Premonochromator for stray light rejection in Raman spectroscopy

A new apparatus for attenuating the Rayleigh light is described. The main components are two concave gratings mounted in a special nondispersive way. By means of the first grating the entrance slit of the apparatus is focused as an intermediate spectrum upon a sharp-edged movable mirror. This mirror reflects all the spectrum except the Rayleigh light to the second grating, which collects the light to an image of the entrance slit. The result is a sharp cutting of the spectrum at any desired position. The rejection of the Rayleigh line has been measured to less than 10(-5) (optimally 10(-6)). Raman lines can be.measured about 10 cm(-1) from the exciting line. The instrument has been used as a preeliminating unit coupled to a conventional double monochromator. It is expected that a system consisting of the instrument and a good single monochromator can be used for Raman studies.