Development of a Trace Gas Detector using Cavity Ring-Down Detection of Two-Photon Absorption

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With support from the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry, Professor Kevin Lehmann and his research group at the University of Virginia are working to develop and implement a new laser-based method for the selective detection of trace gases. The method takes advantage of a unique process called near-resonant two-photon absorption spectroscopy in order to improve the extent to which different molecules can be distinguished in complex gas mixtures. The improvements are based on significant increases in both the selectivity and the sensitivity of the measurement that occur when a molecule simultaneously absorbs two photons, with one of the photons having an energy that is very close to, but not exactly the same as, one of the vibrations of the molecule. The experiments use an optical cavity that traps the light between two mirrors in order to increase the signal strength. Cavity-enhanced techniques for trace gas detection, especially a method known as cavity ring-down spectroscopy, allow the identification and characterization of molecules in many applications, including ultrahigh purity manufacturing, drug purity and lifetime monitoring, environmental monitoring on both local and global scales, medical diagnostics, and even the time-dating of archeological samples. The research team led by Professor Lehmann is using the new two-photon approach to overcome important complications that have limited the size of the molecules that can be distinguished using traditional cavity-enhanced methods, potentially leading to significant increases in the range and complexity of samples that can be analyzed. This project involves development of new trace gas detection methods based on near-resonant two-photon absorption cavity ring-down spectroscopy (TP-CRDS) in the infrared spectral region. Measurements to detect small molecules, such as nitrous oxide and carbon dioxide, provide important tests of the underlying theory of the technique and the detection limits compared with the predicted shot-noise limit. Test measurements include detection of trace levels of nitrous oxide in synthetic air samples with realistic minor components that are present in ambient air in various environments. Additional measurements examine the two-photon absorption spectra of more complex molecules, such as trans-butadiene. Wide-bandwidth scans survey the two-photon absorption spectra of these larger molecules for the first time, providing experimental spectra of the pure gases at low pressure for comparison with simulated spectra based on anharmonic frequency predictions from electronic structure calculations. The wide-bandwidth survey scans use two detection methods, one based on absorption and one based on emission. In the first case, absorption of infrared light in a multipass Herriott cell is measured based on amplitude or frequency modulation of a counter-propagating wave. The second method detects infrared emission of the sample induced by two counter-propagating waves in order to selectively distinguish two-photon absorption from Lamb dips, based on opposite sign of the signal in the two cases. Measurements covering the spectral region of the fundamental C-H stretching vibrations of polyatomic molecules have the potential to reveal the composition of mixtures of hydrocarbons that cannot be distinguished by their one-photon absorption spectra due to spectrally overlapping bands. Overall, this new approach to trace gas detection using near-resonant two-photon absorption spectroscopy holds the promise of more sensitive and more selective detection, including detection of isotopically distinct species and identification of larger molecules in more complex mixtures than current methods allow. These developments could have impact across many areas, including atmospheric science, industrial and environmental monitoring, and radiocarbon dating. The research project also will provide advanced training opportunities for graduate and undergraduate research students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.