He-Ne laser frequencies near 2.4 µm and their application to hydrogen fluoride detection

He-Ne lasers can provide three frequencies near 2.4 μm, one of which coincides with the
1R(5)
frequency of HF at 4173.98 cm
^-1
. Either of the other frequencies may serve as a reference for absorbance measurements. One of these, at 4169.33 cm
^-1
, is a new lasing transition not previously observed (
4d'_{4}-3P_{9}
). Operating conditions are discussed.