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2.5.5 Wavelength standards

Laser standards

Wavelength values of optical radiations may be expressed either as the values in standard air (see below) or in vacuum.

One of the principal means of realization of the metre, according to the 1983 definition, is through the wavelength values of stabilized laser radiations. These vacuum wavelength values are determined by the relation λ = c/f, from the fixed value for the speed of light and the measured frequencies of the radiations. Eight such laser standards are recommended by the CIPM. They form the most precise and stable references for interferometric measurement and spectroscopic investigations. They are emitted by single-mode lasers stabilized to transitions of absorbing atoms or molecules contained in cells within or external to the lasers (or in an atomic beam). Their frequency and wavelength values, and their estimated relative standard uncertainties (equivalent to a standard deviation), are given in the table. These values apply only when the associated conditions and accepted good practice are followed.

Source conditions

Discharge tube sources

The former primary wavelength standard and the twelve former secondary standards have been retained as recommended standards for interferometric measurement, with unchanged wavelength values and uncertainties. These values were specified by the International Committee for Weights and Measures in 1963, and are also recognized by the International Astronomical Union for use in spectroscopy (Trans. IAU, vol. XII A, 1965). The table shows their vacuum wavelength values in nanometres.

Source conditions

Krypton 86. The wavelength of the 605 nm radiation, when emitted by a lamp conforming to the specification below, has an estimated uncertainty (99% confidence) of ±4 parts in 10⁹. The other radiations, under similar conditions, have uncertainties of ± 2 parts in 10⁸.
   The source is a hot-cathode discharge lamp containing krypton-86 (purity 99%) in sufficient quantity to assure the presence of solid krypton at 64 K, the lamp having a capillary portion with the dimensions: internal diameter 2–4 mm, wall thickness 1 mm. The conditions of operation are:
   (i) The capillary is observed end-on from the anode side of the lamp;
   (ii) the lower part of the lamp, including the capillary, is immersed in a refrigerant bath maintained within 1K of the triple point of nitrogen;
   (iii) the current density in the capillary is 3 ± 1 mA mm⁻².

Mercury-198. The uncertainties of the wavelengths are ± 5 parts in 10⁸ when emitted by a high-frequency electrodeless discharge lamp, operated at moderate power with the radiation observed through the side of the capillary. The lamp should be maintained at a temperature below 10 °C and contain mercury-198 (purity 98%) with argon as carrier gas at a pressure between 65 and 133 Nm⁻². The internal diameter of the capillary should be about 5 mm, with the volume of the lamp preferably >20 cm³.

Cadmium-114. The standard wavelengths have an estimated uncertainty of ±7 parts in 10⁸ when emitted by an electrodeless discharge lamp source, maintained at a temperature such that the green line is not reversed and containing cadmium-114 (purity 95%) with argon as carrier gas (pressure about 150 N m⁻² at ambient temperatures). The radiations should be observed through the side of the capillary part of the tube, having an internal diameter of about 5 mm.

W.R.C.Rowley

Practical wavelength standards for calibration of spectrophotometers

Ultraviolet and visible region. There are several extensive tables of emission lines intended for spectroscopic use but these are not very suitable for practical users of general purpose spectrophotometers. Many of the lines cited cannot be detected or resolved because the dispersion and light grasp of such instruments are not high enough, or because the pressure or electron temperatures in commercial discharge lamps may not allow a line to be separated from neighbours or the continuum background. Some elements, like neon, iron or iodine, have too many lines for easy identification.
   For this reason a practical table of useful atomic emission lines is given below. These lines can usually be found and recognized using atomic emission lamps of the kind available commercially for wavelength calibration. Certain doublet or triplet emissions, such as the 365 nm mercury emission group, are included to aid recognition of other lines: these doublets or triplets should only be used for calibration when they are fully resolved by the instrument. Often included in lists of lines recommended by textbooks are strong resonance lines such as the 253.65 nm mercury line. In most mercury lamps this particular line is broadened to the point of being nearly inverted, so that it seems like two broadened lines at perhaps 251 and 256 nm, for example, and can be mistaken for the 253.65 nm and much weaker 257.63 nm lines. However, if the mercury lamp is a genuinely low-pressure one for UV use, this line at 253.65 nm is satisfactory.

Useful emission lines for wavelength calibration (air values)

Infrared region. The mid-infrared spectral region is extensively used for analytical and structural chemistry, and a number of atlases and compilations of molecular rotational and vibrational absorption peaks have been published. However, for calibration of mid-infrared spectrophotometers by users who are not spectroscopists, the most practical aid is published by the International Union of Pure and Applied Chemistry, as it gives spectral profiles and tabulated data separately for low, medium and high resolution spectrophotometers. This is needed as many of these absorption peaks are not as sharp and separate as typical atomic emission lines, and they tend to be clustered so that the number of usable peaks depends markedly on the instrumental resolution.

Reference

A. R. H. Cole (1977) IUPAC: Tables of Wavenumbers for the Calibration of Infrared Spectrophotometers, 2nd edn, Pergamon Press, Oxford.

F.J.J.Clarke

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