Knowledge Bank

Hirota and Endo reported observations of the rotational transitions of HCO$+$ and DCO$+$ in excited vibrational states.} The lowest three rotational transitions of $(0220)$ could not be detected in their experiment. They ascribed this non-observation to the Stark broadening caused by the electric field in a hollow cathode discharge. More recently Dore and coworkers} also did not see the $\ell =2$ lines in an extended negative glow discharge and interpreted this result in terms of the Stark effect. However, symmetric top ions such as CH$3$CNH$+$ } and SD$\mathrm{3+}$ } were observed with no difficulty. 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 up to 800 GHz. The HCO$+$ and DCO$+$ ions were produced in an extended negative glow discharge in a gas mixture of H$2$ or D$2$ and CO ( a couple of mTorr each ) in Ar buffer ( 12 mTorr ). The measurements were done mostly at liquid nitrogen temperature. For HCO$+$, the rotational lines in the excited vibrational levels up to (040) and (002) which are located at about 4300 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ above the ground state have been measured. The measurements for DCO$+$ are not as extensive so far. Our observations confirmed that $(0220)$ lines and, in general, larger $\ell$ lines in the (030) and (040) states were weaker than expected. However, a most notable result obtained in the present investigation is that some low-$J$ lines of $(0220)$ have been detected as induced emission for both HCO$+$ and DCO$+$. This observation clearly leads to a conclusion that the previous non-observation of low-$J$ lines in $(0220)$ is not due to the Stark effect, but due to specificity of the reaction mechanism and subsequent collisional relaxation processes. %More details will be presented in the talk.