Ultrasensitive Spectroscopy

There are many problems of both fundamental and of practical importance that requires measurement of extremely low concentrations of certain impurities. Molecular spectroscopy provides one approach that excels in the high specificity provided by the detailed structure in the spectrum, particularly for molecules in the gas phase. Lehmann’s group has been working on the development of new trace sensors, largely based upon the method of cavity ring-down spectroscopy (CRDS). In CRDS, one forms a stable optical cavity using mirrors with reflectivity > 99.99 percent and observes absorption of a sample contained inside the cavity by an increase in the rate of decay of light that is trapped between the mirrors. Sample absorption as low as 1 part in 109 per pass of the cell can be measured in this way. The Lehmann group pioneered the use of low cost and rugged diode lasers developed for the telecom industry in CRDS and has demonstrated detection of a number of small molecules, such as H2O, NH3, NO, and CH4 at levels below one part per billion in a sample gas. Tiger Optics, Inc. is now selling instruments based upon this work to several industries. 

A current focus of the research is development of a version of CRDS that exploits near-resonant two-photon IR absorption of a low-pressure gas.  A low-loss optical cavity can enhance the intensity of light up to ten thousand times over the intensity of the laser used to excite the cavity.  Two-photon absorption of counter propagating photons is with out Doppler Broadening, resulting in transitions with widths ~103 times narrower than the Doppler width of transitions.   While the spectrum of polyatomic molecules often have crowded spectra due to the large number of thermally populated rotational and vibrational states, the two-photon spectrum is dominated by only a few transitions where there is a pair of transitions, sharing an intermediate level, that are very nearly at the same frequency.   For modest size polyatomic molecules, the two-photon absorption spectrum has less than 0.1% as many strong lines at the one photon spectrum.  The combination of the small number and small widths of the lines mean that the typical two-photon spectrum is ~106 times more selective than the one photon spectrum in the same spectral region.  In addition, the two-photon loss of a cavity can be separated from linear loss contributions to the cavity from mirrors, scattering, and one-photon absorption, which results in increased sensitivity as well.

Double Resonance Spectroscopy

There are many situations, such as in combustion and for “Hot Jupiter” Exoplanets, where simulation of spectra of hot samples of polyatomic molecules is needed.  Traditionally, such spectra are difficult to study experimentally due to spectral congestion and sample decomposition.  Great strides have been made in theoretical modeling of such spectra but these theoretical spectra need to be tested experimentally to assess their accuracy, particularly with respect to transitions starting from excited vibrational states.   The Lehmann group, in collaboration with a group in the Univ. of Umea in Sweden, has developed a novel double resonance method that involves using a narrow bandwidth infrared Optical Parametric Oscillator to pump a single ro-vibrational transitions of a molecule (so far we have focused on methane) and then probe absorption transitions from this excited state with a frequency comb, which allows us to simultaneously detect absorption has tens of thousands of discrete frequencies, each determined with ~1 KHz frequency accuracy.  As the pump laser only pumps molecules with a narrow width of Doppler Shift, the probe transitions are sub-Doppler with a width of only a few MHz. 

Meta-Science

There is currently a “replication crisis” is several fields of science, such as psychology and cancer research, where it has been found that a majority of published experiments fail to give the reported results when carefully repeated by independent laboratories.   There are more isolated reports that a substantial fraction of chemical syntheses cannot be reproduced using only what is in the published papers.    Spectroscopy is most often more about measurement than hypothesis testing and has substantially redundancy in that many transitions are fit to high accuracy with a Hamiltonian with a limited number of constants.   However, the accuracy of derived molecular parameters are often dependent upon the details of statistical analysis and poorly understood model errors.  The lab is currently doing systematic investigations the historical reproducibility of spectroscopic constants and the molecular properties derived from them, such as equilibrium structures.

Recent Publications

1. Simon Lobsiger, Cristobal Perez, Luca Evangelisti, Kevin K. Lehmann, and Brooks H. Pate, Molecular Structure and Chirality Detection by Fourier Transform Microwave Spectroscopy, J. Phys. Chem. Lett. 6, 196-200 (2015)
2. Y. Chen, P. Mahaffy, V. Holes, J. Burris, P. Morey, K.K. Lehmann, B. Sherwood Lollar, G. Lacrampe-Couloume, and T.C. Onstott, Near Infrared Cavity Ring-Down Spectroscopy for Isotopic Analysis of CH4 on Future Martian Surface Missions, Planetary and Space Sciences 105, 117-122 (2015).
3. Vitali I. Stsiapura, Vincent K. Shuali, Benjamin M. Gaston, and Kevin K. Lehmann, Detection of S‑Nitroso Compounds by Use of Midinfrared Cavity Ring-Down Spectroscopy, Analytical Chemistry 87, 3345-53 (2015).
4. Z Meng, G.I. Petrov, S Cheng, J.A. Jo, K.K. Lehmann, V.V. Yakovlev, M.O. Scully, Lightweight Raman Spectroscope using time-correlated photon counting detection, Proceedings of the National Academy of Sciences, 112, 12315-12320 (2015).
5. Chen, Y., K. K. Lehmann, Y. Peng, L. M. Pratt, J. R. White, S. B. Cadieux, B. S. Lollar, G. Lacrampe-Couloume and T. C. Onstott, Hydrogen Isotopic Composition of Arctic and Atmospheric CH4 Determined by a Portable Near-Infrared Cavity Ring-Down Spectrometer with a Cryogenic Pre-Concentrator, Astrobiology 16(10): 787-797. (2016).
6. Lehmann, K. K., Influence of resonant collisions on the self-broadening of acetylene, Journal of Chemical Physics 146(9): 094309 (2017).
7. Seifert, N. A., D. P. Zaleski, R. Fehnel, M. Goswami, B. H. Pate, K. K. Lehmann, H. O. Leung, M. D. Marshall and J. F. Stanton, The gas-phase structure of the asymmetric, trans-dinitrogen tetroxide (N2O4), formed by dimerization of nitrogen dioxide (NO2), from rotational spectroscopy and ab initio quantum chemistry, Journal of Chemical Physics 146(13): 134305 (2017).
8. K.K. Lehmann, Theory of Enantiomer-Specific Microwave Spectroscopy, in Frontiers and Advances in Molecular Spectroscopy, pgs 713-743 (2018).  Edited by Jaan Laane, Elsevier publisher. 
9. K.K. Lehmann, Influence of spatial degeneracy on rotational spectroscopy: Three-wave mixing and enantiomeric state separation of chiral molecules, Journal of Chemical Physics 149(9): 094201 (2018).
10. J. Karhu, K.K. Lehmann, M. Vainio, M. Metsala, and L. Halonen, Step-modulated decay cavity ring-down, Optics Express 26(22): 094201 (2018)
11. K. K. Lehmann, Resonant enhanced two-photon cavity ring-down spectroscopy of vibrational overtone bands: a proposal, Journal of Chemical Physics 151: 144201(2019), https://doi.org/10.1063/1.5122988.
12. Gang Zhao, D. Michelle Bailey, Adam J. Fleisher, Joseph T. Hodges, and Kevin K. Lehmann, Doppler-Free two-photon cavity ring-down spectroscopy of a nitrous oxide (N2O) vibrational overtone transition, Physical Review A 101, 062509 (2020).
13. Kevin K. Lehmann, Optical cavity with intracavity two-photon absorption, Journal of the Optical Society of America B 37, 3055 (2020).
14. Aleksandra Foltynowicz, Lucile Rutkowski, Isak Silander1, Alexandra C. Johansson, Vinicius Silva de Oliveira, Ove Axner, Grzegorz Soboń, Tadeusz Martynkien, Paweł Mergo, and Kevin K. Lehman, Sub-Doppler Double-Resonance Spectroscopy of Methane Using a Frequency Comb Probe, to be published in Physical Review Letters.
15. Aleksandra Foltynowicz, Lucile Rutkowski, Isak Silander1, Alexandra C. Johansson, Vinicius Silva de Oliveira, Ove Axner, Grzegorz Soboń, Tadeusz Martynkien, Paweł Mergo, and Kevin K. Lehman, Measurement and assignment of double-resonance transitions to the  8900--9100 cm-1 levels of methane, to be published in Physical Review A.

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