Gas-phase Laser as a Source of Light for an Optical Diffractometer

Abstract

THE commercial availability of gas-phase lasers giving a continuous output in the visible region of the spectrum solves a problem that has been troubling users of optical diffractometers for some time. It is sometimes necessary to study a region of a diffraction pattern which is so far removed from the centre that it subtends an angle which is large compared with that subtended by the detail to be resolved. If, for example, the distance between the region to be studied and the centre is 100 times the size of the detail to be resolved, it is clear that λ/100 is the maximum bandwidth that can be tolerated. The usual combination of mercury-vapour lamp and interference filter often gives too wide a band because of both the high-pressure broadening of the spectral lines and the problems of narrow-band filtering without undue reduction in effective source brightness. If the output of a helium–neon laser is focused on the pinhole of the diffractometer the necessary brightness is easily obtained with an extremely small bandwidth. Using a laser with an output of about 3 mW, photographic exposures are comparable with those for a 250-W compact-source mercury-vapour lamp in conjunction with a 70-Å bandwidth interference filter, even though the film is appreciably less sensitive to the laser wave-length (6328 Å) than to the usual mercury–yellow wave-length (5780 Å).