Application of a CW helium neon laser for measurement of gaseous hydrogen fluoride concentration

The concentration of gaseous hydrogen fluoride in air can be measured by the use of a close coincidence between the 3P ₄–2s ₂ cascade cw helium neon laser line and the 1R5 hydrogen fluoride absorption line. A nearby cw HeNe laser line (3p ₂–2s ₂) is not absorbed by hydrogen fluoride and may be used to compensate for transmission losses in a long-path measurement system. Most of the HFv=0 tov=1 absorption lines lie in a region in which there is strong attenuation caused by atmospheric water vapour, but the 1R5 line is almost free of interference from H₂O, CO₂ or CO.