The Millimeter/sub-millimeter Spectrum of the Lis Radical in Its P I Ground State

The pure rotational spectrum of LiS …X 2 P i † has been recorded in the v ˆ 0 and v ˆ 1 states in the region 130±540 GHz using millimeter/sub-millimeter direct absorption methods. This radical was synthesized from lithium vapor and CS 2 in a DC discharge. Transitions arising from the two orbit components were measured, both which exhibited lambda-doubling and, for the X ˆ 1=2 ladder, hyper®ne interactions. Rotational, lambda-doubling, and hyper®ne parameters have subsequently been determined. These data prove unambiguously that the ground state of LiS is 2 P i and suggest that the A 2 R ‡ state lies $ 4300 cm À1 higher in energy. The bonding in this radical appears to be predominantly ionic.