Anonymous,2025-10-142025-10-142025Physical Review Applied, 2025; 24(1):014037-1-014037-112331-70192331-7019https://hdl.handle.net/2440/147778We present a metrological technique to characterize the thermodynamic state (temperature, pressure, and density) of gaseous molecules in a single spectroscopic measurement. We demonstrate the accuracy and precision of our approach by using a low-pressure (number density approximately 1.76 × 10¹⁸ molecules/cm³) acetylene gas in a sealed glass cell over a temperature range of −70 °C up to +60 °C. Under these experimental conditions the sample is in the gaseous state, displaying a behavior very close to that of an ideal gas. This allows us to predict the thermodynamic state with high accuracy, and thus rigorously test the performance of our approach. Our spectroscopic measurement uses a tunable wide-band optical frequency comb to acquire the full ro-vibrationally broadened acetylene spectrum between 1512 and 1538 nm with approximately 80-kHz spectral resolution. Through nonlinear fitting of this entire spectrum we obtain high accuracy (<1%) and high-precision (approximately 0.1%) estimates of the temperature, pressure, and density of the gas across the full examined temperature range. This combination of broadband probing and ultra-high-resolution spectrum ensures that we obtain reliable and precise measurements on the thermodynamic state.en© 2025 American Physical SocietyAnalytical chemistry; Atomic & molecular collisions; Frequency combs & self-phase locking; Metrology; Molecular spectra; Molecules; Infrared spectroscopy; Optical absorption spectroscopyJournal article10.1103/vbfp-2yvt745216