Laboratory Microwave Spectrum of HCO+

The $J=0\mathrm{}\to 1$ rotational transition of the HC${\mathrm{O}}^{+}$ ion has been observed with the same apparatus used earlier for detection of C${\mathrm{O}}^{+}$. A discharge in various mixtures of hydrogen and carbon monoxide cooled to near liquid-nitrogen temperature was employed in a slow-flow system. The frequency we have obtained is 89 188.545±0.020 MHz, but because of the ion drift velocity in the dc discharge this exceeds, by an unknown but small amount, the true rest frequency. This observation confirms the long-standing contention that the radio astronomical $X$-ogen line is in fact due to HC${\mathrm{O}}^{+}$.