The Spectrum of Thorium from 250 nm to 5500 nm: Ritz Wavelengths and Optimized Energy Levels
Stephen L. Redman, Gillian Nave, Craig J. Sansonetti
We have made precise new observations of the spectrum emitted by a thorium-argon hollow cathode lamp in the region 350 nm to 1175 nm using a high-resolution Fourier transform spectrometer. Our measurements are combined with results from seven previously published thorium line lists (Giacchetti et al. 1974; Zalubas & Corliss 1974; Zalubas 1976; Palmer & Engleman 1983; Engleman et al. 2003; Lovis & Pepe 2007; Kerber et al. 2008) to re-optimize the energy levels of neutral, singly-, and doubly-ionized thorium (Th I, Th II, and Th III). Using the optimized level values, we calculate accurate Ritz wavelengths for 19646 thorium lines between 250 nm and 5 500 nm (40000 cm−1 to 1800 cm−1). We have also found 102 new thorium energy levels. A systematic analysis of previous measurements in light of our new results allows us to identify and propose corrections for systematic errors in Palmer & Engleman (1983) and typographical errors and incorrect classifications in Kerber et al. (2008). We also found a large scatter in the thorium line list of Lovis & Pepe (2007). We anticipate that our Ritz wavelengths will lead to improved measurement accuracy for current and future spectrographs that make use of thorium-argon or thorium-neon lamps as calibration standards.