Main Index: Main Index WG1 Index: WG1 Index

Wavelength Standards

Nave et al. (1996) have determined very high-accuracy Ritz wavelengths for 473 vacuum-ultraviolet Fe II lines (927$-$1995 Å). They recommend the new values as standards, the uncertainties lying mainly in the range $0.0001\; to\; 0.0003\; \AA .These\; new\; Fe$II standards, like several sets of high-accuracy wavelength determinations cited in our previous reports, depend on visible-region Ar II reference standards from Norlén (1973). New measurements of the Ar spectrum from a hollow-cathode discharge by Whaling et al. (1995) give Ar II wavenumbers systematically greater than Norlén's values by about 7 parts in $108$; adjustment of the new Fe II results of Nave et al. to the Ar II wavenumber values of Whaling et al. would decrease the Fe II wavelengths by $0.00006\; to\; 0.00014\; \AA .$

The paper by Whaling et al. gives high-precision Ritz wavelengths for hundreds of Ar II lines over the range 2225 Å to 5.33 m. The wavenumber uncertainties of most of the lines are less than $0.001\; cm$ -1$,but\; the\; values\; are\; uncorrected\; for\; any\; pressure\; shifts.\; A\; number\; of\; the\; lines\; in\; the\; range\; 3600\; \AA \; to\; 3.27$m are indicated as especially appropriate for use as reference standards because of their small pressure shifts. The systematic disagreement of 7 parts in $108$ between these Ar II wavenumber determinations and those by Norlén ``is roughly twice the uncertainty assigned to the wavenumbers in either experiment'' (Whaling et al. 1995). The indicated need for additional measurements is, according to Nave et al., being met by experiments underway in other laboratories.