Knowledge Bank

Observations of highly deuterated molecules in prestellar cores and protostars in recent years have aroused new interest in deuterium ion chemistry. The widely accepted interpretation of this phenomenon as due to extraordinarily high deuterium fractionation of H$\mathrm{3+}$ to H$2$D$+$, HD$\mathrm{2+}$ and D$\mathrm{3+}$} \textbf{591}, L41 (2003).} implies that deuterated variants of other fundamental molecular ions also play pivotal roles in the deuterium fractionation. Aiming at providing approximate rotational constants for millimeter wave spectroscopists to identify these deuterated species, we are continuing our project to study the laboratory spectra of the deuterated CH$\mathrm{2+}$ molecular ions. CH$\mathrm{2+}$ has been chosen as our first target ion because it is the intermediate between the abundant CH$+$ and yet to be observed but very important CH$\mathrm{3+}$ in the ``tree'' of interstellar chemistry. Its abundance is expected in diffuse clouds although our search for interstellar CH$\mathrm{2+}$ based on our infrared} \textbf{153}, 738 (1992).} and near-infrared} \textbf{121}, 11527 (2004).} laboratory spectra has not been successful. CH$\mathrm{2+}$ and its deuterated species are also of special interest for theoretical studies because of their unique intramolecular dynamics, i.e., the Renner-Teller interaction and quasi-linearity. Using He-dominated liquid-N$2$ cooled plasmas ($\sim$10Torr)containingasmallamount($\sim$0.1Torr)ofCD$_4$,wehavemeasuredthespectraofCD$_2^+$ in the near-infrared from 11,000 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ to 12,500 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ with our Ti:sapphire laser spectrometer that combines velocity modulation and phase modulation with heterodyne detection for near shot-noise-limited sensitivity. In this talk, we will review our analysis of the $A~(0,5,0)1\leftarrow X~(0,0,0)0$ band, presented last year}$ OSU International Symposium on Molecular Spectroscopy, MJ02 (2007).} and examine the $A~(0,5,0)0\leftarrow X~(0,0,0)1$ and $A~(0,4,0)2\leftarrow X~(0,0,0)1$ bands of CD$\mathrm{2+}$ based on the \textit{ab initio} calculations by Bunker and colleagues}. A scan for CHD$+$ is in preparation.