Submillimeter-wave spectroscopy of HCO+ in the excited vibrational states

The pure rotational transitions of HCO+ in excited vibrational states located below 5000 cm−1 over the ground state have been investigated with a high-sensitivity frequency/magnetic field double modulation submillimeter-wave spectrometer in the frequency range of 280-810 GHz. The ions were generated in an extended negative glow discharge through a gas mixture of a few millitorrs of H2 and CO and 12 mTorr of Ar buffer gas. Throughout the experiments, the cell was maintained at liquid nitrogen temperature. In the present study, we have determined accurate molecular constants for the excited vibrational states. Our analysis suggests that there may be a higher order Coriolis interaction between the (0 3 1) and (1 2 0) states. In previous investigations, the Stark effect caused by the electric field present in the discharge plasma was cited as a reason for non-observations of low-J lines in the (0220) and for the systematic shifts observed for low-J lines in the (0110), (0220), (0310), and (0420) states of HCO+ as well as DCO+. In the present investigation, some low-J lines in the (0220) and (0420) states have been observed in emission. Furthermore, J = 8-7, J = 9-8 lines in (031e1) were detected in emission. This finding indicates that missing low-J lines for the Δ sublevel obtained in the past is not due to the Stark effect but due to small population differences in those levels.