Rapid Precision Wave Number Measurements from Fabry-Perot Interferograms*

In order to obtain the more accurate wave numbers needed for studies of the very rich heavy element spectra, a new measuring and calculating procedure has been developed. A modern sharp line source, such as a hollow cathode discharge or an electrodeless metal-halide lamp, is used to illuminate a vacuum Fabry-Perot interferometer (5, 10, or 20 mm spacer). The interferometer is crossed with a spectrograph resolving the free spectral range of the interferometer. The resulting interferogram is measured with a two-coordinate photoelectric comparator. The measurements are punched on IBM cards, and vacuum wave numbers are directly calculated with a high-speed digital computer. Only one standard line is needed and the index of refraction of air correction is used only to obtain air wavelengths. The phase change correction is obtained from measurements with two different spacers or from measurements on several standard lines. Only a few minutes reading time are needed for each line. This procedure is being used for a further study of the uranium spectrum with sources containing separated uranium isotopes. Over 8000 lines near 4100 A have been measured with a precision better than 0.005 cm−1.