Wavelength Standards

Accurate wavelengths have been determined for about 2500 spectral lines of Th and Ar in the range 691 nm to 5804 nm. In general the density of lines drops significantly towards longer wavelengths. The wavenumbers and wavelengths are strictly valid only for a Th/Ar hollow cathode similar to the one used, operated at a current of 20 mA. However, wavelengths of Th I, Th II and Ar II lines show little sensitivity to operating conditions and our results can thus be reliably applied to lamps run at lower currents. Low-excitation lines of Ar I are also reliable, but high-excitation lines (upper level higher than 115 000 cm⁻¹) may show large wavelength shifts when the pressure or current in the lamp is different from ours; these lines have an asterisk in column 13 of the tabulated data in the atlas.

A description of the tabulated data is given in table 1. The total wavenumber uncertainty in column 2 and corresponding wavelength uncertainty in column 12 have been obtained by combining the statistical uncertainty in quadrature with the estimated calibration uncertainty of 2×10⁻⁸ σ_c.

The integrated intensity of each line is given in column 5. It has been adjusted for the response of the spectrometer using the calibration spectra taken with the tungsten standard lamp. Since intensities are highly dependent on the running conditions of the lamp they should be used with caution when comparing our results to those obtained under other operating conditions. The identification of the line is in columns 6-10. Identifications of Ar lines have been taken from [Whaling et al. (1995)] and [Whaling et al. (2002)], and Th lines from [Palmer, Engleman (1983)] and [Engleman, Hinkle, Wallace (2003)]. We have identified some additional lines of Ar using spectra of electrodeless discharge lamps or hollow cathode lamps taken from archival spectra [National Solar Observatory Digital Library (2007)]. In addition, 5 impurity lines of Ca I have been observed.