Knowledge Bank

A photoelectric recording Raman spectrograph has been constructed and is now in use in this laboratory. The main component of the instrument is an autocollimating monochromator using a single five-inch off-axis parabolic mirror. The dispersing elements consist of two large glass prisms, sixty and thirty degrees respectively, in a Littrow arrangement so that they are equivalent to three sixty-degree prisms. The spectrum is scanned by rotating the prisms as a unit. A variety of conventional Raman sources have been used. Before entering the monochromator, the scattered light is chopped by a shutter at a frequency of thirty cycles. The photomultiplier at the exit slit produces an alternating-current signal which is amplified and then rectified by synchronous switches operating from the same shaft as the chopping disk. The resulting direct-current signal is supplied to a Leeds and Northrup recording potentiometer. A second photomultiplier may be used to monitor the mercury arcs and regulate the slidewire current of the recorder, thus making the recorded spectrum substantially independent of fluctuations in the exciting lamps.