Knowledge Bank

A Fabry-Perot Interferometer (FPI) with 10 cm diameter $CaF2$ etalons whose multilayer diclectric coatings having reflectivities in excess of 0.90 between 3.9 and 5.2 $\mu m$ has been constructed. Parallelism of the etalons to better than 100 \AA is maintained by an active control system over the 3 $\mu m$ scan range of one etalon. The scan is made by piezoelectric transducers and is monitored by a He-Ne laser for frequency calibration of the spectrum. An Ebert spectrometer is used to isolate a particular FPI fringe and is scanned in phase with the FPI to obtain a spectrum. A resolving power of about 180 000 near 4.5 $\mu m$ has been achieved with an etalon spacing of 1.5 cm. Examples of the use of this instrument will be presented. These will include the measurement of some of the halfwidths of lines in the $N2O$ $v2$ band, pressure broadened with $N2$, and the observation of $l$-type doubling in the $\nu 3+\nu 2-\nu 3$ band of $N2O$.