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

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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.