Terahertz free-electron laser radiation to determine water concentration in flames

The possibility of measuring the concentration of H$_2$O molecules in flames based on the absorption of terahertz free-electron laser radiation was studied. These measurements were performed using the 177.32 cm$^{-1}$ absorption line in the rotational spectrum of H$_2$O. This line has a low intensity at room temperature, and at about 1000 K, its intensity is comparable to that of the strongest lines. The temperature dependence of the radiation absorption coefficient at a frequency of 77.32 cm$^{-1}$ was studied theoretically and experimentally. It is shown that the method can be used for measurements in a sooty C$_2$H$_4$/O$_2$/Ar flame, which strongly scatters visible and UV radiation.