Absolute Infrared Absorption Cross Section of the Simplest Criegee

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Absolute Infrared Absorption Cross Section of the Simplest Criegee Intermediate Near 1285.7 Cm –1

Yuan-Pin Chang, Yu-Lin Li, Meng-Ling Liu, Ting-Chun Ou, and Jim Jr-Min Lin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06759 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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The Journal of Physical Chemistry

Absolute Infrared Absorption Cross Section of the Simplest Criegee Intermediate near 1285.7 cm–1 Yuan-Pin Chang1*, Yu-Lin Li2,3, Meng-Ling Liu2,4, Ting-Chun Ou2, Jim Jr-Min Lin2,3 1. Department of Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan 2. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan 3. Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan 4. Air Quality Control, Solid Waste and Waste Water Process Engineering, Universität Stuttgart, Stuttgart 70569, Germany

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ABSTRACT

The ν4 fundamental of the simplest Criegee intermediate, CH2OO, has been monitored with highresolution infrared (IR) transient absorption spectroscopy under total pressures 494 Torr. This IR spectrum provides an unambiguous identification of CH2OO and is potentially useful to determine the number density of CH2OO in various laboratory studies. Here we utilized an ultraviolet (UV) and IR coupled spectrometer to measure the UV and IR absorption spectra of CH2OO simultaneously; the absolute IR cross section can then be determined by using a known UV cross section. Due to significant pressure broadening in the studied pressure range, we integrated the IR absorption spectra between 1285.2 cm–1 and 1286.4 cm–1 (covering the Q branch) and then we converted this integrated absorbance to the absolute integral IR cross section of CH2OO (for the Q branch); its absolute value is (3.7±0.6)×10–19 cm·molecule–1 or 2.2±0.4 km·mol–1. The whole rotational band (P, Q and R branches) can be adequately simulated by using the precise spectroscopic parameters from the literature, yielding the absolute integral IR cross section (full ν4 band) to be 19.2±3.5 km·mol–1. For a practical detection of CH2OO, this work also reports the peak cross section as a function of total pressure (494 Torr O2). At low pressure (≤ 4 Torr) where the pressure broadening is insignificant, the absorption cross section of the highest peak is (6.2±0.9)×10–18 cm2·molecule–1 (at the system linewidth of 0.004 cm–1 FWHM).


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INTRODUCTION Criegee intermediates (CIs) are highly reactive carbonyl oxides, which are produced by the ozonolysis of unsaturated hydrocarbons in our atmosphere.1–7 Recent laboratory studies on various prototypical CIs have found that they play important roles in atmospheric chemistry such as oxidizing atmospheric species like SO2, NO2, volatile organic compounds and water vapor to form precursors of secondary organic aerosols;8–36 and their unimolecular processes may generate OH radicals.37–42 The present work focuses on CH2OO, because it is the simplest prototypical CI, which is important for elucidating issues of structures, spectroscopic properties and reactivity of CIs. In laboratory studies, CH2OO can be created efficiently by using a well-established preparation method, demonstrated by Welz et al.8 in 2012: CH2I + O2 →CH2OO + I, where CH2I is produced from the UV photolysis of diiodomethane (CH2I2). This method can generate a significant amount of stabilized CH2OO due to the smaller exothermicity compared to other preparation method.43 Various spectral methods have been utilized to detect CH2OO in the ultraviolet (UV), infrared (IR) and microwave regions. In the UV region, CH2OO has a strong absorption band that corresponds to its B1A'−X1A' electronic transition, which has been studied extensively. Beames et al.44 reported absolute UV absorption cross sections in a molecular beam via laser depletion of CH2OO. The absolute UV absorption cross sections under bulk conditions were reported by following groups: Shep45 utilized cavity-enhanced absorption spectroscopy; Ting et al.46 utilized single-pass absorption spectroscopy; Foreman et al.47 utilized both singlepass absorption and cavity ring-down spectroscopy. However, when compared to the UV spectra obtained from those Beer's law absorption measurements,21,45–47 that from the depletion measurement44 lacks the absorption feature with an oscillatory structure at longer wavelengths (>


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360 nm). A recent study of velocity-map imaging by Vansco et al.48 investigated the photoexcitation of CH2OO in this long wavelength region (364 to 417 nm), and they observed that the action spectrum of O(1D) photofragments also exhibits the similar oscillatory structure, thus justifying the direct absorption measurements. This broad oscillatory structure could be attributed to the short-lived vibrations in the excited B1A' state.48,49 Finally, note that Ting et al.46 determined the absolute values of their cross sections at 308 nm and 352 nm via laser-depletion method,50–52 which can determine the absolute absorption cross sections accurately based on the direct measurements of absolute laser fluences. And their results have a fair agreement with the later results of Foreman et al., where the estimation of absolute cross sections was based on the indirect determination of absolute [CH2OO].47 As a summary, the UV absorption spectrum of CH2OO is intense and broad with a peak cross section of (1.23±0.18)×10–17 cm2·molecule–1 at 340 nm, according to Ting et al.46 In the IR region, Su et al.43 utilized step-scan Fourier-transform infrared (ss-FTIR) spectrometer to extract the absorption spectra of CH2OO between 800 and 1500 cm–1 at a resolution of 1.0 cm–1. Later from the same group, Huang et al.53 reported the ss-FTIR absorption spectra at a better resolution (0.32 cm–1 FWHM), showing a partially resolved rotational structure. These two studies show that the ν4 (CO stretching coupled with CH2 scissoring motion) and ν6 (OO stretching) fundamental bands are the strongest fundamental transitions. Both studies also reported the theoretically predicted IR intensity of each fundamental band. The effect of isotope substitution was also discussed in a follow-up paper.54 Recently, Chang et al.55 utilized a high-resolution mid-infrared distributed-feedback quantum cascade laser (QCL) spectrometer with a spectral resolution of 0.004 cm–1 to study the spectrum of the ν4 fundamental band of CH2OO in region 1273−1290 cm-1; the well resolved ro-vibrational spectra and precise rotational


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spectroscopic constants were obtained. Very recently, Luo et al.56,57 employed an external-cavity QCL and a Herriott cell to study the spectra of CH2OO and syn-CH3CHOO in region 880−932 cm-1 at resolution 0.0015 cm–1. For laboratory studies or even field studies, it is often desired to know the absolute concentration of CH2OO. For example, it can be used to quantify the yield of CH2OO or the contribution of the extremely fast self-reaction,11,12,14,20 etc. Thus, the information of the absolute absorption cross sections is needed. The absolute UV absorption cross sections have been measured extensively as described before,44–47 but no absolute value for IR absorption cross sections has been experimentally determined yet. Many laboratory kinetic studies have utilized the absolute UV cross sections to determine the absolute [CH2OO]. However, compared to the conventional UV spectroscopic probing of CH2OO, the high-resolution IR spectroscopy of CH2OO has the following advantages:55,56 (i) it can avoid byproduct interferences because of a distinct structure of a rovibrationally-resolved IR spectrum, thus promising a higher selectivity; (ii) while the current UV photolysis laser beam may contribute some kind of background (scattered light, absorption of optics, etc.) to the UV probe system, there is essentially no such interference to the IR probing system due to their large wavelength difference.33 In this work, we measured the UV and IR absorption spectra of CH2OO simultaneously. Thus, the absolute value of the IR absorption cross section can be deduced from the known absolute UV cross section of CH2OO directly.46 We further measured the IR absolute absorption cross section as a function of total pressure. We also compared our measurements with several theoretically predicted values and discussed the potential applications of this work.


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EXPERIMENTAL METHODS The experimental details have been described elsewhere,33,55 and thus only a brief description is provided here. The schematic of the combined UV and IR transient absorption spectrometer is shown in Figure 1. Firstly, a small stream of oxygen (when total pressure is 4 torr) or nitrogen flowed above liquid CH2I2 (heated to 306 K) to carry CH2I2 vapor. The concentration of CH2I2 was monitored via its UV absorption by using an absorption cell (75 cm), a light emitting diode (LED) peaked at 290 nm (cross section of CH2I2 ≈ 3.8×10–18 cm2 @ 290 nm) and a spectrometer (Ocean Optics, USB2000+UV-VIS-ES). A 75 cm-long, 19 mm-inner-diameter glass tube with two BaF2 windows at both ends was used as a reaction cell. The windows were purged with nitrogen, which also reduced the effective path length to 67 cm. The mixture gas flow including CH2I2 and O2 or N2 (0-53 sccm) and additional oxygen (0-1450 sccm) was guided into this reaction cell. The flow rates of gases were controlled by mass flow controllers (Brooks, 5850E). In the reaction cell, CH2I2 was photodissociated by a 248 nm excimer laser (KrF, Lambda Physik, LPX-210i, repetition rate: 4 Hz, laser fluence: (3−13) × 1015 photon·cm–2) to form CH2I, which subsequently reacted with oxygen to produce CH2OO. The photolysis laser was introduced into the cell by a dielectric mirror (HR1 in Figure 1) (Eksma Optics, BaF2 substrate, custom item), and then it was reflected away by another one (HR2 in Figure 1); the laser fluence was monitored after HR2 by an energy meter (Gentec-EO, UP19K-30H-VM-D0).


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Figure 1. Schematic of the multi-pass absorption cell, the IR spectrometer (solid line area) and the UV spectrometer (dot line area). AL: aspheric lens; AH: achromatic lens; RAP: right angle prism; CM: concave mirror; WG: optical wedge; MR: mirror; BS beam splitter; HR: dielectric mirror; ND: neutral density filter; FL: bandpass filter; CG: cover glass; OPR: off-axis parabolic mirror. For the IR measurement, the QCL was driven by an intermittent CW driver (Alpes Lasers SA, ICW driver), powered by a DC-power supply (HAMEG Instruments, HMP2020). The IR laser was in pulsed operation mode with a 600 μs pulse period and 40% duty cycle (240 μs pulse duration) for scanning the spectrum of the Q branch of CH2OO, or with a 60 μs pulse period and 40% duty cycle (24 μs pulse duration) for measuring the population time profile of CH2OO. The laser frequency was down-chirped (1286.4 to 1285.2 cm–1) in each laser pulse. The spectral linewidth was estimated to be about 0.002 cm–1. The temperature of QCL was controlled by a temperature controller (Alpes Lasers SA, TC-3) and a Peltier (TEC) cooling element inside the laser housing. The output beam of QCL was projected by a ZnSe aspheric lens (Thorlabs, AL72525-G, f = 25.4 mm) to a 1 mm spot before the reaction cell. A concave mirror (CM in Figure 1) (R = 1016 mm, Edmund Optics, part # 43549) and a BaF2 right angle prism (RAP in Figure 1) (length of leg = 7 mm, Eksma Optics, custom item) were used to focus and reflect both


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the IR and UV probe beams. These two probe beams were fully overlapped with each other, and both of them propagated through the reaction cell four times, achieving an optical path of 0.67 x 4 = 2.7 ± 0.1 m. After leaving the reaction tube, the IR probe beam was detected by a HgCdTe (MCT) detector (Kolmar technologies, KMPV11-1-J2). To determine the absolute IR laser wavelength, an optical wedge was used to split a small portion of the IR laser beam, which was then split again by a beam splitter into 50:50 for the reference gas (3 Torr N2O) and the etalon (Ge etalon 3 in. in length, FSR = 0.0163 cm–1) measurements. We measured the UV transient absorption of CH2OO via laser driven light source (EQ-99, ENERGETIQ) based UV spectrometer. As shown in Figure 1, the output light beam of EQ-99 was projected by an achromatic lens into the reactor cell. A cover glass (0.17 mm thick) was used as a beam splitter to give a reference beam to one channel of the balanced photodiode detector (Thorlabs, PDB450A). We used a dielectric mirror (HR3 in Figure 1) (Eksma Optics, CaF2 substrate, custom item) to combine the UV and IR probe beams before the reactor cell, and both beams passed through the reactor cell four times. After leaving the reactor cell, the IR and UV beams were separated by a similar dielectric mirror (HR4 in Figure 1). And the separated UV beam was guided to the other channel of the balanced photodiode detector. Two 340 nm bandpass filters (10 nm bandwidth, Edmund Optics, 65129) were placed before the two channels of the balanced photodiode detector. The UV signal from the balanced photodiode detector and the IR signals from the DC outputs of all the MCT detectors were acquired by a digital oscilloscope (Lecroy, HDO4034, 14 bits vertical resolution). For each measurement, we averaged the data for 120 photolysis laser shots.


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RESULTS AND DISCUSSION Figure 2 shows the intensity profile of the UV probe light (EQ-99 emission after the bandpass filter) and the absolute UV absorption cross section of CH2OO reported by Ting et al.46 The intensity profile of the probe light centers at 340 nm and has a bandwidth about 10 nm. Thus, the effective UV absorption cross section used in our data analysis was the weighted value according to Equation 1 within 320 to 360 nm, and the corresponding value is (1.21±0.18)×10–17 cm2·molecule–1.

1

1400 1200

CH2OO (Ting et al.) EQ-99 (with filter)

1000

20

2

Absorption cross section / 10 cm molecule

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800 600 400 200 0 -200

300

350

400

Wavelength / nm

450

Figure 2. EQ-99 intensity profile after the 340 nm bandpass filter (black) and UV absorption spectrum of CH2OO (blue) measured by Ting et al.46

𝜎𝑎𝑣𝑔 =

∫𝜎(𝜆)𝐼𝐸𝑄99(𝜆)𝑑𝜆 ∫𝐼𝐸𝑄99(𝜆)𝑑𝜆

(1)

Figure 3 shows the UV and IR absorption time profiles of CH2OO. In this work, we probe the change of absorbance, ΔA, with respect to that before the photolysis laser pulse for both the UV and IR probes. After each photolysis pulse, the fast increase and slow decay of the absorbance indicate that CH2OO was formed within a few microseconds and subsequently diminished to a


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negligible amount after 8 milliseconds (ms) due to self-reaction and reaction with byproducts such as I atoms.11 The total pressures (494 Torr O2) would slightly affect the yield of CH2OO, but they do not affect the lifetime of CH2OO significantly. The UV and IR absorption time profiles have an excellent agreement in terms of decay time, indicating that they probed the same [CH2OO], which can affect its decay time due to self-reaction. The potential interference of byproducts at the UV probe wavelength (340 nm) or at the IR wavelength in both time profiles should be negligible. Finally, note that the short IR pulse duration (24 μs) used in Figure 3 cannot cover the entire Q branch. Thus, to determine the absorbance of the entire Q branch, we employed a longer IR pulse duration (240 μs) for most of our measurements, as described later. To determine the UV absorbance of CH2OO at time zero, we averaged the UV data between 20 and 40 μs photolysis-probe delay time. As the lifetime of CH2OO is much longer (about few ms), we regarded [CH2OO] at the above short time period as [CH2OO] at time zero, [CH2OO]0. As the absorption cross section of the precursor CH2I2 at 340 nm is about 8×10–19 cm2·molecule1, the depletion of CH2I2 by the UV photolysis laser could contribute a negative absorbance in the measured UV absorbance, AUV.11,17,19,24 We estimated that the depleted [CH2I2] is equal to the product of [CH2I2], the laser fluence and the absorption cross section of CH2I2 at the wavelength of the photolysis laser (248 nm). And then we estimated that the negative absorbance due to depleted [CH2I2] is equal to the product of depleted [CH2I2], the optical path length of the UV probe beam and the absorption cross section of CH2I2 at the wavelength of the UV probe beam (340 nm). Based on the experimental conditions provided in Table S1, we estimated this negative absorbance for each experimental data, and the value of this negative absorbance is typically about 5% of the maximum absorbance signal. In the data analysis, we took this contribution of precursor depletion into account, and then all the UV


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absorbance values reported in this work are solely attributed to the absorption of CH2OO. Finally, the absolute [CH2OO]0 was determined from the time-gated absorbance by using the effective cross section described above and the known optical path length. The absolute [CH2OO]0 in the present work was typically in the range of 0.1x1012 to 7.0x1012 molecule·cm–3, as provided in Table S1 of Supporting Information. The variation of [CH2OO]0 can be achieved by changing [CH2I2] or the photolysis laser fluence.11 Finally, note that we also performed the kinetic model simulation described by Ting et al.11 to simulate the [CH2OO] decay curves (see Supporting Information for details), and the modeled [CH2OO]0 and the time profile confirmed the measured values, as shown in Figure S6 of Supporting Information.

Figure 3. Representative time profiles of [CH2OO] at 4 Torr and 298 K probed by UV (red) and IR (black circle) (expt. 91, see Table S1). IR Pulse period: 60μs; duty cycle: 40%. UV and IR measurements: total 161 pulses were averaged.

For the present UV and IR combined setup, both UV and IR measurements were performed simultaneously, and they have the same optical path length. Figure 4 shows the representative


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UV and IR transient absorption spectra in the pump-probe delay time domain. Each IR spectrum was scanned between 12 and 252 μs pump-probe delay time via intra-pulse mode of QCL. As shown in Figure 4, most of the IR spectral feature appears within 100 μs, and the [CH2OO]UV decays by about 5% at 100 μs. Thus, later when we obtained the peak or integral IR absorbance of [CH2OO]0, we also normalized the IR spectrum based on the corresponding UV measurement before further data analysis. To convert the IR spectrum in Figure 4 from the time domain to a frequency domain, we utilized the etalon signal and the reference gas signal measured at the same time to carry out a frequency calibration, as described in our previous work.55

Figure 4. Representative time profiles of UV (blue) and IR (black) absorbance changes (ΔA) due to CH2OO, at 30 Torr and 298 K (expt. 74, see Supporting Information). Figure 5 shows the representative IR transient absorption spectra under five different total pressures. The spectral range of 1286.4 to 1285.2 cm–1 covers the Q branch transitions of the ν4 fundamental band of CH2OO, which corresponds to the motions of CO-stretching coupled with CH2-scissoring mode.43 When the total pressure (O2) increased, the transitions became broader, as shown in Fig 5. This pressure broadening coefficient for O2 has been determined to be (1.46 ± 0.07)x10–1 cm–1·atm–1.55 Under the high pressures, such as above 60 Torr, the byproduct


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ICH2OO could also form in the CH2I2/O2 photolysis system.11,58 Huang et al. have measured the ss-FTIR spectra of ICH2OO,58 They observed several fundamental bands of ICH2OO, and they did not observe any spectral feature of ICH2OO in the spectral range of the present work, i.e., 1285.225–1286.4 cm–1. Indeed, as shown in Figure 5, we do not observe any spectral feature only appeared in the spectra of high pressures, implying no interference of ICH2OO. Finally, we analyzed the IR data in two ways. First, we obtained the peak absorbance of the largest peak of the Q branch, which means the absorbance at 1285.725 cm–1 (see Figure 5). Secondly, we also determined the integral IR absorbance with an integration range of 1285.225–1286.4 cm–1, covering the Q branch.

0.10

0.08

0.06

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.04

0.02

0.00 1286.4

94 Torr 80 Torr 50.4 Torr 30.1 Torr 4 Torr 1286.2

1286.0

1285.8

1285.6

Wavenumber / cm

1285.4

1285.2

1

Figure 5. Infrared absorption spectra of CH2OO at total pressures of 4 torr, 30 torr, 50 torr, 80 torr and 94 torr (O2). The transitions correspond to the Q branch of the ν4 fundamental band of CH2OO. For clarity, the spectra at pressures higher than 4 Torr have been shifted upwards.


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Figure 6. The relationship between the UV absorbance ( Δ AUV) and (a) the integral IR absorbance (integral over Q branch, 1285.225–1286.4 cm1) (Int. ΔAIR) or (b) the peak IR absorbance (at 1285.725 cm–1) (Peak ΔAIR), of the data (symbol) under total pressure 30 Torr. The slope of the fitted line (red) represents the fitted ratio of IR absorbance and UV absorbance. Figure 6 plots the integral or peak IR absorbance versus UV absorbance for the total pressure of 30 Torr. The similar plots of other total pressures are shown in Supporting Information (Figures S1–S5). In Figure 6, note that we deliberately varied [CH2I2] or laser fluence to have a wide range of ΔA. The slope from Figure 6b, as shown as Equation 2, represents the ratio of the peak IR cross sections and the UV cross sections, as both results correspond to a same concentration of CH2OO and the same optical path length. Similarly, the slope from Figure 6a represents the ratio of the integral IR cross sections and the UV cross sections, as shown in Equation 3. Slope =

𝑃𝑒𝑎𝑘 ∆𝐴𝐼𝑅 ∆𝐴𝑈𝑉

𝜎𝐼𝑅

= 𝜎𝑈𝑉

(2)

𝜆

Slope =

𝐼𝑛𝑡. ∆𝐴𝐼𝑅 ∆𝐴𝑈𝑉

=

∫𝜆1𝜎𝐼𝑅𝑑𝜆 2

𝜎𝑈𝑉

(3)

As described before, 𝜎𝑈𝑉 represents the known effective UV cross section, (1.21±0.18)×10–17 cm2·molecule–1, and thus we can obtain the IR cross section 𝜎𝐼𝑅 directly from the slope in Figure


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6. The peak and integral IR absorption cross sections for all total pressures are summarized in Table 1. The error bars in Table 1 come from the propagation of the error bars of slope fitting and 𝜎𝑈𝑉. As shown in Figure 6a, the measured integral ΔA have larger deviations than the peak ones, and they results in the larger errors of the fitted slopes (see Figure S1–S5 in Supporting Information) and the integral cross sections. We attribute these to the noisy backgrounds of the spectra. The error analysis in details is described in Supporting Information. According to Table 1, while the peak cross section significantly depends on the total pressure due to pressure broadening, the integrated cross sections do not depend on the pressure significantly. Thus, we also plot the integral IR absorbance versus UV absorbance for all total pressures, as shown in Figure 7. According to the fitted slope in Figure 7, the integral cross section (averaged integral over Q branch, 1285.225–1286.4 cm1) for all pressures is (3.7±0.6)×10–19 cm·molecule–1 or 2.2±0.4 km·mol–1. Table 1. Integral and peak IR absorption cross sections at different total pressures, with 1-σ error. Pressure (Torr)

Integral absorption cross section

Peak absorption cross section

(10–19 cm·molecule–1)

(10–18 cm2·molecule–1)

4

3.9 ± 0.6

6.2 ± 0.9

30

3.6 ± 0.5

4.6 ± 0.7

50

3.6 ± 0.5

3.9 ± 0.6

80

3.7 ± 0.6

3.2 ± 0.5

94

3.1 ± 0.5

3.0 ± 0.5


15


7

-1

6

-4

Int. AIR / 10 cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5 4 3 2 1 0 0

4

8

12 AUV / 10

16

20

-3

Figure 7. The relationship between UV absorbances ( Δ AUV) and integral IR absorbances (integral over Q branch, 1285.225–1286.4 cm1) (Int. ΔAIR) of all total pressures. The slope of the fitted line (red) represents the fitted ratio of IR absorbance and UV absorbance.

The results of the previous theoretical investigations for the IR intensities of the ν4 and ν6 fundamental bands are summarized in Table 2. Su et al.43 or Huang et al.53 predicted that the absolute intensity of the ν4 mode is 124 km·mol–1 or 70 km·mol–1, respectively. Both Su et al.43 or Huang et al.53 also measured the ss-FTIR spectra of both modes, and the measured intensity ratios of the ν4 and ν6 fundamental bands are 0.42 : 1.0 or 0.37 : 1.0, respectively. We can see that the relative theoretical intensities in Table 2 (ν4 mode versus ν6 mode) do not reproduce the experimental ones; there are also large variations in the absolute theoretical intensity (methoddependent) for the ν4 mode. Such disagreement may imply that the associated uncertainty could be large. Note that these predictions considered the whole ν4 fundamental band, while our measured value only corresponds to the Q-branch of this band due to the limitation of the scan


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range of the QCL spectrometer. To make a fair comparison, we utilized PGopher59 to simulate the whole rovibrational spectrum of the ν4 fundamental band under our experimental conditions (temperature = 298 K and FWHM = 0.02 cm-1 from measurements of the total pressure 94 Torr). And then we estimated the ratio between the intensity the Q-branch (in the spectral range of the present work) and the intensity of the whole band. Based on this ratio and our measured Qbranch integral cross section at 94 Torr, we can predict that the integral cross section of the whole band would be 19.2±3.5 km·mol–1, which is significantly smaller than both theoretically predicted values. Such difference in cross section surprised us, as our previously reported spectroscopic constants of CH2OO measured with the present mid-IR laser spectrometer have a fair agreement with these theoretical studies.53,55 The further investigations, such as measuring the absolute IR intensities of other fundamental bands of CH2OO, may be necessary.

Table 2. Integrated absorption cross sections (unit: km·mol–1) of the ν4 and ν6 modes (two strongest fundamental transitions) of CH2OO from this work and theoretical predictions of previous works. See text for details.

mode

Su et al.43

Huang et al.53

This work

theory

theory

experiment

ν4

ν6

ν4

ν6

ν4

124

124

70

129

19.2±3.5

The measured intensity ratios of the ν4 and ν6 fundamental bands via ss-FTIR studies indicate that the integrated intensity of the ν6 band is larger than that of the ν4 band. However, because the ν4 band has much more congested transitions in the Q branch (see Figure 5 in this work and Figures 2 and 4 from Huang et al.53), the peak intensity of the ν4 band is actually a few times


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larger than that of the ν6 band. As a result, to detect CH2OO, it would be most sensitive to probe the Q branch of the ν4 band. In the recent studies by Luo et al.,56,57 the high resolution spectra for the ν6 band CH2OO in the region of 880−932 cm-1 also show that the intensity of the Q branch is similar to those of the P and R branches.

Finally, the absolute absorption cross section can be utilized to directly determine the absolute concentration of stabilized CH2OO created in alkene ozonolysis. While CH2OO has been detected in laboratory ethylene ozonolysis by means of microwave spectroscopy60 and chemical ionization mass spectrometry,61 its absolute concentration is still not fully clarified. The typical concentration of CH2OO in such experiment is expected to be on the order of 108 molecule·cm–3. According to the peak cross section of the largest peak of the ν4 Q branch at various pressures in Table 1, we can estimate the detection sensitivity for probing CH2OO. Assuming the minimum detectable ΔA is 1×10–4 and the optical path length is 1000 meter, the detection limit of [CH2OO] is about 3×108 molecule·cm–3 at low pressures (< 100 Torr). The detection sensitivity would become worse at a higher pressure because of pressure broadening. Nevertheless, the required optical path length for probing CH2OO in ozonolysis with such a concentration (108 molecule·cm–3) is expected to be on the order of 103 meter, which can be achieved via cavity ring-down spectroscopy or cavity enhanced spectroscopy.

CONCLUSIONS In this work, we built a combined UV and IR transient absorption spectrometer and utilized it to measure the absolute IR absorption cross sections of ν4 fundamental of CH2OO at 298 K from 4 Torr to 94 Torr. The value of the Q-branch-integrated IR cross section is (3.7±0.6)×10–19


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cm·molecule–1. The values of the peak IR cross section at the largest peak of the Q branch range from 6.2×10–18 to 3.0×10–18 cm2·molecule–1 at the pressure range of 4 to 94 Torr.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website: Experimental conditions of all data, error analysis and kinetic modeling.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions Y.-P. C. and J. J. L. designed the experiments. Y.-P. C., Y.-L. Li, M.-L. L. and T.-C. O. performed the experiments. Y.-P. C., M.-L. L. and Y.-L. Li carried out the data analysis. Y.-P. C., M.-L. L. and Y.-L. Li wrote the manuscript. Funding Sources Ministry of Science and Technology, Taiwan, including: MOST107-2113-M-110-004-MY3, MOST106-2113-M-001-026-MY3. Academia Sinica.


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ACKNOWLEDGMENT This work is supported by the Ministry of Science and Technology, Taiwan (MOST107-2113M-110-004-MY3, MOST106-2113-M-001-026-MY3) and Academia Sinica. We thank Mr. CheYu Chiang for helping the IR intensity simulation. We also thank Mr. Wen Chao for building the UV spectrometer.

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(61) Berndt, T.; Herrmann, H.; Kurtén, T. Direct Probing of Criegee Intermediates from GasPhase Ozonolysis Using Chemical Ionization Mass Spectrometry. J. Am. Chem. Soc. 2017, 139 (38), 13387–13392.


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TOC:


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QCL EQ-99

AL

WG

HR3 CG

AH Ref. gas cell

RAP MR

MR BS

FL

Flow cell Balanced photodiode MCT detector 1

MCT detector 2

OPR

MCT detector 3 Etalon

RAP HR4 ND

HR1

photolysis laser

MR ACS Paragon Plus Environment

HR2 CM

1400

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1200

CH2OO (Ting et al.) EQ-99 (with filter)

1000

−20

2

Absorption cross section / 10 ⋅cm ⋅molecule

−1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37


800 600 400 200 0 -200

300

350


400

Wavelength / nm

450

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2.5

Scaled Δ AUV Peak ΔAIR

2.0

−3

1.5

ΔA / 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45


1.0

0.5

0.0

-0.5 0

2

4

Time / ms


6

8

10

∆ A / 10-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

9 8 7


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UV data IR data

6 5 4 3 2 1 0

50

ACS Paragon Plus Environment 100 150

Time / µs

200

250

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0 .1 0

0 .0 8

0 .0 6

∆A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45


0 .0 4

0 .0 2

0 .0 0 1 2 8 6 .4

9 4 T o rr 8 0 T o rr 5 0 .4 T o rr 3 0 .1 T o rr 4 T o rr 1 2 8 6 .2

1 2 8 6 .0

1 2 8 5 .8

1 2 8 5 .6

W a ACSv Paragon e n u Plus m Environment b e r / c m

−1

1 2 8 5 .4

1 2 8 5 .2

6

(a)

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(b)

5 6 -3

4

Peak ΔAIR / 10

-4

Int. ΔAIR / 10 cm

-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

8


3 2

4

2

1 0

0

4

8

12 -3

ΔAUV / 10

16

20

0


0

4

8

12 -3

ΔAUV / 10

16

20

Page 33 of 33

7 6

c m

-1

5

/ 1 0

-4

4 IR

3

I n t . ∆A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44


2 1 0 0

4

8

1 2 -3 ACS Paragon Plus Environment

∆A

U V

/ 1 0

1 6

2 0

Absolute Infrared Absorption Cross Section of the Simplest Criegee

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