A simple grating-prism dispersive arrangement which provides improved resolution over a wide spectral range

Abstract

Abstract For many years designers of spectrometers have generally utilized variations of the Littrow-mounted prism monochromator as a means of providing adequate dispersion over the spectral range of interest. The recent availability of inexpensive replica diffraction gratings has brought forth instruments of increased resolving power which utilize coupled fore prism and grating monochromators. Where accurate broad spectral range scanning is desired, the fore prism-grating system requires precise and rather complex tracking linkages. Observations of several phenomena, which are in part predictable from the grating equation nλ = d(sin i ±sin r), as well as an examination of the requirements of infrared analysis of liquids and solids, has led to the construction of a simplified grating and prism dispersive arrangement which provides improved performance over a broad spectral range. The system incorporates a diffraction grating blazed in the first order at 3·75 μ. and mounted in the Littrow position with respect to a 60° rock salt prism. The grating is rotated over an angle such that its dispersion augments the prism dispersion, scanning the range, 4000-1400 cm−1. Since the grating is used only in the first order, higher order interference is easily eliminated by filters. Near 1400 cm−1, the grating is rotated to normal incidence and acts as a simple mirror as the range 1400-650 cm−1 is scanned. Over the total range, the predicted spectral slit widths vary from 1 to 5 cm−1 as compared to 2 to 18 cm−1 for the rock salt prism alone. The performance observed indicates the practicability of the arrangement. Present prism spectrometers might easily be converted to the new arrangement to realize the performance gain.