Optical Determination of Thermodynamic Temperatures from a C2H2 Line-Doublet in the Near Infrared

This paper reports the implementation of Doppler-broadening gas thermometry by use of line-shape analysis of a line-doublet. The two spectral components are vibration-rotation transitions belonging to a pair of acetylene combination bands at a wavelength of 1.39 μm. Characterized by an extraordinary spectral fidelity in combination with high resolution, the spectrometer is based on two phase-locked extended-cavity diode lasers, one of them being referenced to an optical-frequency-comb synthesizer. The global analysis of 1180 spectra, which are recorded as a function of the C2H2 pressure at the constant temperature of the triple point of water, yields an optical determination of the thermodynamic temperature with a combined relative uncertainty (type A plus type B) of 23 parts per million. Similar results are obtained at the melting point of gallium (approximately 303 K). Furthermore, we apply line-absorbance analysis to the acquired spectra, demonstrating a reduction by a factor of approximately 6 of the statistical uncertainty for the retrieved gas temperature.