Spectra of the Th-Ar lamp was recorded on the NIST 2-m FTS. The FTS [Nave et al.(1997)] was fitted with a CaF2 beamsplitter, silver coated mirrors, and InSb detectors. The optimum alignment of the spectrometer depends slightly on wavelength. Hence two interferograms optimized for different wavelength regions were recorded - the first for wavelengths between 800 nm and 2000 nm and the second for wavelengths greater than 2000 nm. A resolution of 0.01 cm −1 was used for the short wavelength region and a resolution of 0.005 cm−1 for the long wavelength region.
The Th-Ar hollow cathode lamp and a calibrated tungsten ribbon lamp that served as a radiometric standard were mounted on a foreoptics system that was purged with dry, CO2-free air to avoid IR absorption. This foreoptics system incorporated a remotely actuated rotating mirror that was used to switch between the two light sources and a concave mirror that imaged either source to the entrance aperture of the FTS.
To obtain good signal-to-noise ratio many interferograms were co-added for each spectrum, corresponding to data acquisition times of up to 20 hours. For each hollow cathode spectrum a spectrum of the radiometric standard lamp was also recorded. This calibration spectrum was used to determine the instrumental response function for the foreoptics/FTS/detector combination. The response function was used to place the line intensities on a consistent scale.
This atlas is based on spectra recorded using a commercial Th/Ar lamp with a closed end cathode and sapphire window. The lamp was run at a current of 20 mA. No significant increase in signal-to-noise ratio was observed when the current was increased to 30 mA.
The interferograms were transformed with the program Xgremlin [Nave et al.(1997)]. Careful attention was paid to the phase correction of the spectra, particularly when the alignment of the instrument was optimized for wavelengths above 2 μm. Voigt profiles were fitted to all the lines in the resulting spectra to obtain wavelengths, intensities and line widths.