Sensing of molecular hydrogen using direct tunable diode laser absorption spectroscopy

This study presents sensitive and precise measurements of molecular hydrogen using direct tunable diode laser absorption spectroscopy (TDLAS). The S(1) H₂ quadrupole transition at 2122 nm (4713 cm^-1) was investigated using an 11.4-meter multipass cell. A minimum detectable concentration of 12 ppmv was achieved with a 1-second integration time, and 1.2 ppmv with a 100-second integration time. The measurements demonstrate excellent linearity across a broad concentration range in binary H₂-N₂ mixtures, from low ppmv levels to pure H₂ at 1 atm. The results emphasize that accurate quantification requires a line profile that incorporates collision-induced narrowing. We underscore the importance of accounting for collision-induced absorption (CIA) and intercollisional dips, the latter posing significant challenges for sensing H₂ at elevated pressures. Additionally, we present the first direct TDLAS measurement of the Q(1) H₂ absorption line at 2406 nm (4155 cm^-1), which exhibits an exceptionally strong intercollisional dip. This work highlights the potential of TDLAS technology for applications requiring accurate and sensitive H₂ measurements, including industrial process monitoring, safety, and control.