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

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.

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.