Indirect rotational spectroscopy of HCO⁺

Spectroscopy of the ν₁ band of the astrophysically relevant ion HCO⁺ is performed with an optical parametric oscillator calibrated with an optical frequency comb. The sub-MHz accuracy of this technique was confirmed by performing a combination differences analysis with the acquired rovibrational data and comparing the results to known ground-state rotational transitions. A similar combination differences analysis was performed from the same data set to calculate the previously unobserved rotational spectrum of the ν₁ vibrationally excited state with precision sufficient for astronomical detection. Initial results of cavity-enhanced sub-Doppler spectroscopy are also presented and hold promise for further improving the accuracy and precision in the near future.