The accuracy of predicted radioastronomical frequencies and the spectrum of hydroxyl

Abstract

IF radioastronomers are to use predicted frequencies to search for new interstellar species, they require some fairly precise indication of the accuracy of the theoretical calculations. Calculations of rotational constants are not very accurate by the standards of radiofrequency spectroscopy, but computations of Λ-doublet splittings have seemed remarkably accurate^1–3. However, these calculations were presented without error limits and as it is not possible to produce these from pure theory we have used the hydroxyl radical to probe the limits of accuracy. The experimental frequencies are known from observations^4,5 of the interstellar maser action in OH to a high precision, and it is a particularly suitable case for computation. We show here that the prediction for the lowest rotational level is accurate to 1 MHz; an accuracy higher than conventional terrestrial experimental spectroscopy.