Depletion of the vibrational ground state of CH4 in absorption spectroscopy at 3.4 µm in N2 and air in the 1–100 Torr range

Thomas Hausmaninger, Gang Zhao, Weiguang Ma, Ove Axner

Research output: Contribution to journalArticleScientificpeer-review

1 Citation (Scopus)

Abstract

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 1–100 Torr 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 2 W and for buffer gases of N2 or dry air, and verifies the applicability of the model. It was found that the degree of depletion is significantly larger in N2 than dry air, and that it increases with pressure for pressures up to around 10 Torr (attributed to a decreased diffusion rate) but decreases with pressure for pressures above 20 Torr (caused by an increased collisional vibrational decay rate). The maximum degree of depletion (∼80%) was obtained for methane in N2 at around 15 Torr. 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 13CH4, 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.

Original languageEnglish
Pages (from-to)59-70
Number of pages12
JournalJournal of Quantitative Spectroscopy and Radiative Transfer
Volume205
DOIs
Publication statusPublished - 1 Jan 2018
MoE publication typeA1 Journal article-refereed

Fingerprint

Absorption spectroscopy
Ground state
absorption spectroscopy
depletion
ground state
air
Methane
Air
Buffers
Gases
methane
buffers
Lasers
Cavity resonators
lasers
gas composition
Spectrometry
cavity resonators
gases
decay rates

Keywords

  • Absorption spectroscopy
  • Cavity enhanced
  • Depletion
  • Methane
  • Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy
  • Optical saturation

Cite this

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title = "Depletion of the vibrational ground state of CH4 in absorption spectroscopy at 3.4 µm in N2 and air in the 1–100 Torr range",
abstract = "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 1–100 Torr 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 2 W and for buffer gases of N2 or dry air, and verifies the applicability of the model. It was found that the degree of depletion is significantly larger in N2 than dry air, and that it increases with pressure for pressures up to around 10 Torr (attributed to a decreased diffusion rate) but decreases with pressure for pressures above 20 Torr (caused by an increased collisional vibrational decay rate). The maximum degree of depletion (∼80{\%}) was obtained for methane in N2 at around 15 Torr. 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 13CH4, 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.",
keywords = "Absorption spectroscopy, Cavity enhanced, Depletion, Methane, Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, Optical saturation",
author = "Thomas Hausmaninger and Gang Zhao and Weiguang Ma and Ove Axner",
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language = "English",
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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 1–100 Torr range. / Hausmaninger, Thomas; Zhao, Gang; Ma, Weiguang; Axner, Ove.

In: Journal of Quantitative Spectroscopy and Radiative Transfer, Vol. 205, 01.01.2018, p. 59-70.

Research output: Contribution to journalArticleScientificpeer-review

TY - JOUR

T1 - Depletion of the vibrational ground state of CH4 in absorption spectroscopy at 3.4 µm in N2 and air in the 1–100 Torr range

AU - Hausmaninger, Thomas

AU - Zhao, Gang

AU - Ma, Weiguang

AU - Axner, Ove

PY - 2018/1/1

Y1 - 2018/1/1

N2 - 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 1–100 Torr 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 2 W and for buffer gases of N2 or dry air, and verifies the applicability of the model. It was found that the degree of depletion is significantly larger in N2 than dry air, and that it increases with pressure for pressures up to around 10 Torr (attributed to a decreased diffusion rate) but decreases with pressure for pressures above 20 Torr (caused by an increased collisional vibrational decay rate). The maximum degree of depletion (∼80%) was obtained for methane in N2 at around 15 Torr. 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 13CH4, 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.

AB - 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 1–100 Torr 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 2 W and for buffer gases of N2 or dry air, and verifies the applicability of the model. It was found that the degree of depletion is significantly larger in N2 than dry air, and that it increases with pressure for pressures up to around 10 Torr (attributed to a decreased diffusion rate) but decreases with pressure for pressures above 20 Torr (caused by an increased collisional vibrational decay rate). The maximum degree of depletion (∼80%) was obtained for methane in N2 at around 15 Torr. 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 13CH4, 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.

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KW - Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy

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