Spectroscopic Measurements of Methane Solid-Gas Equilibrium Clapeyron Curve between 40 and 77 K

Area of Science:

• Thermodynamics and Spectroscopy
• Planetary Science and Astrochemistry

Background:

• Methane (CH4) is a key molecule in planetary atmospheres and interstellar ices.
• Accurate thermodynamic data, particularly the solid-gas equilibrium, is crucial for modeling these environments.
• Previous measurements of methane's vapor pressure have limitations in temperature range and accuracy.

Purpose of the Study:

• To experimentally determine the Clapeyron solid-gas equilibrium curve for methane.
• To accurately measure methane's vapor pressure and sublimation enthalpy across a wide temperature range (40-77 K).
• To validate experimental results using two distinct optical setups and compare with existing literature data.

Main Methods:

• Utilized infrared gas-phase absorption spectroscopy to probe methane.
• Employed two cryogenically cooled experimental setups: a Fourier transform infrared spectrometer and a tunable laser source with a Herriott cell.
• Recorded methane absorption features from 40 K to 77 K.

Main Results:

• Derived the methane vapor pressure curve: ln(p/Pa) = -(1191.92 ± 8.92)/(T/K) + (22.49 ± 0.16) in the 40-77 K range.
• Calculated a sublimation enthalpy of 9910 ± 75 J mol⁻¹.
• Extended knowledge of methane saturation pressure by two orders of magnitude down to 40 K.

Conclusions:

• The derived vapor pressure relation provides a reliable thermodynamic dataset for methane.
• The experimental results align well with literature values and thermodynamic models.
• This study enhances our understanding of methane's behavior in low-temperature environments relevant to astrochemistry and planetary science.