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Depletion of the vibrational ground state of CH4 in absorption spectroscopy at 3.4 μm in N2 and air in the 1-100Torr range
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
2018 (English)In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 205, p. 59-70Article in journal (Refereed) Published
Abstract [en]

A model presented in an accompanying work predicts that mid-IR absorption signals from methane in trace concentrations in various buffer gases detected at pressures in the 1-100Torr range can be reduced and distorted due to depletion of the vibrational ground state if the molecules are exposed to laser powers in the tens of mW range or above. This work provides experimental evidence of such depletion in a resonant cavity under a variety of conditions, e.g. for intracavity laser powers up to 2W and for buffer gases of N-2 or dry air, and verifies the applicability of the model. It was found that the degree of depletion is significantly larger in N-2 than dry air, and that it increases with pressure for pressures up to around 10Torr (attributed to a decreased diffusion rate) but decreases with pressure for pressures above 20Torr (caused by an increased collisional vibrational decay rate). The maximum degree of depletion (similar to 80%) was obtained for methane in N-2 at around 15Torr. This implies that absorption spectrometry of methane can experience significant non-linear dependencies on laser power, pressure, as well as buffer gas composition. It is shown that depletion takes place also in (CH4)-C-13, which verifies the applicability of the model also for this isotopologue, and that NICE-OHMS signals detected in absorption phase are less affected by depletion than in dispersion. It was concluded that the absorption mode of detection can provide concentration assessments that are virtually free of influence of depletion for intracavity powers below 0.8 W. 

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD , 2018. Vol. 205, p. 59-70
Keywords [en]
Absorption spectroscopy, Cavity enhanced, Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, Methane, Depletion, Optical saturation
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:umu:diva-143714DOI: 10.1016/j.jqsrt.2017.10.007ISI: 000417665000008OAI: oai:DiVA.org:umu-143714DiVA, id: diva2:1171802
Available from: 2018-01-08 Created: 2018-01-08 Last updated: 2018-06-09Bibliographically approved

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Hausmaninger, ThomasZhao, GangMa, WeiguangAxner, Ove

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