Identification, monitoring, and reaction kinetics of reactive trace species using time-resolved mid-infrared quantum cascade laser absorption spectroscopy: Development, characterisation, and initial results for the CH 2 OO Criegee intermediate

. The chemistry and reaction kinetics of reactive species dominate changes to the composition of complex chemical 10 systems, including Earth’s atmosphere. Laboratory experiments to identify reactive species and their reaction products, and to monitor their reaction kinetics and product yields, are key to our understanding of complex systems. In this work we describe the development and characterisation of an experiment using laser flash photolysis coupled with time-resolved mid-infrared (mid-IR) quantum cascade laser (QCL) absorption spectroscopy, with initial results reported for measurements of the infrared spectrum, kinetics, and product yields for the reaction of the CH 2 OO Criegee intermediate with SO 2 . The instrument presented 15 has high spectral (< 0.004 cm -1 ) and temporal (< 5 µs) resolution, and is able to monitor kinetics with a dynamic range to at least 20,000 s -1 . Results obtained at 298 K and pressures between 20 and 100 Torr gave a rate coefficient for the reaction of CH 2 OO with SO 2 of (3.83 ± 0.63) × 10 -11 cm 3 s -1 , which compares well to the current IUPAC recommendation of ( 3.70+0.45-0.40 ) × 10 -11 cm 3 s -1 . A limit of detection of 4.0 × 10 -5 , in absorbance terms, can be achieved, which equates to a limit of detection of ~2 × 10 11 cm -3 for CH 2 OO, monitored at 1285.7 cm -1 , based on the detection pathlength of (218 ± 20) cm. Initial results, 20 directly monitoring SO 3 at 1388.7 cm -1


Introduction
The behaviour of reactive intermediates is critical to understanding the chemistry of complex systems. In the gas phase, react ive intermediates govern the chemistry and composition of planetary atmospheres (Blitz and Seakins, 2012), the interstellar 30 medium and star-forming regions (Herbst, 2001), as well as controlling combustion processes and autoignition (Pilling et al., 1995;Zador et al., 2011). In the Earth's atmosphere, the chemistry of reactive intermediates determines the rate at which compounds emitted into the atmosphere are removed and transformed into other species, and thus drives changes to air quality and climate (Monks, 2005;Von Schneidemesser et al., 2015).

35
Experimental investigation of the spectroscopy, kinetics, and reaction mechanisms of reactive intermediates is key to understanding the behaviour of such species, requiring sensitive and specific detection techniques capable of monitoring changes in concentrations during the course of reactions commonly occurring on microsecond to millisecond timescales. Flash photolysis experiments, in which a reactive species is generated rapidly by a brief pulse of light from a flashlamp or, more commonly, a laser and then monitored throughout its subsequent reactions, have enabled the study of many reactions of 40 interest. However, while the flash photolysis method can be coupled to a wide range of techniques to determine the kinetics o f a reaction, limitations remain, particularly surrounding the identification of reaction products and measurements of product yields (Seakins, 2007).
For the study of fast reactions, spectroscopic or mass spectrometric techniques are typically required to provide the necessary 45 time resolution. Laser-induced fluorescence (LIF) spectroscopy has been demonstrated to have both high sensitivity and specificity for the measurement and identification of reactants and products, but is not an absolute technique and so requires either calibration or the use of an internal standard to determine yields (Carr et al., 2007). Moreover, LIF can only be applied to the relatively small number of species that exhibit fluorescence spectra. Mass spectrometry can be applied more widely, particularly if soft ionisation techniques such as photoionisation are employed, but such techniques can be costly and require 50 sampling of a reaction mixture into an ionisation and detection region which can limit investigations to low pressure regimes (Fockenberg et al., 1999;Blitz et al., 2007;Osborn et al., 2008;Middaugh et al., 2018). Absorption techniques can therefore be beneficial as these can provide absolute measurements over a wide range of temperatures and pressures.
Absorption measurements based on ultraviolet (UV) spectroscopy have been used successfully to measure the kinetics of a 55 broad variety of reactions, with advantages for radical-radical reactions where absolute concentrations are required. While UV absorption spectra can be relatively broad and featureless (Orlando and Tyndall, 2012), developments in the use of broadband light sources have enabled the separation of multiple absorbing species with overlapping spectra (Cossel et al., 2017), particularly if any of the spectra display distinctive vibronic structure. Multipass (Lewis et al., 2018) and cavity enhanced techniques (Cossel et al., 2017) can also offer significant improvements to sensitivity. However, UV absorption experiments 60 are typically employed to monitor changes in reactant concentrations to determine reaction kinetics, but identification of products and measurement of product yields is often not possible owing to a lack of suitable UV absor ption features or low UV absorption cross-sections for product species. There is thus interest in the development and use of infrared (IR) techniques which, despite lower absorption cross-sections compared to the UV, have the potential to be implemented more extensively since most species exhibit some features in the IR region of the spectrum, with IR spectroscopy offering the potential for 65 structural determination and unique identification of reactants and products. Infrared absorption techniques are also often advantageous over other spectroscopic methods, particularly at relatively low temperatures and pressure, since IR transitions are typically not dissociative (Taatjes and Hershberger, 2001;Hodgkinson and Tatam, 2013), and as a result do not suffer lifetime broadening or probe-induced photochemistry. Additionally, Doppler broadening in the IR is less problematic than in the UV or visible, leading to better resolution of closely spaced features (Taatjes and Hershberger, 2001;Hodgkinson and 70 Tatam, 2013).
For many reactions of atmospheric interest, products have been determined by long-path Fourier transform infrared (FT-IR) absorption spectroscopy in atmospheric simulation chambers used to study reactions at a mechanistic level (Doussin et al., 1997;Glowacki et al., 2007;Nilsson et al., 2009;Seakins, 2010), but this method has relatively poor time resolution compared 75 to direct studies of elementary reactions. Such studies can be influenced by secondary chemistry and wall reactions which may transform reactive products into more stable species on the timescale of the experiment.
Step-scan FT-IR experiments (Su et al., 2013;Huang et al., 2007), in which spectra are recorded at successive time points during the course of a reaction, offer improved time resolution and aid direct identification of reaction products. However, step-scan FT-IR has relatively poor sensitivity and spectral resolution compared to other techniques and is not typically employed to investigate reaction kineti cs 80 owing to the length of time required to record a suitable time profile.
Diode lasers (Taatjes and Hershberger, 2001), quantum cascade lasers (QCLs) (Faist et al., 1994;Hofstetter and Faist, 2003;Yao et al., 2012;Zhang et al., 2014;Pecharroman-Gallego, 2013), and frequency comb lasers (Fleisher et al., 2014;Bjork et al., 2016;Roberts et al., 2020) can be used to give both high spectral resolution and high temporal resolution, and can be 85 applied to measurements of transient species. Frequency comb lasers can provide extremely high resolution spectra, but there are, at present, relatively few examples (Bjork et al., 2016) of their application in chemistry and chemical kinetics owing to relatively high cost and complexity of the experimental setup. In contrast to mid-IR lead salt diode lasers, mid-infrared QCLs do not require cryogenic operation and do not suffer issues related to mode-hopping, uneven diode quality, or unpredictable tunability (Hodgkinson and Tatam, 2013). The spectral output of QCLs can be tailored to transitions over a wide range, 90 providing access to regions of the spectrum not readily accessible with diode lasers or frequency comb lasers (Shahmohammadi et al., 2019;Yao et al., 2012;Zhang et al., 2014). QCLs have high output powers, and can reach several hundred mW to ~1 W (Hodgkinson and Tatam, 2013;Pecharroman-Gallego, 2013;Yao et al., 2012;Zhang et al., 2014), compared to typical powers of a few mW for diode lasers (Hodgkinson and Tatam, 2013), and absorbances on the order of 10 -6 have been reported for QCL-based infrared absorption experiments for stable species (Borri et al., 2006). The high resolution, specificity, and 95 sensitivity of QCL-based absorption techniques has led to the development of a number of field instruments for detection of trace species in the atmosphere (Li et al., 2013;Du et al., 2019), including CH4 Kostinek et al., 2019), CO2 (Kostinek et al., 2019), OCS , N2O Banik et al., 2017;Kostinek et al., 2019), H2O , HCOOH (Herndon et al., 2007), and HONO (Cui et al., 2019) which can be challenging to monitor by other methods. The high power and spectral resolution of QCL light sources, enabling sensitive and specific 100 experiments in regions of the spectrum characterised by strong fundamental transitions, have also led to interest from the chemical kinetics community.
QCLs consist of layers of semiconductor material that create a series of coupled quantum wells in which the layer thickness determines the depth of the quantum well and thus the energy of emitted photons (Yao et al., 2012;Faist et al., 1994). In 105 contrast to diode lasers, which involve electronic transitions between conduction and valence bands, laser action in QCLs involves intersub-band transitions within the conduction band, typically via a three-level system (Yao et al., 2012;Gmachl et al., 2001;Curl et al., 2010). In the absence of an electric field, electrons are confined in the quantum wells within an injector region. When an electric field is applied, the quantum wells within the injector region align and electrons are injected into an upper intersub-band state, creating a population inversion with an intermediate level. Relaxation of electrons to a lower 110 intersub-band state, resulting in photon emission, followed by rapid tunnelling of electrons from the lower intersub-band state into the injector region of the next layer creates a cascade of electrons as the layer structure is traversed, leading to inc reased photon emission and significant optical gain (Yao et al., 2012). Broadband emission can be achieved using external cavity (EC) QCLs, which can give coverage of several hundred cm -1 , or Fabry-Perót (FP) QCLs, giving coverage of ~50 cm -1 , while single mode emission relevant to this work is achieved using distributed feedback (DFB) QCLs (Shahmohammadi et al., 2019). 115 For emission in the mid-IR, DFB QCLs can be operated at room temperature, with control of the temperature of the semiconductor and the applied current allowing fine tuning of the laser output within a range of ~5 cm -1 (Shahmohammadi et al., 2019).
Pulsed QCLs have been used to measure the production of CO in the combustion of n-heptane (Nasir and Farooq, 2019), and 120 have been used to determine the IR absorption spectrum and cross-sections of the Criegee intermediate CH2OO (Chang et al., 2017;Chang et al., 2018b;Luo et al., 2018a). CH2OO is a reactive species produced in the atmosphere during the ozonolysis of unsaturated volatile organic compounds (VOCs) that has been of recent interest as a result of developments in photolytic sources (Welz et al., 2012) for detailed laboratory studies which have revealed a more significant role in atmospheric chemistry than previously expected (Chhantyal-Pun et al., 2020;Percival et al., 2013). Quasi-continuous QCLs, pulsed QCLs in which 125 the pulse period is relatively long compared to the lifetime of the species under investigation, have also been used to investigate the spectra and kinetics of CH2OO (Chang et al., 2018a), and cw QCLs have been used to investigate the kinetics of CH2OO (Luo et al., 2018b;Luo et al., 2019;Li et al., 2019;Li et al., 2020) and other larger Criegee intermediates (Luo et al., 2018b), as well as the spectroscopy and kinetics of the atmospherically important peroxy radicals HO2 (Miyano and Tonokura, 2011;Sakamoto and Tonokura, 2012) and CH3O2 (Chattopadhyay et al., 2018). While mid-IR QCLs have been employed to study 130 the kinetics and spectroscopy of reactive species relevant to atmospheric chemistry, there are still few examples of the use of QCL-based techniques to identify reaction products and to determine product yields.
In this work, we report the development, characterisation, and initial results from a robust and economical experiment using laser flash photolysis coupled with time-resolved mid-infrared QCL absorption spectroscopy that can be applied to a wide 135 range of problems in atmospheric chemistry and beyond. We describe the experimental setup (Sect. 2), time-averaged measurements of absorption spectra of stable species (Sect. 3), and time-resolved measurements of photolytically generated species and reaction products (Sect. 4) that can be used to determine spectra of reactive species as well as reaction kinetics and product yields. Factors affecting the limit of detection are also discussed (Sect. 5).

Experimental 140
A schematic of the experimental setup is given in Figure 1. The reaction cell has been used previously in IR diode laser experiments (Qian et al., 2000(Qian et al., , 2001Choi et al., 2006)  pressures from < 5 Torr to above atmospheric pressure achievable. Experiments reported in this work were performed at room temperature, although temperature control of the reaction cell is possible by surrounding the cell with ceramic heaters (Watlow, WATROD tubular heater), a bath filled with an appropriate solvent (e.g. methanol) chilled by a refrigerated immersion probe (LabPlant Refrigerated Immersion Probe, RP-100CD), or a dry ice slush bath (Choi et al., 2006), all of which are available in this laboratory. 165 For experiments involving reactive species generated by photolysis, chemistry within the cell was initiated by the fourth harmonic of an Nd:YAG laser (Continuum Powerlite 8010), giving 266 nm (typical fluence 30 mJ cm -2 ). The photolysis beam has diameter ~ 1 cm and was aligned through the centre of the reaction cell using a pair of mirrors (ThorLabs NB1-K04). For all experiments reported in this work, the repetition rate of the photolysis laser was set to 1 Hz and the flow rate through the 170 reaction cell was sufficiently high to ensure that a fresh gas mixture was photolysed for each photolysis shot.
Infrared probe radiation was provided by one of two cw mid-IR DFB QCLs, depending on the application, providing radiation at wavenumbers of ~1286 cm -1 (~7.77 µm) (Alpes Lasers) and ~1390 cm -1 (~7.19 µm) (ThorLabs) with a tuning range of ~ 5 cm -1 around each centre. Temperature and current control of the QCL, which determines the precise output wavenumber of a 175 given QCL, were controlled by a combined laser current and thermoelectric (TEC) controller (ThorLabs, ITC4002QCL). The current and TEC controller was operated in constant current mode, which provides current control up to 2 A in 0.1 mA steps with accuracy ±(0.1 % + 800 µA) and stability < 150 µA, and temperature control between 123 and 423 K in 0.001 K steps with stability < 0.002 K.

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QCLs were housed in high heat load (HHL) packages, which were each integrated with a ZnSe aspheric lens to collimate the output beam (divergence < 6 mrad), and mounted on a heatsink (Hamamatsu HHL mount, A117909-1) to dissipate excess heat. Mounted QCLs were positioned on a pitch and yaw stage (ThorLabs, PY003/M) on an adjustable height platform (ThorLabs, C1519/M) to aid alignment through two adjustable aperture irises (ThorLabs, ID8/M) which were separated by > 70 cm. A collimated laser diode operating at 635 nm (ThorLabs, CPS635R) was co-aligned through the irises via a mirror 185 (ThorLabs, PF10-03-G01) on a flip-mount (ThorLabs, FM90/M) to guide alignment of the QCL beam through the reaction cell. A pair of 1" Al mirrors (ThorLabs, PF10-03-G01) situated after the second iris were used to direct the beam into the reaction cell, with the beam focussed into the centre of the cell by a 1" ZnSe plano-convex lens (focal length 1000 mm, ThorLabs, LA7753-G).

190
The probe IR beam was aligned through the cell in a multipass arrangement to increase the total pathlength and sensitivity, with the mirrors reflecting the probe beam external to the cell. Custom built mirror mounts with a central 12 mm hole were located exterior to the cell and were used to mount six Ag mirrors (12 mm diameter, 2.4 m radius of curvature, Knight Optical) in a circular arrangement at each end of the cell, similarly to the arrangement previously described for the multipass UV absorption experiment in this laboratory (Lewis et al., 2018). These mirrors can be aligned independently of each other via 195 three alignment screws for each mirror, and enable up to 13 passes through the reaction cell. On the final pass of the IR probe beam through the cell the beam was directed onto a 1" Au off-axis parabolic mirror (reflected focal length 4", ThorLabs, MPD149-M01) and focussed onto the detector. The detector was a DC-coupled photovoltaic Mercury-Cadmium-Telluride The signal was transferred to an oscilloscope via a sheathed BNC cable for data collection and processing, with the settings on the oscilloscope dictating the time resolution of the experiment. For measurements of stable species, all spectra reported in this work were recorded using a traditional oscilloscope (LeCroy Waverunner-2, LT262, 350 MHz, 1 GS/s sample rate, 8 bit resolution), while for reactive species produced in photolytic experiments, the use of a PicoScope (Pico Technology, PicoScope 205 6402C, 250 MHz, 5 GS/s sample rate, 12 bit resolution) was also investigated. Data acquisition is discussed further in Sect. 4 and 5.
Synchronisation of the photolysis laser and oscilloscope was achieved using a custom-built digital delay generator based on National Instruments hardware, with the overall experiment and data collection controlled by custom LabVIEW software. In 210 addition to measurements of the probe intensity as a function of time at a fixed QCL output wavenumber, the software enabled stepping of the current applied to the QCL at a set temperature to vary the output wavenumber, which is used to measure the variation in the probe intensity across the tuning range with and without a sample present to determine the spectra of stable species. For reactive species produced by, or following, photolysis, the average pre-and post-photolysis intensities were determined over specified time ranges at each current setting to give the absorption spectrum. Further details are given in Sect. 215

Time averaged experiments
Characterisation of the output wavenumber of the QCL was achieved through measurement of the absorption spectra of stable species with well-defined rovibrational spectra. Spectra for SO2 and CH3I were recorded by measuring the variation in QCL 220 intensity across the tuning range of the QCL for the cell filled with N2 and for the cell filled with a mixture of N2 and the species of interest under otherwise identical conditions. The absorbance, ̃, at each wavenumber ̃, was calculated from the Beer-Lambert law (Equation 1): where ̃,0 and ̃ are the intensities at wavenumber ̃ without and with the species of interest present, respectively, ̃ is the 225 absorption cross-section at wavenumber ̃, [C] is the concentration, and l is the path length of the IR probe beam through the sample. In measurements of stable species in experiments in which no photolysis takes place, the total path length can be determined from the geometry of the cell and optical arrangement, and was ~13 m for the measurements discussed here. Figure 2 shows the normalised observed absorbance spectrum for SO2 alongside the comparison to normalised spectrum 230 available on the HITRAN database (Gordon et al., 2017;Kochanov et al., 2019) to illustrate the calibration of the QCL output wavenumber with the current. The measured spectrum indicates the capacity for high resolution measurements made possible by the narrow laser linewidth.

Stable species
Photolytic experiments were initially performed for a system demonstrating a step-change in the time-resolved signal intensity 240 at a given wavenumber on photolysis, such that photolysis led to the removal of the species under investigation with no significant further chemistry on the timescale of the measurements. Figure 3 shows the time-resolved absorbance observed on 266 nm photolysis of CH3I/N2 mixtures and the average post-photolysis change in absorbance as a function of the initial CH3I concentration. Photolysis of CH3I leads to a decrease in concentration, and thus an increase in signal intensity and a negative absorbance. The extent of change in absorbance reflects the absorption cross-section at the measurement wavenumber, the 245 change in concentration, and the effective path length resulting from the overlap between the UV photolysis beam and the IR probe beam. For the 266 nm laser fluence of 30 mJ cm -2 and CH3I absorption cross-section of 9.7 × 10 -19 cm 2 at the photolysis wavelength (IUPAC) (Atkinson et al., 2008), a change in CH3I concentration of 4 % is expected on photolysis. For the absorbance data shown in Figure 3 and an estimated infrared cross-section of CH3I at ~1287 cm -1 of 2 × 10 -21 cm 2 (HITRAN) (Gordon et al., 2017;Kochanov et al., 2019), the effective path length of the IR probe beam for these measurements can thus 250 be estimated as (290 ± 30) cm. The effective path length is discussed further in Sect. 4.2.

Reactive species
The behaviour of reactive species was investigated through 266 nm photolysis of CH2I2/O2/N2 and CH2I2/O2/N2/SO2 mixtures 260 (Welz et al., 2012), resulting in the rapid production (kCH2I+O2[O2] > 2 × 10 5 s -1 ) of the Criegee intermediate CH2OO ( Figure 4 shows the spectrum obtained in the absence of SO2 by measuring the average pre-and post-photolysis absorbances for an observed time-profile for a given QCL current setting, and thus a given wavenumber, and then stepping to the next current setting and repeating. The step in current for these experiments was 0.1 mA, the smallest step-size available with the current controller used, giving steps of < 0.002 cm -1 across the range investigated. The pre-photolysis region was defined as -275 4000 μs to -500 μs, owing to detection of some radiofrequency noise associated with the Q-switch delay of the photolysis laser which was set to 280 μs (i.e. the Q-switch fires at t = -280 μs), and the post-photolysis region as 500 μs to 6000 μs, where t = 0 is the time at which the photolysis laser is fired. Experiments were performed at 50 Torr and each time-resolved trace was averaged for 1000 photolysis shots. The concentration of CH2I2 for these experiments, determined from the flow rates of gases in the cell, the vapour pressure of CH2I2 and measurements of the saturation of the flow with CH2I2 in previous experiments 280 with similar flow rates, was ~2 × 10 14 cm -3 , and the 266 nm laser fluence was 30 mJ cm -2 , giving an expected initial CH2OO concentration of ~7 × 10 12 cm -3 using previous measurements of the yield of R2a (Stone et al., 2013). The spectrum measured in this work is in good agreement with that reported previously ( Chang et al., 2017;Chang et al., 2018a). The resolution observed in this work is similar to the resolution of < 0.004 cm -1 reported in previous work (Chang et al., 2017) based on the observation of non-overlapped peaks, although the linewidth of the QCLs used in this work and in that in the previous 285 measurement of the spectrum (Chang et al., 2018a) should enable higher resolution of < 0.002 cm -1 when observing species with more closely spaced features. Kinetics describing the loss of CH2OO were monitored in separate experiments with the QCL set at the peak in the CH2OO spectrum (~1285.73 cm -1 ). Figure 5 shows an example CH2OO decay obtained at a pressure of 50 Torr in the absence of SO2.
The data shown in Figure 5 indicate the ability to tune the QCL can be tuned to a particular spectral feature , and for the QCL 300 to and remain tuned to that feature for prolonged periods of time during which kinetics experiments, and repeat measurements, can be performed without any drift in spectral position. QCLs thus offer significant advantages for kinetics experiments over alternative mid-IR sources such as lead salt diode lasers which can suffer from mode-hopping, uneven diode quality, and unpredictable tuneability (Hodgkinson and Tatam, 2013). Such behaviour of alternative mid-IR sources can require sophisticated techniques to scan over a spectral feature, identify the peak, and then wait until the laser remains stable for a 305 sufficient period of time to perform the desired experiment (Qian et al., 2000).
In the absence of any additional co-reactant such as SO2, the loss of CH2OO (as shown in Figure 5) is dominated by reactions R3 and R4 (Mir et al., 2020). Since R3 and R4 are not first-order, the kinetics describing the loss of CH2OO are dependent on absolute CH2OO concentrations. The kinetics of R3 and R4 have been determined in our previous work (Mir et al., 2020), and the absorption cross-section of CH2OO has been measured (Chang et al., 2018b) to be (3.9 ± 0.6) × 10 -18 cm 2 at the peak of its 310 spectrum at ~1285.73 cm -1 at a total pressure of 50 Torr. The numerical integration package FACSIMILE (MCPA Software, 2014) was therefore used to fit to the measured absorbances to determine the physical losses of CH2OO owing to diffusion out of the probe beam (approximated by a first-order rate coefficient kphys) and the effective path length (l) of the probe beam required to convert the absorbances to concentrations compatible with the measured CH2OO absorption cross-section of (3.9 ± 0.6) × 10 -18 cm 2 (Chang et al., 2018b) and the known kinetics for the mechanism used in the model (Table 1). Fits were 315 performed for experiments in which precursor concentrations were varied in the range 0.1 -5.0 × 10 14 cm -3 . Initial concentrations of CH2I, iodine atoms, and CH2IO2 were calculated in the model relative to those for CH2OO using the yields of R1 and R2 determined in our previous work (Stone et al., 2013).

Reaction number Reaction
Rate coefficient, k a / cm 3 s -1 or k b / s -1
In the presence of excess SO2 the observed decays of CH2OO are dominated by R5 and the loss of CH2OO can be described by pseudo-first-order kinetics (Equation 2).
where At is the absorbance at time t, A0 is the absorbance at time zero, and k' is the pseudo-first-order rate coefficient, given by k' = k5[SO2] + kphys where k5 is the bimolecular rate coefficient for reaction between CH2OO and SO2 and kphys is the rate coefficient representing physical losses of CH2OO such as diffusion out of the probe region.
Fits of Equation 2 to the observed CH2OO decays obtained in the presence of SO2 were performed to determine the pseudo-345 first-order rate coefficients, k', which can plotted against the known SO2 concentration (Sect. 2) to give the bimolecular rate coefficient k5. Figure 6 shows typical bimolecular plots of k' against the SO2 concentration, giving k5 = (3.73 ± 0.19) × 10 -11 cm 3 s -1 at 20 Torr, k5 = (3.84 ± 0.27) × 10 -11 cm 3 s -1 at 50 Torr, and k5 = (3.95 ± 0.28) × 10 -11 cm 3 s -1 at 100 Torr, in good  (Cox et al., 2020). The data shown in Figure 6 indicate that the experiment developed in this work is able to measure the spectra and kinetics of reactive species to 350 a high degree of accuracy and precision, and that a high dynamic range up to at least 20,000 s -1 can be achieved. Further experiments were performed with the CH2I2/O2/N2/SO2 system to monitor the production of SO3 at ~1388.7 cm -1 . 365 Previous work in this laboratory has demonstrated that HCHO is produced in R5 (Stone et al., 2014), and theory has predicted the co-production of SO3 (Vereecken et al., 2012;Kuwata et al., 2015). Experimental work using step-scan FT-IR spectroscopy with a resolution of 1 -4 cm -1 have indicated the production of SO3 following photolysis of CH2I2/O2/N2/SO2 mixtures (Wang et al., 2018), but kinetics of SO3 production have yet to be reported to confirm direct production through R5. The QCL operating at ~1390 cm -1 was tuned to an absorption feature in the SO3 spectrum using a sample of gaseous SO3/N2 in an infrared absorption cell prepared from solid SO3 (Sigma-Aldrich, >99 %) in a glove box purged with N2. For experiments to monitor the kinetics of SO3 production following photolysis of CH2I2/O2/N2/SO2 mixtures, a PicoScope was used to collect the signal owing to lower absorbance signals for SO3 compared to CH2OO for given experimental conditions. The faster sampling rate and memory (5 Gs/s and 12 bit, respectively) of the PicoScope compared to the traditional LeCroy oscilloscope 375 (sampling rate 1 Gs/s and 8 bit memory) effectively decreases the limit of detection. Factors affecting the limit of detection, and the comparison between the Picoscope and the traditional oscilloscope, are discussed further in Sect. 5. Figure 7 shows examples of the time-resolved SO3 absorbance, which can be described by a pseudo-first-order growth combined with a first-order loss (Equation 3) and used to provide an alternative determination of k5. 380 where At is the absorbance at time t, A0 is the maximum absorbance which relates to the initial radical concentration and yield of SO3, kgrowth is the pseudo-first-order rate coefficient describing the growth of SO3 which is equal to k5 [SO2], and kloss is the first-order rate coefficient describing the loss of SO3 which is expected to be dominated by physical losses such as diffusion out of the probe region. 385

390
The bimolecular plots obtained through SO3 measurements at total pressures of 20, 50, and 100 Torr are shown in Figure 8 and demonstrate the capability of the instrument to measure the kinetics describing product formation to at least 25,000 s -1 .

Limit of detection
The limit of detection can be determined from the variability of the baseline absorbance, (i.e. in the absence of any absorbing 405 species), which should be equal to zero and for which deviations from zero are determined only by noise. In order to detect an absorbance signal above the baseline, the signal must be greater than the noise (i.e. the signal-to-noise ratio must be greater than one) and the limit of detection can thus be defined as the standard deviation of the noise. Figure 9 shows how the 1σ li mit of detection varies with the number of samples, which for measurements involving reactive species (Section 4) is equal to the number of photolysis shots and is determined for the baseline given for the pre-photolysis period (-4000 μs to -500 μs, where 410

420
The limit of detection is given for measurements with the traditional oscilloscope and the PicoScope, with some improvement to the limit of detection achieved on using the PicoScope, owing to greater memory and sampling rate (12 bit and 5 Gs/s) compared to the traditional oscilloscope (8 bit and 1 G/s), which effectively increases the number of measurement points within a sample within a given time period. For the traditional oscilloscope, a limit of detection of 8.3 × 10 -5 , in absorbance terms, is achieved for 250 samples, which is reduced to 5.4 × 10 -5 for 1000 samples. For the estimated path length of (218 ± 20) cm and 425 typical IR absorption cross-sections of ~10 -19 cm 2 , the limit of detection for these data in terms of concentration can thus be estimated as ~3.8 × 10 12 cm -3 for 250 samples and ~2.5 × 10 12 cm -3 for 1000 samples, which compares well with alternative IR-based techniques (Taatjes and Hershberger, 2001;Roberts et al., 2020). For measurements of CH2OO, which has relatively high IR absorption cross-sections on the order of 10 -18 cm 2 at ~1286 cm -1 , the limit of detection for 250 samples using the traditional oscilloscope is thus ~2.5 × 10 11 cm -3 . For the PicoScope, the limit of detection is significantly better than that for 430 the traditional oscilloscope when the number of samples is low, with the limits of detection becoming more comparable as the number of samples is increased. For 1000 measurements, the PicoScope gives a limit of detection of 4.0 × 10 -5 in absorbance terms, which for species with IR cross-sections on the order of ~10 -19 cm 2 gives a limit of detection of ~1.8 × 10 12 cm -3 in terms of concentration. For CH2OO, with relatively high IR cross-sections of ~10 -18 cm 2 at ~1286 cm -1 , the limit of detection is thus on the order of ~1.8 × 10 11 cm -3 . 435

Conclusions and future improvements
This work presents the characterisation and initial experiments performed using a new instrument based on mid-infrared QCL absorption spectroscopy to investigate the chemistry of reactive species with high spectral and temporal resolution. We have presented details of the experimental setup (Section 2), results obtained for time-averaged measurements of stable species (Section 3), and those for time-resolved measurements of reactive species (Sect 4). 440 We have demonstrated the application of the instrument to measurements of the IR spectra of reactive species which can be used to identify reactive species and their reaction products, as well as to monitor reaction kinetics. The capabilities of the instrument have been demonstrated through measurements of the ν4 band of the infrared spectrum of the CH2OO Criegee intermediate, produced by laser flash photolysis of CH2I2/O2/N2 gas mixtures at λ = 266 nm, and through measurements of the 445 kinetics of the reaction between CH2OO and SO2 under a range of conditions. The results have demonstrated the ability to measure reaction kinetics through monitoring of either reactants or reaction products, with the potential for the identification of reaction products and measurements of product yields. Results have shown that SO3 is a reaction product in the reaction of CH2OO with SO2, with preliminary results indicating that there is no pressure dependence in the yield of SO3.

450
The instrument described in this work has applications in atmospheric chemistry and chemical kinetics, with wider potential uses in trace gas analysis in industrial processes and medical diagnostics. Future development of the instrument will focus on improvements in the signal-to-noise ratio and limit of detection. Improvements to the limit of detection could be achieved through further increases to the sampling rate and through the use of a balanced detector or lock-in amplification techniques to improve the signal-to-noise ratio. Additional improvements to the limit of detection in concentration terms are achievable 455 through the use of a mirror arrangement for the probe beam in which the optics are internal to the reaction cell, thereby reducing intensity losses as the probe beam passes through the cell windows on each pass through the cell and enabling significant increases to the effective pathlength.