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 CH₂OO Criegee intermediate

<p>The chemistry and reaction kinetics of reactive species dominate changes to the composition of complex chemical 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 <span class="inline-formula">CH₂OO</span> Criegee intermediate with <span class="inline-formula">SO₂</span>. The instrument presented has high spectral (<span class="inline-formula">&lt;</span> 0.004 cm<span class="inline-formula">⁻¹)</span> and temporal (<span class="inline-formula">&lt;</span> 5 <span class="inline-formula">µ</span>s) resolution and is able to monitor kinetics with a dynamic range to at least 20 000 s<span class="inline-formula">⁻¹</span>. Results obtained at 298 K and pressures between 20 and 100 Torr gave a rate coefficient for the reaction of <span class="inline-formula">CH₂OO</span> with <span class="inline-formula">SO₂</span> of (3.83 <span class="inline-formula">±</span> 0.63) <span class="inline-formula">×</span> 10<span class="inline-formula">⁻¹¹</span> cm<span class="inline-formula">³</span> s<span class="inline-formula">⁻¹</span>, which compares well to the current IUPAC recommendation of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><mfenced close=")" open="("><mrow><msubsup><mn mathvariant="normal">3.70</mn><mrow><mo>-</mo><mn mathvariant="normal">0.40</mn></mrow><mrow><mo>+</mo><mn mathvariant="normal">0.45</mn></mrow></msubsup></mrow></mfenced></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="22pt" class="svg-formula" dspmath="mathimg" md5hash="d583b63ae9fb740190bf16448b830808"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-2875-2022-ie00001.svg" width="58pt" height="22pt" src="amt-15-2875-2022-ie00001.png"/></svg:svg></span></span> <span class="inline-formula">×</span> 10<span class="inline-formula">⁻¹¹</span> cm<span class="inline-formula">³</span> s<span class="inline-formula">⁻¹</span>. A limit of detection of 4.0 <span class="inline-formula">×</span> 10<span class="inline-formula">⁻⁵</span>, in absorbance terms, can be achieved, which equates to a limit of detection of <span class="inline-formula">∼</span> 2 <span class="inline-formula">×</span> 10<span class="inline-formula">¹¹</span> cm<span class="inline-formula">⁻³</span> for <span class="inline-formula">CH₂OO</span>, monitored at 1285.7 cm<span class="inline-formula">⁻¹</span>, based on the detection path length of (218 <span class="inline-formula">±</span> 20) cm. Initial results, directly monitoring <span class="inline-formula">SO₃</span> at 1388.7 cm<span class="inline-formula">⁻¹</span>, demonstrate that <span class="inline-formula">SO₃</span> is the reaction product for <span class="inline-formula">CH₂OO</span> <span class="inline-formula">+</span> <span class="inline-formula">SO₂</span>. The use of mid-IR QCL absorption spectroscopy offers significant advantages over alternative techniques commonly used to determine reaction kinetics, such as laser-induced fluorescence (LIF) or ultraviolet absorption spectroscopy, owing to the greater number of species to which IR measurements can be applied. There are also significant advantages over alternative IR techniques, such as step-scan FT-IR, owing to the coherence and increased intensity and spectral resolution of the QCL source and in terms of cost. The instrument described in this work has potential applications in atmospheric chemistry, astrochemistry, combustion chemistry, and in the monitoring of trace species in industrial processes and medical diagnostics.</p>