Frequency Comb Spectroscopy of Methylmercapto Radical Reactions

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


The purpose of this PhD project is to measure the CH3S + NO and CH2SH + NO reaction rate constants at room temperature using laser photolysis combined with vibrational absorption spectroscopy, measured using a novel cavity-enhanced mid-infrared frequency comb. This set of reactions are being studied in order to further our fundamental understanding of the role of sulfur in akyl-nitrate reactions. For example, how does sulfur change the rates of these reactions or the dynamics of these reactions as compared to its oxygen analogues? How does the change in the radical site (carbon versus sulfur centred) change the dynamics or rates of the reactions? The first reaction is also being studied due its atmospheric and biological relevance in both Earth and exoplanetary atmospheres. The interpretation of previous data for this reaction and similar ones are often hindered by poor selectivity or sensitivity. This project aims to use this new technique to record more accurate rate coefficients, and hence gain a better understanding of gaseous reactions involving methylmercapto radicals.
Cavity-enhanced frequency comb spectroscopy can be used to simultaneously obtain a broadband, high-resolution vibrational absorption spectrum, as well as reaction rate constants for a desired gas-phase reaction. This novel technique involves a femtosecond frequency comb laser, gas-cell enhancement cavity and VIPA + grating spatial dispersion optics coupled to an IR camera. This system will be used to measure the vibrational absorption spectra of reactants, products, and intermediates in the CH3S + NO and CH2SH + NO reactions. These spectra can then be used as a detection method to monitor the reaction progress and derive room temperature reaction rate coefficients.
The first part of this project is to construct, optimize, and characterize a cavity-enhanced frequency comb spectrometer, using a VIPA etalon, grating, and infrared camera detection method and methane gas for calibration of the set-up. To the best of our knowledge, this will be the second such spectrometer in the world, and the only one in use in a School of Chemistry. The second part of the project involves creating the corresponding gas mixtures for analysis and observing the rates of reaction of the reactions highlighted above. The new spectrometer will have the necessary sensitivity and selectively to simultaneously measure the loss of reactants and growth of products during a reaction, giving coupled or multiplexed information that is typically unavailable in traditional kinetics experiments. The high resolution vibrational absorption spectra of the highly reactive CH3S and CH2SH radicals, and any transient intermediates, will also be measured. The laboratory measurements will be interpreted with the aid of theoretical methods, including quantum chemical calculations, spectroscopic modelling, and the opportunity to collaborate within the Atmospheric and Planetary Chemistry group for the potential to use the results in atmospheric models.


10 25 50