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

Highlights: • Depletion of vibrational ground state of CH₄ in N₂ & air in absorption is shown (81) • A model for depletion in CH₄ and ¹³CH₄ in N2 and air at 1–100 Torr is verified (80) • The depletion has a strong pressure dependence; less strong in air than in N2 (79) • Considerable degrees of depletion (> 80%) can be obtained for Watt-levels of light (84) • Guidelines for how to avoid depletion are given (49) 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.