Directly-excited laser-induced thermal grating spectroscopy and thermometry with carbon dioxide vibrational transition

Laser-induced thermal grating spectroscopy (LITGS) has been proved for accurate thermometry and measurement of energy transfer processes in molecules. While electronic transitions are often used for excitation in previous LITGS works, here we report laser-induced thermal gratings formed by direct excitation of CO₂ with an infrared (IR) laser at wavelength near 2 μm, with which high signal-to-noise ratio LITGS signals are generated (SNR ∼ 300 at room temperature). A theoretical LITGS model assuming a ‘two-steps’ energy transfer process is used to describe the recorded signal waveform, with relative fitting residuals of less than 10%. Quantitative thermometry is performed in CO₂ gas flows between 293 K and 420 K, with a relative uncertainty of 1.6% and a precision of 1.1% defined as the 1-σ standard deviation of 30 repeated measurement. Furthermore, the time constants of vibrational energy transfer at different temperatures are extracted from the temporal signal waveform with a precision better than 80 ns, and the measured results are consistent with the simulation using a detailed vibrational energy transfer model. These results demonstrate IR LITGS as a potential tool for spatially-resolved measurement of the thermophysical properties of fluids, as well as molecular vibrational energy transfer processes.