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Title:3μm - 1.6μm double resonance spectroscopy of CH4
Author(s):Schwartz, George
Contributor(s):Lehmann, Kevin; Yang, Shaoyue; Belaas, Erik
Subject(s):Spectroscopy in Atmospheric Chemistry
Abstract:The Near-IR Spectrum of CH$_4$ is dense with many overlapping bands that perturb each other by vibrational and ro-vibrational interactions. Assignments of the individual lines are needed in order to simulate the spectrum as a function of pressure and temperature, as needed in the search for CH$_4$ in extrasolar planets. Both the group at the University College, London$^1$ and that at the University of Reins$^2$ have produced theoretical spectra that allows simulation up to the high temperatures expected on ``Hot Jupiters’’. The accuracy of these theoretical spectra need to be further tested. Because CH$_4$ is a light spherical top, assignment of its perturbed spectra is a formable challenge as none of the lines allowed in the rigid rotor approximation have ground vibrational state combination differences. We are using IR-IR double resonance to observe modulation in the strength of near-IR absorption caused by a modulation of a 3 $\mu$m OPO beam that is tuned to a particular transition in the C-H stretching fundamental of CH$_4$. This produces V-type double resonance transitions (which share the lower state with the pump transition), which provides firm assignments for lines normally observed in absorption in the near-IR. We also observe sequential double resonance which reveals transitions that have a known rotational level of the $\nu_3$ fundamental as the lower state and reaches final states in the 9000\,cm$^{-1}$ spectral region. These are states of {\it A, E, F}$_1$ vibrational symmetries which are forbidden in transitions from the ground vibrational state. These 3 level double resonance transitions are Doppler Free and have a linewidth of $\sim$10\,MHz due to a combination of near-IR laser jitter and power broadening of the mid-IR transition. We also observed many 4-level double resonance transitions that we have tentatively assigned as arising from the $\nu_4$ fundamental level. These are distinguished from the 3-level double resonance transitions by they being Doppler broadened and having a large phase shift relative to the intensity modulation. 1. S.N. Yurchenko, PNAS {\bf 111} 9379-83 (2014)
2. M. Rey, JQSRT {\bf 18}, 207-220 (2015), PCCP {\bf 18}, 176-189 (2016)
Issue Date:2016-06-20
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper/Presentation
Rights Information:Copyright 2016 by the authors
Date Available in IDEALS:2017-01-26

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