# Detection and quantification of hydroperoxides and radicals within stabilized cool flames by means of SVUV-PEPICO spectroscopy

Sébastien Batut, Laure Pillier, Jérémy Bourgalais, Thomas Panaget, Pascal Demaux, Olivier Herbinet, Frédérique Battin-Leclerc, Gustavo Adolfo Garcia, Laurent Nahon, Guillaume Vanhove

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Sébastien Batut, Laure Pillier, Jérémy Bourgalais, Thomas Panaget, Pascal Demaux, et al.. Detection and quantification of hydroperoxides and radicals within stabilized cool flames by means of SVUV-PEPICO spectroscopy. Physical Chemistry Chemical Physics, In press, 10.1039/D5CP00887E. hal-05131050

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Physical Chemistry Chemical Physics

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# Detection and quantification of hydroperoxides and radicals within stabilized cool flames by means of SVUV-PEPICO spectroscopy

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DOI: 10.1039/D5CP00887E

Sébastien Batut<fnref n="1" />, Laure Pillier<fnref n="1" />, Jérémy Bourgalais<fnref n="2" />, Thomas Panaget<fnref n="1" />, Pascal Demaux<fnref n="1" />, Olivier

Herbinet<fnref n="3" />, Frédérique Battin-Leclerc<fnref n="3" />, Gustavo A. Garcia<fnref n="4" />, Laurent Nahon<fnref n="4" />, Guillaume Vanhove<fnref n="1" />,*

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<footnote n="1">¹Université de Lille, CNRS, PC2A, F-59000 Lille, France</footnote>

<footnote n="2">² Université de Rennes, CNRS, IPR, F-35000, Rennes, France</footnote>

<footnote n="3">³Université de Lorraine, CNRS, LRGP, F-54000 Nancy, France</footnote>

<footnote n="4">⁴Synchrotron SOLEIL, L'Orme des Merisiers, Départementale 128, 91190 St Aubin, France</footnote>
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DOI: 10.1039/D5CP00887E

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<footnote>* corresponding author: guillaume.vanhove@univ-lille.fr</footnote>
Abstract

A novel coupling of a stagnation plate burner and the SAPHIRS instrument located at synchrotron SOLEIL has been performed to enable the observation of labile species and radicals within stabilized cool flames. Gases were extracted from a dimethyl ether/O₂/O₃/N₂ cool flame with a capillary probe and expanded through two consecutive skimmers to reach the Synchrotron Vacuum UltraViolet-PhotoElectron Photolon Coincidence spectrometer. Threshold PhotoElectron Spectra (TPES) and Total Ion Yields (TIY) were recorded as a function of the photon energy, and lead to the identification of species that have so far never been observed in cool flames, including the only expected keto- or aldohydroperoxide (hydroperoxymethylformate), hydrogen peroxide, methyl and hydroperoxyl radicals. The quantification of the detected species has been performed and is compared with the predictions of a kinetic model, demonstrating the validity of the experimental approach and suggesting possible ways for improvement.

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DOI: 10.1039/D5CP00887E

Introduction

Ever since their discovery by Humphry Dav in the 19th century, cool flames have garnered periodic interest due to their practical applications in coal mine safety¹, the formulation of fuels for internal combustion engines ²,³, and more recently in plasma-assisted ignition ⁴, constant volume chamber combustion⁵ and fire propagation under microgravity conditions ⁶,⁷. Early pioneering work performed in the 1960s at the Shell Thornton laboratory ²,⁸ hypothesized that the ignition of cool flames and the first-stage ignition phenomenon in adiabatic reactors are caused by the formation of hydroperoxides (ROOH), following the addition of radicals to oxygen. Most notably, keto- and aldohydroperoxides were expected. These hydroperoxides decompose to yield radicals such as OH, causing indirect radical-chain branching ⁹ which leads to autoignition. While this theory was consistent with indirect observations, such as the detection of cyclic ethers spawning from hydroperoxide decomposition, the high reactivity of hydroperoxides made their experimental observation a challenge for the following decades.

Experimental proof for the formation of such indirect chain-branching agents was finally obtained in a
motored engine and flow reactor using *n*-heptane and *n*-dodecane as the fuels, where Sahetchian et

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al. ¹⁰,¹¹ employed cold trapping, thin-layer chromatography and offline mass spectrometric analysis
This was later confirmed by Battin-Leclerc et al.¹², who used online synchrotron-based photoionization
mass spectrometry (PIMS) coupled with a jet-stirred reactor (JSR) setup. By exposing gases extracted
from the JSR to tuneable VUV radiation, soft ionization was achieved, consistent with the expected
ionization energies of the ketohydroperoxides (KHP) formed by *n*-butane, although the branching
fraction of the formed ketohydroperoxide isomers could not be accessed from these results. The
identification of individual ketohydroperoxide isomers was then made possible through advanced
techniques such as double imaging photoelectron photoion coincidence (i2-PEPICO) spectroscopy ¹³-
¹⁶.

In the past decade, burner-stabilized cool flames have emerged as a promising configuration to study
ignition-extinction regimes ⁷, low-temperature oxidation kinetics ¹⁷⁻²⁰ and laminar burning velocities
of cool flames ²¹,²². These studies have mostly focused on counterflow - in the premixed ²³ and diffusion
⁶ flame regimes – and stagnation plate ¹⁷,²⁴ configurations. Detailed chemical information¹⁷⁻¹⁹ has been
garnered in non-isothermal conditions relevant to the ignition-extinction regimes. Moreover, burner-
stabilized cool flames offer low-dilution conditions conducive to detecting chain-branching agents and
radicals. Sensitivity analysis has shown the importance of different reaction pathways in cool flames
compared to jet-stirred/flow reactors, revealing their untapped potential for validating chemical
kinetic models. However, despite these advances, and because of the numerous associated challenges,
the coupling of burner-stabilized cool flames with synchrotron-based techniques, such as PIMS or i2-
PEPICO, had not been attempted until now.

The present work presents the first coupling of a cool flame configuration with synchrotron-based
techniques, to detect ketohydroperoxides and radicals in ozone (O₃)-seeded cool flames of dimethyl
ether (DME, CH₃-O-CH₃). DME is a model oxygenated fuel candidate for renewable energy systems, as
it can be derived from biomass or captured CO₂ and renewable electricity²⁵,²⁶. Its symmetrical
structure, small molecular size (m/z 46), and high reactivity in the low-temperature combustion regime

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DOI: 10.1039/D5CP00887E

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lead to the formation of a single aldohydroperoxide, hydroperoxymethylformate (HPMF, HOO-CH₂-O-). This makes DME an ideal reference compound for studying low-temperature oxidation chemistry and ignition processes.

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Although some studies have investigated DME oxidation, particularly aiming at the quantification of KHPs, these efforts remain mostly limited to JSR and flow reactors coupled with SVUV-PIMS. Moshammer et al.<sup>27,28</sup> detected, and reported significant fragmentation of HPMF near its adiabatic ionization energy (AIE~ 10 eV), primarily resulting in the loss of a CO molecule to produce a fragment at m/z 64 (HOO-CH<sub>2</sub>OH) with a much higher signal intensity than the parent KHP at m/z 92. Later, Couch et al.<sup>29</sup> confirmed m/z 64 as the main fragment of HPMF but showed that this holds only at IEs below 10.5 eV. At higher energies, the additional loss of an OOH group produces a previously unreported fragment at m/z 31 (CH<sub>2</sub>OH), with partial photoionization cross sections (PICS) comparable to those of m/z 64 around 11 eV. Couch et al.<sup>25</sup> also derived a total PICS for HPMF that was experimentally twice as low as the theoretical value explicitly calculated by Moshammer et al.<sup>23,24</sup>.

Simultaneously, Liao et al.<sup>30</sup> investigated the effect of ozone addition—known as a combustion promoter—on the low-temperature oxidation of DME in a JSR with SVUV-PIMS. They corroborated the identification of m/z 64 as the primary fragment of HPMF but observed differences in the total ion yield (TIY) curves compared to those reported by Couch et al. and Moshammer et al. Moreover, like Moshammer et al., they did not identify m/z 31 as a major fragment.

Thus, while this reactive system represents a promising tool for studying low-temperature oxidation processes, some ambiguities remain, particularly concerning the fragmentation patterns and photoionization behavior of HPMF. This makes it an excellent reference system for ongoing experimental development and refinement, building on previous studies of stabilized cool flames under similar conditions<sup>17,21</sup>. These prior works have contributed to the creation of a well-validated kinetic mechanism tailored to these systems.

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Physical Chemistry Chemical Physics Accepted Manuscript
# Experimental, Numerical, and Theoretical Methods

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DOI: 10.1039/D5CP00887E

## Experimental Method

The stagnation plate burner configuration developed in PC2A¹⁷ has been duplicated and adapted to enable the coupling at the undulator-based DESIRS³¹ VUV beamline of synchrotron SOLEIL, using the SAPHIRS end-station³², equipped with the double-imaging photoelectron/photoion (i²PEPICO) spectrometer DELICIOUS III³³. Only the details relevant to this coupling are provided here, with a full description of the burner given in Panaget et al.¹⁷. As depicted in Figure 1, the burner assembly is kept within a vessel, the pressure of which is fixed at 96.3 kPa (ie. 0.95 atm) by means of both the pumping of the exhaust gases and a regulated flow of N₂. This flow also helps flush the vessel and protect electrical connections from the burnt gases.

<figure><img src="image_8.png" /><figcaption>Figure 1: Experimental setup used in this study.</figcaption></figure>

The composition of the fresh DME/O₂/O₃/N₂ mixture entering the burner is given by the mole fractions x in Table 1, corresponding to a fuel-lean premixed flame with an equivalence ratio φ (i.e., the molar

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ratio of fuel to oxidizer with regards to stoichiometry) of 0.161. It exits the 10 mm diameter central nozzle, which is maintained at a temperature of 283 K, and flows toward a stagnation plate heated to 650 K, with the burner-plate distance set to 12 mm. A capillary probe with a 150 µm inner diameter and 220 µm outer diameter protrudes 800 µm from the stagnation plate into the flame gases, the total length of the probe being approximately 3 mm. An uncertainty of ± 200 µm is assumed for the reported position, based on relevant studies of the spatial resolution of sampling probes in flames ³⁴. This probe emerges on the other side of the stagnation plate within a 45° half-angle conical section, which allows the expansion of the sampled gases into the jet chamber, inside which the vessel is held and whose pressure is about 10⁻³ mbar.

DOI: 10.1039/D5CP00887E

<table><caption>Table 1: Experimental conditions of the investigated flame</caption><thead><tr><th>X<sub>DME</sub></th><th>X<sub>O2</sub></th><th>X<sub>O3</sub></th><th>X<sub>N2</sub></th><th>Q<sub>burner</sub> / SL.min<sup>-1</sup></th><th>Φ</th><th>Geometric Strain rate α / s<sup>-1</sup></th></tr></thead><tbody><tr><td>0.043</td><td>0.759</td><td>0.026</td><td>0.173</td><td>2.32</td><td>0.161</td><td>41</td></tr></tbody></table>

The sampled gases further expand across two consecutive, 2 mm diameter skimmers, to first reach the differential chamber where the pressure is about 10⁻⁵ mbar before arriving to the spectrometer chamber whose pressure was around 10⁻⁷ mbar. Such expansion conditions facilitate a reduced collision environment, freezing the ongoing reactions and enabling the observation of transient molecular species and radicals. In the ionization chamber the gases are exposed to the VUV beam of DESIRS. Prior to entering the ionization chamber, the light supplied by DESIRS passes through a monochromator, equipped for this experiment with a 200 grooves per mm grating, resulting in a flux of ca. 10¹³ photons per second with a spectral resolution of ca. 25 meV at 10 eV. A gas filter located upstream of the monochromator, filled with Kr, effectively removes high harmonics from the undulator, providing a high spectral purity across the 9.5–12.7 eV range used during the experiments. The photon flux is recorded as a function of photon energy using a dedicated photodiode (AXUV from

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Opto Diode) and used to correct the spectra, while the photon energy scale was calibrated using the well-known ionization energy of the methyl radical³⁵.

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DOI: 10.1039/D9CP00887E

The molecular beam crosses the VUV synchrotron radiation at the centre of the i²PEPICO spectrometer DELICIOUS III ³³. The coincident electrons and ions resulting from the ionization process are accelerated in opposite directions by a 88 V/cm continuous electric field and analysed by a Velocity Map Imaging (VMI) spectrometer and a modified Wiley-McLaren time-of-flight (TOF) imaging spectrometer, respectively. The coincidence operation generates mass-filtered photoelectron images simultaneously for any peak in the TOF mass spectrum which are further treated by Abel inversion³⁶ to yield mass-selected photoelectron spectra as a function of photon energy. The Threshold PhotoElectron Spectra (TPES) are obtained from these data according to the method outlined by Poully et al. ³⁷ and Briant et al., ³⁸ here with an electron energy resolution of 17 meV. The TPES show resonant transitions from the neutral ground state toward vibronic states of the cation and serve as the vibronic fingerprint of the ionic species ¹⁴. These were used for the identification of the structures observed at different mass-to-charge ratios. Whenever possible, the TPES were preferred for identification because of their more distinctive spectral features to decipher isomers. However, when the signal-to-noise ratio does not allow an electron kinetic energy analysis for a specific mass-to-charge channel, the identification was based on the analysis of the respective total ion yield (TIY) curve obtained through integration over all electron energies. The signals as a function of photon energy were normalized by the photon flux, as measured offline with a dedicated AXUV photodiode.

Quantification of the relevant species was attempted from the fixed photon energy mass spectra obtained at selected quantification photon energies E<sub>q</sub> above their respective ionization energies. The selected quantification energies, as well as the source for the used total and partial PICS are provided in Table 2. In the permanent regime, the acquired signal for species *i* at mass-to-charge ratio *m* can be calculated from Equation 1:

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Physical Chemistry Chemical Physics Accepted Manuscript
$$S_i^m(E) = A \cdot D^m \cdot \sigma_i^m(E) \cdot n_i$$

Equation Article Online
DOI: 10.1039/D5CP00887E

Where A is the instrument function, D^m the mass discrimination factor, σ_i^m(E) the cross section at a given photon energy, and n_i the concentration. Given that the PICS of DME are well reported in the literature, and that DME could be detected at all values of E_q, the signal of DME was used as an internal standard. A separate mass spectrum was acquired for a DME/N_2 mixture flowing through the burner to determine the value of A in the current conditions. The calculated n_DME in the flame conditions was further used to determine the concentrations of all quantified species using Equation 2.

$$n_i = n_{DME} \cdot \frac{\sigma_{DME}^{46}(E) \cdot D^{46}}{\sigma_i^m \cdot D^m \cdot S_{DME}^{46}} \cdot \frac{S_i^m}{S_{DME}^{46}} \qquad \text{Equation 2}$$

The mass discrimination factors of the SAPHIRS instrument have been measured previously³⁹, and have been used here. Given the uncertainty of the used PICS and the indirect measurement, an estimated overall uncertainty on n_i of +-50% is assumed. In the case of the m/z = 31 fragment of HPMF, the contribution of ¹³CH₂O was subtracted on the basis of the isotopic ratio, and assuming an identical PICS for ¹²CH₂O and ¹³CH₂O.

<table><caption>Table 2: Quantified species in the current study, along with the photon energy at which quantification was performed E_q, the used Photolization Cross Section (PICS) and its reference.</caption><thead><tr><th>m/z</th><th>Formula</th><th>Species</th><th>E<sub>q</sub> /eV</th><th>PICS(E<sub>q</sub>) /Mb</th><th>Ref.</th></tr></thead><tbody><tr><td>15</td><td>CH<sub>3</sub></td><td>Methyl</td><td>11.0</td><td>6.5</td><td>39</td></tr><tr><td>18</td><td>H<sub>2</sub>O</td><td>Water</td><td>12.7</td><td>5.9</td><td>40</td></tr><tr><td>30</td><td>CH<sub>2</sub>O</td><td>Formaldehyde</td><td>11.0</td><td>9.6</td><td>41</td></tr><tr><td>31</td><td>C<sub>2</sub>H<sub>4</sub>O<sub>4</sub></td><td>HPMF fragment</td><td>11.0</td><td>0.93</td><td>29</td></tr><tr><td>32</td><td>CH<sub>3</sub>OH</td><td>Methanol</td><td>11.0</td><td>4.4±0.9</td><td>41</td></tr><tr><td>33</td><td>HO<sub>2</sub></td><td>Hydroperoxyl</td><td>11.5</td><td>1.5±0.3</td><td>41</td></tr><tr><td>34</td><td>H<sub>2</sub>O<sub>2</sub></td><td>Hydrogen Peroxide</td><td>11.0</td><td>0.37±0.09</td><td>42</td></tr><tr><td rowspan="3">46</td><td rowspan="3">C<sub>2</sub>H<sub>6</sub>O</td><td rowspan="3">DME</td><td>11.0</td><td>11.4±2.3</td><td>29</td></tr><tr><td>11.5</td><td>11.0±2.2</td><td>29</td></tr><tr><td>12.7</td><td>13.2±2.6</td><td>29,43</td></tr><tr><td>48</td><td>O<sub>3</sub></td><td>Ozone</td><td>12.7</td><td>3.2</td><td>44</td></tr><tr><td>60</td><td>C<sub>2</sub>H<sub>4</sub>O<sub>2</sub></td><td>Methyl Formate</td><td>11.0</td><td>2.78</td><td>29</td></tr><tr><td rowspan="2">64</td><td rowspan="2">C<sub>2</sub>H<sub>4</sub>O<sub>4</sub></td><td>HPMF fragment</td><td>Non quantified</td><td></td><td>29</td></tr><tr><td>+ hydroperoxymethanol</td><td></td><td></td><td></td></tr></tbody></table>

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Kinetic modelling

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DOI: 10.1039/D5CP00887E

The investigated flame was modelled using the stagnation plate burner model implemented in Cantera
⁴⁵, following the method described in Panaget et al.¹⁷. Following the results of a recent characterization
of the flow field in the same configuration via Particle Imaging Velocimetry ²¹, the computational
domain was restricted to a length of 6.5 mm relative from the stagnation plate. This length was chosen
as the best compromise between achieving an accurate description of the axial velocity field and
minimizing the constraint on the occurrence (or absence) of a cool flame. Simulations were performed
using the DME/O₃ model developed at PC2A ¹⁷,²¹, which has been extensively validated with cool flame
speciation and laminar burning velocities results. Correlations derived from previous PIV data²¹ were
used to calculate the velocity at the centre of the burner exit nozzle. Finally, in the absence of flow
field data for the horizontal burner configuration, the flame spread rate in the modelling was adjusted
to best fit the measured reactants and major products mole fractions at the measurement point,
thereby allowing for a comparison of the branching between cool flame products.

Theoretical Quantum Calculations and Vibronic Spectrum Simulations

To facilitate the identification of the observed m/z channels, the spectra for hydroperoxymethanol, dimethyl ether, dioxirane and methyl formate were calculated. The simulations began with calculating the lowest-energy conformer for each isomer using Density Functional Theory (DFT) with the M06-2X-D3 functional⁴⁶. This DFT functional was paired with the aug-cc-pVTZ Dunning's correlation-consistent basis set⁴⁷. Additionally, the zero-damped D3 dispersion correction developed by Grimme et al.⁴⁸ was integrated into M06-2X. The lowest-energy structure was then used as the starting point for the cation optimization. All these calculations were performed within the harmonic approximation.

Next, the adiabatic ionization energies of each isomer, both in its neutral and ionized forms, were
calculated using the CBS-QB3 composite method⁴⁹,⁵⁰ with an accuracy of less than 100 meV. The PES
were then simulated at 0 K using the time-independent adiabatic Hessian Franck-Condon (TI-AH|FC)

<figure><img src="image_13.png" /></figure>
model⁵¹. The resulting stick spectrum was combined with a Gaussian profile featuring a 200 µm bandwidth to match the experimental resolution. The 0-0 transition of the computed stick spectra was shifted to align with the experimental spectra, within the uncertainty of the energy calculations.

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DOI: 10.1039/D5CP00887E

# Results and discussion

## Identification

A representative time-of-flight mass spectrum acquired at a PI energy of 11.5 eV with the proposed identification is shown in Figure 2. A list of all the detected and quantified species is also provided in Table 2, but the detailed identification process will only be provided for selected m/z ratios. For the most abundant species, for which the identification has already been reported elsewhere, a comparison of the experimental and literature spectra is nevertheless provided in the Supplementary Material (SM). The relatively low electric field in the extraction region allows distinction of peaks originating from fragmentation events since those present a broader peak shape than the ones from parent ions due to the larger translational energies of the former.

<figure><img src="image_14.png" /><figcaption>Figure 2: Typical TOF-MS recorded by synchrotron photoionization of sampled gas from the DME/O₂/O₃/N₂ cool flame, acquired at a fixed 11.5 eV photon energy during 2 hours for the flame conditions reported in Table 1, at a distance of 800 ± 200 µm of the stagnation plate. The proposed identification is based on the following analysis.</figcaption></figure>

<figure><img src="image_15.png" /></figure>
$$ \cdot CH_3, m/z = 15 $$

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The experimental TPES measured for m/z 15 is shown in Figure 3 and is in good agreement with the one reported for the methyl radical by ³⁵ using the same instrument. This demonstrates the possibility to observe radicals in such a configuration after probe sampling, which is partly due to the low dilution (or high concentration of the reactants) conditions at which the experiments were performed. Further comparison of the TIY spectrum with data from the literature is provided in the SM and confirms the attribution of the signal at m/z = 15 to the methyl radical.

<figure><img src="image_16.png" /><figcaption>Figure 3: Comparison of the TPE spectrum recorded at m/z = 15 (black symbols and lines) and the one reported for the methyl radical by Cunha de Miranda et al. ³⁵</figcaption></figure>

$$ HOO^-, m/z = 33 $$

As visible from Figure 2, the signal at m/z = 33 is very weak, because of the low concentration of HO₂, but also because of the low Franck-Condon factors, as reported by Tang et al.⁵². This prevents the determination of a resolved TPES. Figure 4 however shows a comparison between the experimental TIY and the available literature data for the HO₂ radical⁵² in the common photon energy range of both studies, showing good agreement. Despite the fact that the low Franck-Condon factors also translate into a slow increase of the TIY signal at the ionization limit, this peak can be identified as originating

<figure><img src="image_17.png" /></figure>
from the HO₂ radical. The combination of this partial spectral agreement and the expected formation of HO₂ under our experimental conditions support a confident assignment. Furthermore, the absence of other plausible candidates at m/z 33 under our flame conditions strengthens this identification.

DOI: 10.1039/D5CP00887E

<figure><img src="image_18.png" /><figcaption>Figure 4: Comparison of the TIY spectrum recorded at m/z = 33 (black symbols and lines) and the one reported for the hydroperoxyl radical by Tang et al.⁵²</figcaption></figure>

$$H_2O_2, m/z = 34$$

Hydrogen peroxide is expected to be formed in quantities as high as 0.5% inside the investigated cool flames, and is expected at m/z = 34. The qualitative comparison of the experimental TPES with literature data, as shown in Figure 5, shows notable agreement with the TPES data from Schio et al.⁵³. Here also, the low Franck-Condon factors result in a shallow increase of the signal at the ionization limit. Additional validation can be observed from the TIY spectrum, and is provided in the Supplemental Material.

<figure><img src="image_19.png" /></figure>
<figure><img src="image_20.png" /><figcaption>Figure 5: Comparison of the TPE spectrum recorded at m/z = 34 (black symbols and lines) and the one reported for hydrogen peroxide by Schio et al.⁵³.</figcaption></figure>

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DOI: 10.1039/D5CP00887E

## HPMF fragments (m/z 31 and 64)

Couch et al.²⁹ reported the m/z = 64 and 31 as the most abundant fragments from HPMF (m/z 92) following the loss of CO and subsequent release of HOO:

<figure><img src="image_21.png" /></figure>

In Couch et al.'s work, the m/z 31 and 64 channels were reported to yield significantly more signal than the parent ion and other fragmentation products (m/z = 92, 91, 59) at all investigated photon energies. In agreement with this previous work, no signal was observed at m/z 92, 91 and 59 under our conditions.

$$HO_2CH_2OH, m/z = 64$$

The observed peak at channel m/z=64 of the mass spectrum shown in Figure 2 is broad, which confirms its origin as a fragment. Figure 6 shows a comparison between the recorded TIY at m/z 64 and the TIY of the fragment reported by Couch et al., along with a simulated TIY of hydroperoxymethanol (OHCH₂OOH) calculated in this work. The convolution of both curves agrees relatively well with the

<figure><img src="image_22.png" /></figure>
experimental TIY using a respective HPMF:hydroperoxymethanol signal branching ratio of 1:0.30. Article Online DOI: 10.1039/D5CP00887E should be noted that the TIY at m/z 64 is similar for the lower part to the one reported by Liao et al.³⁰ in ozone-seeded conditions, which is also plotted in Figure 6. The divergence in the higher energy part (ie., above 10.5 eV) can be attributed to a lower HPMF:hydroperoxymethanol ratio in the current work, consistent with a much higher ozone loading (2.6% O₃ in the mixture). On the opposite, the data acquired by Couch et al. at this m/z in the absence of ozone does not feature the lower energy contribution associated with hydroperoxymethanol. In comparison to the conditions of the present work and of the work of Liao et al., the results of Couch et al. were obtained in highly-diluted conditions to target the primary reactivity of DME. It is therefore likely that the formation of hydroperoxymethanol is caused by the reaction of secondary products of DME. Given that the mole fractions of HO₂, H₂O₂ and CH₂O are high in the current conditions, one can suggest that it originates from reactions between CH₂O and H₂O₂, or between CH₃O and HO₂.

<figure><img src="image_23.png" /><figcaption>Figure 6: Comparison of the TIY recorded at m/z = 64 (black symbols and lines), the ones reported for the HPMF fragment by Couch et al.²⁹, and Liao et al.³⁰, and the one calculated for hydroperoxymethanol in this work.</figcaption></figure>

$$CH_2OH, m/z = 31$$

Figure 7 shows the TIY of m/z 31 recorded in this work compared to a convolution of reference TIY curves from the ¹³C isotope of CH₂O from Tang et al.⁴⁹, which is abundantly formed in DME cool flames (as visible from the m/z = 30 peak in Figure 2), the methoxy radical from the same authors, and the fragment of HPMF (HOOCH₂OCHO) recorded by Couch et al. The convolution curve fits the experimental TIY with a HPMF:¹³CH₂O:CH₃O signal branching ratio of 1:0.20:0.08. The signal of m/z 31

<figure><img src="image_24.png" /></figure>
is dominated by the fragment of HPMF, which is consistent with the fact that it is the main fragment of the KHP along with m/z 64. This is also confirmed by the broad shape of the TOF peak which is correlated to dissociative ionization generating fragments with larger kinetic energy release distributions. The contribution from ¹³CH₂O is also confirmed by the distinctive peak at 10.9 eV visible in the TPE spectrum shown in the SM. A contribution of the methoxy radical to the TPES cannot be ruled out, in accordance with the convolution presented in Figure 7. The fragmentation of methanol could also contribute to the observed signal. This, however, corresponds to an appearance energy of around 11.5 eV based on Cool et al. ²⁹, which is at the edge of our current energy range and cannot be properly quantified.

DOI: 10.1039/D5CP00887E

<figure><img src="image_25.png" /><figcaption>Figure 7: Comparison of the TIY recorded at m/z = 31 (black symbols and lines) and the ones reported for the HPMF fragment by Couch et al.²⁹, for the ¹³CH₂O isotope by Tang et al.⁵⁴, and the methanol fragment by Cool et al.⁵⁵.</figcaption></figure>

Quantification and kinetic modelling

The determined mole fractions at the sampling probe location are compared with simulation results in Figure 8. While measurements were only performed at one position of the flame, one can observe the mole fractions of the major species (left) lie within the experimental uncertainty. Notably, the determined mole fraction of HPMF is in fair agreement with the predictions of the kinetic model, and further validates this experimental approach for the observation and quantification of organic hydroperoxides. One can observe higher discrepancies for ozone and water, for which the

<figure><img src="image_26.png" /></figure>
quantification was performed at a photon energy of 12.7 eV. This could be due to the use of DME as an internal standard, while other species than DME could contribute to the m/z = 46 signal at this photon energy. Methanol is underpredicted by about a factor of two, which is consistent with the performance of the kinetic model in previous work on DME cool flames by means of gas chromatographic analysis¹⁷, and could be correlated with the underestimation of the mole fraction of the methyl radical by three orders of magnitude (see Table 2). The discrepancy between the experimental and modelled mole fractions of the methyl radical can also partly originate from the contribution of the fragmentation of heavier species to the TIY signal at the quantification photon energy. Further work on the methanol submodel will therefore be needed to improve the DME kinetic model. Finally, as visible from Table 2, the agreement is however not as satisfying in the case of HO₂ and H₂O₂, which could suggest that losses could still take place, for example through reactivity in the capillary probe, and requires further experimental development.

DOI: 10.1039/D9CP00887E

<figure><img src="image_27.png" /><figcaption>Figure 8: Simulated (lines) and measured mole fraction at the probe position in the investigated flame.</figcaption></figure>

<table><caption>Table 3: Comparison for the measured and simulated mole fractions for the quantified species, at a distance of 800 ± 200 µm of the stagnation plate.</caption><thead><tr><td>Species</td><td>Measured mole fraction</td><td>Simulated mole fraction</td></tr></thead><tbody><tr><td>HPMF</td><td>4 ± 2 x10<sup>-4</sup></td><td>7 x10<sup>-4</sup></td></tr><tr><td>Methyl Formate</td><td>1.8 ± 0.9 x10<sup>-3</sup></td><td>5 x10<sup>-3</sup></td></tr></tbody></table>

<figure><img src="image_28.png" /></figure>
<table><tr><td>Ozone</td><td>0.017 ± 0.008</td><td>0.009</td></tr><tr><td>Dimethyl Ether</td><td>0.03 ± 0.02</td><td>0.02</td></tr><tr><td>Hydrogen peroxide</td><td>3 ± 2 x10<sup>-4</sup></td><td>2 x10<sup>-3</sup></td></tr><tr><td>Hydroperoxyl</td><td>7 ± 4 x10<sup>-5</sup></td><td>3 x10<sup>-4</sup></td></tr><tr><td>Methanol</td><td>6 ± 3 x10<sup>-4</sup></td><td>1 x10<sup>-4</sup></td></tr><tr><td>Formaldehyde</td><td>3 ± 2 x10<sup>-3</sup></td><td>2 x10<sup>-3</sup></td></tr><tr><td>Water</td><td>0.05 ± 0.03</td><td>0.03</td></tr><tr><td>Methyl</td><td>3 ± 2 x10<sup>-5</sup></td><td>1 x10<sup>-8</sup></td></tr></table>

View Article Online
DOI: 10.1039/D5CP00887E

Conclusions

This study provides the first experimental evidence of the presence of keto- or aldo-hydroperoxides within cool flames, by means of a novel coupling of a stagnation plate cool flame burner with the i2-PEPICO SAPHIRS instrument at the DESIRS beamline of synchrotron SOLEIL. Gases extracted upstream from a plate-stabilized DME/O₂/O₃/N₂ cool flame by means of a capillary probe were expanded into the spectrometer, leading to the following conclusions: Unambiguous proof of the formation of hydroperoxymethylformate, the only expected aldohydroperoxide from DME low-temperature gas phase oxidation, has been obtained through the detection of its fragments at mass-to-charge ratios of 31 and 64, as previously reported in flow-reactor conditions in the literature. Through the analysis of the signal collected for m/z = 64, and in accordance with data reported by others, the existence of an ozone-dependent pathway yielding hydroperoxymethanol is suggested. Additional detection of the methyl, methoxy and hydroperoxyl radicals, as well as hydrogen peroxide was also possible. The quantification of most of these species is attempted here for the first time in a cool flame configuration. Kinetic modelling of the studied configuration shows good agreement on the major species, as well as hydroperoxymethyl formate, the aldohydroperoxide responsible for the initiation of the cool flame. The measured mole fractions for HO₂ and H₂O₂ are however lower than those predicted by the kinetic model, suggesting that losses could take place during the sampling. In the case of methanol and the methyl radical, the opposite is true, suggesting that improvement in the model could be needed. Further work will aim at investigating more complex fuels, that yield several KHP
isomers, as well as reaching better jet conditions in order to enable molecular beam conditions prone to quantitative measurement of radicals.

DOI: 10.1039/D5CP00887E

## Author contributions

Sébastien Batut: conceptualization, investigation, methodology. Laure Pillier: conceptualization, investigation, methodology, supervision, writing-review&editing. Jérémy Bourgalais: formal analysis, investigation, validation, writing-review&editing. Thomas Panaget: investigation, writing-review&editing. Pascal Demaux: investigation. Olivier Herbinet: investigation, writing-review&editing. Frédérique Battin-Leclerc: writing-review&editing. Gustavo A. Garcia: formal analysis, investigation, validation, writing-review&editing. Laurent Nahon: resources, writing-review&editing. Guillaume Vanhove: conceptualization, formal analysis, funding acquisition, investigation, methodology, supervision, validation, writing-original draft.

## Acknowledgments

We acknowledge SOLEIL for provision of synchrotron radiation under project 20220879 and we are grateful to Jean-François Gil for his technical help around the SAPHIRS set-up. The authors thank the Région Hauts-de-France, the Ministère de l'Enseignement Supérieur et de la Recherche and the European Fund for Regional Economic Development for their financial support to the CPER ECRIN program. The CaPPA project (Chemical and Physical Properties of the Atmosphere) is funded by the French National Research Agency (ANR) through the PIA (Programme d'Investissement d'Avenir) under contract « ANR-11-LABX-0005-01 » and by the Regional Council « Hauts-de-France » and the « European Funds for Regional Economic Development » (FEDER). Theoretical calculations in this work were performed using HPC resources from the EXPLOR centre hosted by the University of Lorraine (Project: 2021EXTXX2356).

<figure><img src="image_29.png" /></figure>

Physical Chemistry Chemical Physics Accepted Manuscript
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DOI: 10.1039/D5CP00887E

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Physical Chemistry Chemical Physics Accepted Manuscript
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DOI: 10.1039/D5CP00887E

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