Reference lines for dye-laser wave-number calibration in the optogalvanic spectra of uranium and thorium

The optogalvanic spectra of uranium and thorium observed in commercial hollow-cathode lamps provide convenient lines for dye-laser wave-number calibration. We describe a simple procedure by which a single-frequency cw dye laser can be set on such lines with an accuracy of a few parts in 108. We report wave numbers for 16 U and 16 Th lines distributed over the wavelength range 5750–6920 Å. The estimated uncertainty of the measurements is 0.0003 cm−1 for U and 0.0004 cm−1 for Th. The results are compared with previous U and Th measurements and are found to be in good agreement with recently reported emission wave numbers determined by Fourier spectroscopy.