Highly Precise Wavelengths in the Infrared II HCN, N_2O, and CO†

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Abstract
A discussion is given of the effect of mechanical errors in a grating drive on the accuracy with which line frequencies can be measured. It is shown that it is highly improbable that a mechanical device used to rotate a diffraction grating can be constructed with sufficient precision so that the line-frequency error will be determined by the optical information observable in the near infrared spectrum. By making use of the echelle spectrograph 67 lines of the 001–000 absorption band of HCN have been measured. Almost 400 lines from five N2O absorption bands arising from the ground state of the molecule have been measured. In addition, 11 lines from the 1–0 fundamental band of CO have been measured in order to determine ν0 for this band. In all cases only band lines have been measured which appear to be free of blends. The molecular constants of the above bands have been determined by making use of the combination relationships applying least-squares methods to the data. The line frequencies have then been calculated to four decimal places and presented in tables. The tables contain over 600 line frequencies. We believe the calculated line frequencies (lines free of blends) have an absolute accuracy of about 1 part in 5×106 and a relative accuracy referring to lines within a given band somewhat greater.

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