Spectral Purity for Far Infrared Grating Spectroscopy

A far ir grating spectrometer of the Ebert type has been constructed to cover the range from 10 cm(-1) to 160 cm(-1). A technique for quantitatively measuring the spectral purity of this instrument is presented. Filter curves for the reflection filter gratings used with the spectrometer are given for radiations polarized parallel and perpendicular to the groove length. It is found that, with a 20 degrees blaze angle and the plane of incidence of the radiation parallel to the groove length, polarization effects due to the filters are small. In fact, the filters are very efficient and possess sharp cutoff characteristics in both polarizations when used in this fashion. The measured spectral purity of the spectrometer is better than 95%0 throughout most of its operating range. This result is consistent with purity estimates from absorption experiments on water vapor lines in the region from 195 micro to 300 micro. Experimental results pertaining to far ir solidstate physics research are also briefly discussed.