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15.   Spectral Wavelength Ranges, Dispersion of Air

The ranges of most interest for optical atomic spectroscopy are:

~ 2 - 20 m       mid-infrared (ir)
700 - 2000 nm   near ir
400 - 700 nm     visible
200 - 400 nm     near ultraviolet (uv)
100 - 200 nm     vacuum uv or far uv
  10 - 100 nm     extreme uv (euv or xuv)
  < 10 nm       soft x-ray, x-ray

The above correspondence of names to ranges should not be taken as exact; the variation as to the extent of some of the named ranges found in the literature is considerable.

Wavelengths in standard air are often tabulated for the region longer than 200 nm. These wavelengths can be related to energy-level differences by conversion to the corresponding (vacuum) wavenumbers or frequencies [20].

16.   Wavelength (Frequency) Standards

In 2001 the Comit International des Poids et Mesures recommended values for optical frequency standards from stabilized lasers using various absorbing atoms, atomic ions, and molecules [21]. These frequencies range from 29 054 057 446 579 Hz (10.318 436 884 460 m, relative standard uncertainty 1.4  10-13 for a transition in OsO4 to 1 267 402 452 889.92 kHz (236.540 853 549 75 nm; relative standard uncertainty 3.6  10-13) for a transition in the 115In+ ion [21].

Extensive tables of wavenumbers for molecular transitions in the mid-ir range 2.3 to 20.5 m are included in a calibration atlas published in 1991 [22]. Wavenumbers of Ar I [23] and Ar II [24] emission lines having uncertainties as small as 0.0003 cm-1 are included in tables for these spectra covering a broad range from 222 nm to 5.865 m. Measurements of U and Th lines (575 nm to 692 nm) suitable for wavenumber calibration at uncertainty levels of 0.0003 cm-1 or 0.0004$ cm-1 were reported in Ref. [25]. Comprehensive tables of lines for U [26], Th [27], and I2 [28] are useful for calibration at uncertainty levels of 0.002 cm-1 to 0.003 cm-1, the atlas of the Th spectrum extending down to 278 nm.

A 1974 compilation gives reference wavelengths for some 5400 lines of 38 elements covering the range 1.5 nm to 2.5 m, with most uncertainties between 10-5 and 2  10-4 nm [29]. The wavelengths for some 1100 Fe lines selected from the Fe/Ne hollow-cathode spectrum have been recommended for reference standards over the range 183 nm to 4.2 m, with wavenumber uncertainties 0.001 cm-1 to 0.002 cm-1 [30]. Wavelengths for about 3000 vuv and uv lines (110 nm to 400 nm) from a Pt/Ne hollow-cathode lamp have been determined with uncertainties of 0.0002 nm or less [31]. More recent high-accuracy measurements of ultraviolet lines of Fe I, Ge I, Kr II, and Pt III, include some wavelengths with uncertainties smaller than 10-5 nm [32]. The wavelengths tabulated for the Kr and Pt lines in [32] extend from 171 nm to 315 nm, and the accuracies of earlier measurements of a number of spectra useful for wavelength calibration are discussed.

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