AN ULTRASENSITIVE LASER HETERODYNE INTERFEROMETER FOR MOLECULAR ABSORPTION $SPECTROSCOPY^\dag$

Abstract

We describe operation of a cw laser heterodyne interferometer of the type proposed by Davis (1). This device should be useful for probing weakly absorbing molecular species. In preliminary results we have obtained sensitivities of $10^{-8}$ $cm^{-1}$, competitive with photoacoustic spectroscopy (PAS). As a demonstration of its capabilities we have performed Stark spectroscopy of the $\nu_{2}$ band of $NH_{2}D$ and $NH_{3}$. This technique, like PAS, is calorimetric. A grating tuned $CO_{2}$ laser is operated at a wavelength absorbed by the species of interest and as the sample gas is heated via collisional quenching, a small change in density and refractive index is induced. The latter is monitored by placing the sample in an ultrasensitive He-Ne laser heterodyne interferometer capable of detecting refractive index changes as small as $10^{-11}$. When used in Stark studies, this approach has greater sensitivity than that obtained using long path differential absorption and is free from the electric field induced background signal that is encountered when empoying PAS.