The triatomic ion HCO+ is one of the most abundant molecular ions in dense interstellar clouds. It is a key intermediary in the production of carbon monoxide, CO, which is in turn the most abundant constituent of these regions after molecular hydrogen (H2). The abundance ratio of HCO+ to its deuterated counterpart, DCO+, can provide a measure of the electron density in molecular clouds. HCO+ is a linear, closed-shell molecule, so that its pure rotational spectrum consists of a series of harmonically related lines, with the lowest transition in the 3 mm wavelength band.
In 1970, L. Snyder and D. Buhl reported the detection of an unidentified emission line in the spectra of several interstellar molecular clouds and referred to the unknown carrier as “X-ogen.” Laboratory measurements subsequently confirmed that X-ogen was HCO+, in agreement with the theoretical predictions of ion-molecule chemistry by W. Klemperer and E. Herbst that this ion should be...