Improved rest frequencies of HCO+ at 1 THz

The ground-state rotational spectrum of HCO+, produced in a negative-glow discharge cell, has been recorded up to 892 GHz. The sources of systematic error have been carefully accounted for, thus obtaining accurate rotational and centrifugal distortion constants. The present measurements have allowed us to determine a value of the sextic distortion constant, H-J which, unlike the recent determination of Lattanzi et al., is consistent with H-J the values determined for DCO+ and for the isoelectronic molecule HCN. The new set of spectroscopic constants allows us to predict the rotational spectrum of the formyl ion with an accuracy of 1 part in 10(8) near 1 THz. Such an accuracy is important for kinematic studies of dense cores in molecular clouds and for future far-infrared observations.