DESIGN AND PERFORMANCE OF A DOUBLE BEAM FAR INFRARED SPECTROPHOTOMETER

Abstract

Concerning the use of far infrared spectrometers in obtaining molecular data, $Lord^{1}$ has noted “The most serious difficulty remaining is the inaccuracy and inconvenience of spectrophotometry by the single-beam method.” This paper discusses an instrument designed to overcome this difficulty. The instrument is a double beam, ratio recording spectrophotometer which records spectra directly as per cent transmission versus wave number on a linear wave number scale. Three gratings are used each in the first order to cover the region from 400 to 50 $cm{-1}$. Unwanted grating orders are eliminated by crystal choppers and by other easily interchanged filtering optics. The problem of atmospheric absorption is solved, by the combination of double beam operation and a mechanical design which permits effective purging. The instrument has a large sample area, a wide adjustment range for its operating variables and relatively great system flexibility. Spectra will be shown to illustrate performance obtained.