Knowledge Bank

Interferometeric measurements of the wavelengths of 36 argon lines in the region between 1.0 and 1.7 $\mu$ have led to the provisional adoption, by the International Astronomical Union, of all the 2s and 3d levels for the calculation of standards. A total of 106 permitted combinations of these sixteen levels with the ten 2p levels may be calculated, of which at least ninety percent have been observed, and of which more than half are intense enough to be considered useful as Wavelength standards. These combination are distributed between 8875 and 32297 A. Recent revisions of the measurements have been made using Fabry-Perot interferometers with plate separations of 45 and 63 mm. Argon is regarded as especially satisfactory for tins region because of its essentially monoisotopic character, ease of excitation, and number and favorable distribution of the lines. Other atomic emitters will be discussed, including mercury 198, cadmium, and the other noble gases. Interferometric measurements of wavelengths of lines in these spectra will be presented to the extent available. Proposed atomic sources for the region between 2.0 and 3.5 $\mu$ will be considered. Advantages of using lines occurring in this region particularly the requirement of using relatively low grating orders in intercomparisons with infrared regions of greater wavelength, will be discussed.