A Single-Beam, Double-Pass Spectrometer for the Far Infrared

A Perkin-Elmer Model 112 infrared spectrometer has been modified for double-pass operation in the 400 to 173 cm-1 region. This region is scanned with two gratings, 40 lines/mm and 20 lines/mm, used in first or der at the Littrow mirror position. A 14° off-axis paraboloid mirror is employed, and filtering is accomplished with reststrahlen plates, scatter plates, and polyethylene transmission filters. No chopper modifications are required. Atmospheric water absorption is removed by circulation of the air in the optical path through drying columns. The pure-rotation spectrum of atmospheric water recorded throughout the converted range of the instrument is presented.