Interferometrically Measured Thorium Lines between 2747 and 4572 Å

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Abstract
Interferometrically measured wavelengths are given for 278 thorium lines in the range 2747–4572 Å. The light source is a liquid-nitrogen-cooled hollow-cathode lamp with 1 torr of argon as the carrier gas. Each line is measured from interferograms taken with a Fabry–Perot interferometer having 20-, 40-, and 50-mm spacers, and the results are averaged after making the correction for dispersion of optical path. The combination principle is used as a test of the accuracy of the measurements. The average wavelength error is about one part in 3 to 6×107 and the average root-mean-square deviation is 1.6×10−4 Å between the results for the different spacers. Our results are compared with those of different investigators and indicate a relative shift of +4×10−4 Å when the electrodeless discharge tube is compared with the hollow-cathode lamp. The origin of this shift is discussed and attributed partly to differences in the standards used to calibrate the interferometer and partly to differences in the light sources.

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