Mid-Infrared Frequency Comb Lasers for Chemical Kinetics: Applying Physics Technologies to Kinetics, Dynamics, and Molecular Spectroscopy

Lead Research Organisation: University of Leeds
Department Name: Sch of Chemistry


A simple chemical reaction could be described as an interaction between two reactant molecules, A + B, which leads to the formation of two new product molecules, C + D. This process involves the breaking and making of chemical bonds, giving the products inherently different properties than the reactants. One way to identify the product and reactant molecules is by using vibrational spectroscopy. Each bond in a molecule vibrates at a specific frequency, making the vibrational absorption spectrum of one molecule (such as molecule A) different than another molecule (such as molecules B, C, or D), like a "fingerprint" identifying that molecule. However, because bonds in different molecules could vibrate at vastly different frequencies, it is hard to view the fingerprints of all of the molecules in the A + B -> C + D reaction at once. To do so, a simultaneously broadband and high resolution vibrational absorption spectrum would be needed. However, it would also be useful to know the timescale for the reaction. Suppose further that this reaction was competing with another reaction, like A + B -> E. It is then not only important to know the rate at which molecules A and B disappeared, but also the rate at which C, D, and E appeared.

From the above hypothetical chemical reactions, we realize that it is important to know both the identity of molecules involved in a reaction (reactants and products) as well as the rate at which they disappear or appear. Thus, it is essential to use a simultaneously broadband (wide spectral width) and high spectral resolution technique, combined with the time resolution necessary to monitor the kinetics of the chemical reactions. The proposed research uses a technique developed by the optical physics community called cavity-enhanced direct frequency comb spectroscopy and applies it to a fundamentally interesting radical-radical reaction. Here, a frequency comb laser is the source of the infrared radiation necessary to excite molecular vibrations. It is a broadband source, so it can excite a range of different molecular vibrations within a wide spectral region (3 - 3.5 microns). It is unique, though, in that thousands of spectrally narrow "comb teeth" make up this broadband source, each with a known and controllable frequency. This makes it both broadband and high resolution, meeting the criteria for being able to spectrally identify molecules based on their vibrational fingerprints. This light source is passed through a reaction cell, where a chemical reaction takes place (in the proposed experiment, the initial target reaction is the radical-radical reaction CH2SH + NO). Some of the molecules involved in this reaction absorb the infrared radiation, attenuating the amount of infrared light passing through the reaction cell at the specific frequencies ("comb teeth") that the molecules absorbed. In the proposed research, the "comb teeth" of this light source are spatially dispersed onto an infrared sensitive camera, giving a high resolution vibrational absorption spectrum of what is contained in the gas cell. The camera takes images as the reaction occurs, yielding vibrational absorption spectra as a function of reaction time, thus simultaneously identifying and mapping the timescale of the appearance (and disappearance) of molecules involved in the chemical reaction. This is a unique technique to be applied to studying the kinetics and dynamics of chemical reactions, where a significant amount of detail about a chemical reaction is contained in this high resolution, time-resolved spectrum.

Planned Impact

The proposed research generates new knowledge towards the elucidation of chemical reaction kinetics and mechanisms and contains technological advances in the building of the new apparatus. As an innovative and disruptive technology, this new research directly impacts the Productive Nation prosperity outcome. While it primarily benefits academics in the short term (3-5 years), this is a necessary step in the research process particularly at this early stage in Dr Lehman's career, in order to develop a feedback mechanism within the scientific communities that are directly impacted by the proposed work. This research will have a direct impact on fundamental physical chemistry communities, specifically through spectroscopy, reaction kinetics, and dynamics, along with impacting optical physics, astrochemistry, combustion and atmospheric chemistry communities. Information on this research will be disseminated to these communities via publication, involvement at conferences, and further networking. The feedback from these communities, followed by further methodology or instrument improvements, will enable this research to move forward past the tenure of this EPSRC First Grant and onto other types of beneficiaries. In addition, this research is taking place during the formative part of Dr Lehman's career and will directly impact her international success as an academic scientist, as well as impacting the individuals Dr Lehman will mentor.

Underpinning this research is the idea of movement toward non-academic beneficiaries, which is also a natural progression after establishing and further developing this research, and will likely take place in the medium to long term through collaborations with industry. These collaborations will be established through networking with academic colleagues who have industrial ties, along with directly meeting industrial representatives at international conferences. It is anticipated that the method and instrument development stemming from this research will more easily transition to non-academic beneficiaries, leading directly to impacting national importance. For example, trace gas analysis techniques might benefit from this type of sensitive, broadband, high resolution instrument being made portable and more robust in order to make measurements in different settings, which then has the potential to impact the Healthy Nation prosperity outcomes. It is possible to use these tools in medical diagnosis, where a version of this instrument could be used for breath analysis in hospital or elsewhere. In addition, a direct industrial processing application could be likely, particularly through its possible use in trace gas sensing and monitoring impurities or contamination in products that might lead to adverse health effects, which also has an economic impact on industry. In an extreme case, it is also possible for these tools to be used in space exploration, impacting our understanding of planetary atmospheres and their role in planetary evolution. Both of these possibilities require a close collaboration between the academic scientist and the research and development team in an industrial setting, even extending to the manufacturing process, in order to advance the technology. These types of industries (analytical instrumentation, laser development) are often represented at international conferences in physical chemistry, or atmospheric and planetary science. The scientific outcomes of this research also have the potential for a longer term societal impact. It is anticipated that interstellar, planetary, or atmospheric models will incorporate the kinetics results of this research, possibly furthering our understanding of how complex molecules form in space, chemistry of planetary atmospheres, or even the relationship between the chemistry in our atmosphere and environmental changes. This could lead to impacts on policy, which will then influence societal changes related to protecting our environment.
Description A new, state-of-the-art spectrometer was built and applied to studying the spectroscopy of small, atmospherically relevant molecules. This spectrometer is unique in that it uses an infrared frequency comb laser (a simultaneously broadband and high resolution laser) as its light source. The newly built spectrometer was the first of its kind in the UK, and works by spatially dispersing the individual comb teeth of a frequency comb laser and imaging their intensity on an infrared camera. Using direct absorption spectroscopy, the infrared spectra of a complex mixture of gases can be obtained with an excellent (parts-per-billion) level of molecular sensitivity and selectivity due to the nature of the broadband high resolution light source. Beyond building this unique spectrometer, specific key findings include new spectroscopic constants for diiodomethane and initial tests on the photolysis of diiodomethane to form ethylene. The observed high resolution spectrum of diiodomethane has since impacted the interpretation of other dihaloalkane infrared spectra.
Exploitation Route The outcomes of the funding can primarily be taken forward by others within the research group, including myself. As a First Grant, there was quite a bit of learning by myself (the PI) on the research grant cycle, execution of objectives and deliverables, and keeping track of time management and expenditures. Within the research group, the MATLAB code involved in analysing the images is used for any project involving this spectrometer. The optical design and standard operating procedures are also used by the research group. For outcomes to be taken forward outside the research group, the data generated that led to publications are all freely available through the Leeds data repository (with their own DOI). I am also happy to share home written analysis code.
Sectors Chemicals,Environment

Description (HILTRAC) - Highly Instrumented Low Temperature Reaction Chamber
Amount € 2,493,790 (EUR)
Funding ID 948525 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 02/2021 
End 01/2026
Title A Rapid, Spatially Dispersive Frequency Comb Spectrograph aimed at Gas Phase Chemical Reaction Kinetics - dataset 
Description Raw images of spatially dispersed frequency comb laser after absorption by a sample of methane. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL http://archive.researchdata.leeds.ac.uk/642/
Title CH2I2 Spectral Data 
Description Dataset for the full experimental CH2I2 spectrum covering 2932.6 to 3125.4 cm-1, the raw pgopher simulation, extracted spectrum, and line list for each nu_1 simulation and nu_6 simulation, the computational results for 5 different method and basis set combinations, and the computational pgoper simulational and extracted spectrum for the wB97X_def2QZVPP computational results. 
Type Of Material Database/Collection of data 
Year Produced 2022 
Provided To Others? Yes  
Impact This new analysis of diiodomethane infrared spectra could impact the interpretation of other dihalomethane infrared spectra. 
URL https://archive.researchdata.leeds.ac.uk/928/
Description Leeds Festival of Science 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Volunteered for Leeds Festival of Science with the Royal Society of Chemistry science booth at the Leeds City Museum. Significant amount of positive feedback, getting younger generations interested in STEM.
Year(s) Of Engagement Activity 2018
Description Pint of Science 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Pint of Science speaker, with approximately 50 attendees. Significant discussion during and after the event, showing broad, general public interest in the research.
Year(s) Of Engagement Activity 2018
Description UCL Physics Seminar Speaker 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Asked to give a seminar at UCL in their Atomic, Molecular, Optical and Positron Physics school seminar series. This was the second time I've given a talk on frequency comb spectroscopy, and it sparked discussion and possible future collaborations with the high resolution spectroscopists in the audience, primarily those with planetary or interstellar medium research focus.
Year(s) Of Engagement Activity 2019