Submillimeter-wave spectroscopy of DCO+ in the excited vibrational states: Does the Stark effect cause anomalies in the (022) state?

The lowest two rotational transitions of (02(2)0) were not detected in previous investigations. This nonobservation was ascribed to the Stark broadening caused by the electric field in a hollow cathode discharge and an extended negative glow discharge. However, rotational lines of symmetric-top ions such as CH(3)CNH(+) and SD(3)(+) were observed in extended negative glow discharges with no such Stark effect. Also, no anomalies were observed for similar lines for HCN and HNC produced in an extended negative glow discharge. In the present investigation, we extended the measurements of DCO(+) up to 800 GHz. The DCO(+) ions were produced in an extended negative glow discharge in a gas mixture of D(2) and CO (a couple of millitorr each) in Ar buffer ( approximately 12 mTorr). The measurements were made mostly at liquid nitrogen temperature. Our observations confirmed that the lowest rotational lines in (02(2)0) within our frequency coverage, J=4-3, were too weak to be detected. However, a most notable result obtained in the present investigation is that the J=5-4 and J=6-5 lines of (02(2)0) and the J=5-4 line of (04(2)0) have been detected in induced emission. This observation implies that the previous nonobservation of low-J lines in (02(2)0) may not be due to the Stark effect. The l-type splitting in (03(3)0) has been observed for the J=9-8 transition and higher. However, the splittings for the J=7-6 and J=8-7 lines that are expected to be large enough have not been resolved. The reason for this "narrowing" has been unexplained at the present stage. The population inversion suggests that, initially, DCO(+) is formed predominantly in stretching vibrational states, and, subsequently, the energy transfer to bending vibrational states takes place through collisional relaxation processes.