Improvement of the Signal‐to‐Noise Ratio of Photomultipliers for Very Weak Signals

For the photoelectric detection of Raman spectra excited with a He–Ne laser, the dark current of an EMI 9558 QA photomultiplier is reduced by magnetic defocusing of the electrons originating from the inefficient part of the photocathode. A lightpipe cone concentrates the light signal to a spot of 2.7 mm diameter and enhances the quantum efficiency of the photocathode by multiple reflection. A pulse counting system with a deadtime of less than 100 nsec is described and compared with dc detection. It is now possible to detect signals with a power of less than 10−17 W or less than 35 photons/sec at 650 nm with a time constant of 1 sec. The improvement in signal‐to‐noise ratio is demonstrated with the Raman spectrum of polycrystalline azobenzene.