Argon I Standards for Emission Spectrometry

For over a decade, users of the FTS at the McMath Solar Telescope of the National Solar Observatory have had access to a simple but precise working wavenumber standard for absorption spectra in the form of a short sealed cell containing 10 or 20 Torr of N2O. Because of the high pressure, the observed wavenumbers are not those of the unperturbed molecule, but a direct calibration of the 0002 band at 4417 cm−1in this cell against the accurately known P(7) transition in CH4 yielded wavenumbers with a precision of the order of 0.000002 cm−1 and an accuracy thought to be at least 0.00001 cm−1 (Brault [1]). Some years later, this band was observed by Pollock, et al. [2] using laser heterodyne techniques, and when our cell values are corrected using the pressure shift which they observed, the absolute agreement is quite satisfactory: (NSO - Pollock, calculated) = 0.000035 ± .0000035 cm−1; note that the absolute uncertainty in Pollock, et al. is quoted as .000080 cm−1.