Knowledge Bank

The first high resolution rotational spectrum of a weakly bound cluster ion is reported. The millimeter and submillimeter wave spectra of $Ar.H3+,Ar.D3+,Ar.H2D+$ and $Ar.DH2+$ have been observed in a magnetically confined, liquid nitrogen cooled, negative glow discharge and a molecular structure has been derived from their analysis. $Ar.H3+$ is a planar molecule with a $C2v$ symmetry. The Ar atom lies on a symmetry axis of the $H3+$ quasi-equilateral triangle, 2.38 {\AA} from the $H3+$ the triangle. From the centrifugal distortion coefficient $\Delta J$, the dissociations energy has been estimated to about 7 - 8 k cal/mole. Moreover, splitting of some of the lines is strong evidence for tunneling motion.