Double beam spectrophotometry in the far ultraviolet. 1: 1150 A to 3600 A

The instrument described is a double beam spectrophotometer with a wavelength range of 1150 A to 3600 A. Continua are supplied by three microwave-excited, rare-gas discharge lamps and a standard hydrogen lamp. A 1-m, normal-incidence, concave-grating monochromator is used. The beam splitting arrangement is an oscillating toroidal mirror, resonant at 11.3 c/s and arranged so that only a single reflection in each beam is required. The detectors are type EMI 6255S, each with a sodium salicylate phosphor. A ratio recording measuring system is used, with absorbance indicated on a 1.5-cycle logarithmic recorder. The performance data include vapor absorption spectra of several organic compounds and spectral reflectance measurements of magnesium-fluoride-coated aluminum surfaces. Stray light measurements are made by considering the constancy of absorbance vs pressure of water vapor at several wavelengths.