such as shown in the left graph for all four pressures, at four concentrations at each pressure (green circles obtained from the experiments shown in the left graph ). Extrapolation to I = 0 leads to M[DO2]0→∞ = 2k /  From Kircher and Sander2 it can be deduced, that the rate constant is only very slightly pressure

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dependent and an increase of less than 3% can be estimated under our condition (13 to 100 Torr Helium). Therefore, the change in the intercept with pressure as shown in Figure 13 is directly proportional to the change in the peak absorption cross section. Taking the value at 50 Torr as

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reference, we deduce 7019.83 cm-1= 3.91, 2.97, 2.62 and 2.29×10-19 cm2 at 13.3, 51.7, 79.3 and 98.3

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Torr, respectively, summarized in Table 3. This result can be compared with the values obtained by the scanning method, all values are summarized in Table 3: the Lorenztian HWHM, the area A, the heights H such as obtained from the

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fit of the fully scanned lines using the Fityk program (note that the DO2 concentration for the scans at different pressures was arbitrary, therefore the absolute values of A and H are also arbitrary). The line strengths S has then been calculated from the area A and heights H, using the absolute absorption cross section  for scaling. The consistency has been checked by calculating the peak absorption cross sections for all for pressures using the Voigt expression with γHWHM = 0.04382 cm1

atm-1 and the average value of the line strengths S = 6.69×10-21 cm. The agreement is excellent with

less than 2% difference in the peak absorption cross sections obtained by both methods.

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ACCEPTED MANUSCRIPT Table 3: Parameters obtained from a fit using Fityk to a Voigt profile of the scanned lines (Lorentzian HWHM, peak area, peak heights) and peak absorption cross sections from kinetic measurements for the line at 7019.83 cm-1. Lorenztian HWHM / cm-1

A / arb. Units

H / arb. Units

 / cm2 from kinetic

S / cm

 / cm2 from Voigt fit

13.3

8.95×10-4

5.97×10-8

3.37×10-6

3.91×10-19

6.92×10-21

3.84×10-19

51.7

2.62×10-3

1.32×10-7

6.17×10-6

2.97×10-19

6.36×10-21

3.00×10-19

79.3

4.64×10-3

8.90×10-8

3.40×10-6

2.62×10-19

6.86×10-21

2.57×10-19

98.3

5.79×10-3

3.44×10-8

1.19×10-6

2.29×10-19

6.64×10-21

2.33×10-19

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P /Torr

The absorption line at 7022.98 cm-1 has been scanned at four pressures between 20 and 77 Torr, shown in Figure 14. In fact, simulation by PGOPHER shows that this line is a convolution of two

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individual transitions N Ka Kc J = 17 1 17 16.5 <- 17 0 17 16.5 at 7022.982 and N Ka Kc J = 17 1 17 17.5 <- 17 0 17 17.5 at 7022.991 cm-1, not resolved under our conditions. In a first attempt, the lines have been fitted by the Fityk software to two individual lines with the Doppler width fixed to the theoretical value. These fits lead to very reasonable results: the positions of the two lines were separated by 0.0104 – 0.0111 cm-1, very close to the predicted value given by PGOPHER (0.009 cm-1).

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The linestrength of the 7022.991 cm-1 was always slightly higher (between 60-62 % of the total linestrength), also in agreement with the predictions of PGopher, pressure broadening was γHWHM, = 0.0287 cm-1atm-1 and γHWHM, 7022.991 cm-1 = 0.0468 cm-1atm-1.

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7022.982 cm-1

However, for practical purposes, i.e. extrapolation of peak absorption cross sections at different

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pressures, it is more convenient to treat the convolution as one single line. A single Voigt fit reproduces the shape pretty well, as shown in Figure 14. In a first round of fitting all four pressures,

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the Doppler width has not been fixed. As can be expected, a slightly larger value has been obtained by this procedure compared to the theoretical width (the average experimental value being 0.00948

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compared to 0.00744 cm-1 for the Doppler width). With the goal of obtaining consistent results, this value has been used as fixed Doppler width in a second round of fitting all four pressures, the result is shown in Figure 14, where all spectra have been normalized to the fitted surface area.

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40 20.3 Torr 37.3 Torr

 / arb. units

30

51.8 Torr 77.6 Torr

20

10

0

7022.96

7022.98

7023.00

7023.02

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 / cm-1

Figure 14: Absorption lines at 7022.99 cm-1, scanned at 4 different pressures: dots are experimental data, full line shows a Voigt fit to a single line. Black bars are positions of absorption lines such as obtained from PGopher. The line strength has been obtained as S = 7.65×10-21 cm, while the pressure broadening at a given

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pressure is γ = 4.834×10-5 × p/Torr + 2.16×10-3 cm-1. These parameters have no physical meaning, but are useful for extrapolating peak absorption cross sections at different pressures. For the four pressures in this work, we thus obtain 2.85, 2.66, 2.51 and 2.33×10-19 cm2 for 20, 37, 52 and 77 Torr Helium, respectively. The peak absorption cross section for this line is therefore, as expected, less sensitive to pressure broadening: extrapolated to 13 and 98 Torr, the peak absorption cross section

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of this line decreases by only 27%, while it decreases by 42% for the line at 7019.83 cm-1.

Conclusion

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The absorption spectra of the rovibronic band à 2A’ ← ̃ 2A” 000-000 of HO2 and DO2 radicals have been measured by a combination of laser photolysis with time resolved cw-CRDS. Absolute

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absorption cross sections have been obtained through either measuring kinetic decays and retrieving the initial concentration from the known rate constant or by comparison with the well-known

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absorption cross section of an HO2 transition in the vibrational overtone region. The strongest absorption cross sections at 50 Torr Helium are 7000.28 cm-1= 2.12×10-19 cm2 and 7019.83cm-1= 2.97×10-19 cm2 for HO2 and DO2, respectively. The spectrum has been simulated using the spectroscopic data from earlier papers by Fink and Ramsay18,19 and reproduced very well the measurements.

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Acknowledgement This project was supported by the French ANR agency under contract No. ANR-11-LabEx-0005-01 CaPPA (Chemical and Physical Properties of the Atmosphere). The measurements were carried out in Lille thanks to the loan of a diode laser module from the Gerätezentrum ”Cologne Center for Terahertz Spectroscopy”, financed by the Deutsche Forschungsgemeinschaft (DFG). The authors thank the Czech and the French governments for financial support through the Barrande program

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References (1)

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Heard, D. E.; Pilling, M. J. Measurement of OH and HO2 in the Troposphere, Chem. Rev. 2003, 103, 5163-5198 (2) Kircher, C. C.; Sander, S. P. Kinetics and mechanism of hydroperoxo and hydroperoxo-d disproportionations, J. Phys. Chem. 1984, 88, 2082-2091 (3) Thrush, B. A.; Tyndall, G. S. Reactions of HO2 studied by flash photolysis with diode laser spectroscopy, J. Chem. Soc., Faraday Trans. 1982, 78, 1469-1475 (4) Martin, N. A.; Thrush, B. A. The disproportionation of DO2 radicals studied by infrared laser spectroscopy, Chem. Phys. Lett. 1988, 153, 200-202 (5) Beers, Y.; Howard, C. J. The spectrum of DO2 near 60 GHz and the structure of the hydroperoxyl radical, J. Chem. Phys. 1976, 64, 1541-1543 (6) Saito, S. Microwave spectrum of the HO2 radical, J. Mol. Spectrosc. 1977, 65, 229238 (7) Charo, A.; De Lucia, F. C. The millimeter and submillimeter spectrum of HO2: The effects of unpaired electronic spin in a light asymmetric rotor, J. Mol. Spectrosc. 1982, 94, 426-436 (8) Parise, B.; Bergman, P.; Du, F. Detection of the hydroperoxyl radical HO2 toward ρ Ophiuchi A, A&A 2012, 541, L11 (9) Wang, S.; Zhang, Q.; Millán, L.; Li, K.-F.; Yung, Y. L.; Sander, S. P.; Livesey, N. J.; Santee, M. L. First evidence of middle atmospheric HO2 response to 27 day solar cycles from satellite observations, Geophysical Research Letters 2015, 42, 2015GL065237 (10) Hunziker, H. E.; Wendt, H. R. Near infrared absorption spectrum of HO2, J. Chem. Phys. 1974, 60, 4622-4623 (11) Thiebaud, J.; Crunaire, S.; Fittschen, C. Measurement of Line Strengths in the 21 Band of the HO2 Radical using Laser Photolysis / Continous wave Cavity Ring Down Spectroscopy (cw-CRDS), J. Phys. Chem. A 2007, 111, 6959-6966 (12) Ibrahim, N.; Thiebaud, J.; Orphal, J.; Fittschen, C. Air-Broadening Coefficients of the HO2 Radical in the 2v1 Band Measured Using cw-CRDS, J. Mol. Spectrosc. 2007, 242, 64-69 (13) Taatjes, C. A.; Oh, D. B. Time-resolved wavelength modulation spectroscopy measurements of HO2 kinetics, Appl. Opt. 1997, 36, 5817-5821 (14) DeSain, J. D.; Hob, A. D.; Taatjesb, C. A. High-resolution diode laser absorption spectroscopy of the O–H stretch overtone band (2,0,0)(0,0,0) of the HO2 radical, J. Mol. Spectrosc. 2003, 219, 163-169 (15) Thiebaud, J.; Aluculesei, A.; Fittschen, C. Formation of HO2 Radicals from the Photodissociation of H2O2 at 248 nm, J. Chem. Phys. 2007, 126, 186101 (16) Knepp, A. M.; Meloni, G.; Jusinski, L. E.; Taatjes, C. A.; Cavallotti, C.; Klippenstein, S. J. Theory, measurements, and modeling of OH and HO2 formation in the reaction of cyclohexyl radicals with O2, Phys. Chem. Chem. Phys. 2007, 9, 4315 - 4331 (17) Yi, J.; Bahrini, C.; Schoemaecker, C.; Fittschen, C.; Choi, W. Photocatalytic Decomposition of H2O2 on Different TiO2 Surfaces Along with the Concurrent Generation of HO2 Radicals Monitored Using Cavity Ring Down Spectroscopy, J. Phys. Chem. C 2012, 116, 10090-10097 (18) Fink, E. H.; Ramsay, D. A. High-Resolution Study of the Ã2A' → X2A'' Transition of HO2: Analysis of the 000–000 Band, J. Mol. Spectrosc. 1997, 185, 304-324 (19) Fink, E. H.; Ramsay, D. A. High-Resolution Study of the Ã2A' → X2A'' Transition of DO2: Analysis of the 000–000 Band, J. Mol. Spectrosc. 2002, 216, 322-334 (20) Kanno, N.; Tonokura, K.; Tezaki, A.; Koshi, M. Water Dependence of the HO2 Self Reaction: Kinetics of the HO2-H2O Complex, J. Phys. Chem. A 2005, 109, 3153-3158 24

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Hamilton, E. J.; Lii, R.-R. The Dependence on H2O and on NH3 of the Kinetics of the self-reaction of HO2 in the gas-phase formation of HO2·H2O and HO2·NH3 complexes, Int. J. Chem. Kinet. 1977, 9, 875-885

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