Laser Spectroscopy of the3d2Δ←2p2ΠTransition in Molecular LiHe

Observations are reported on the rotationally resolved $3d^2\Delta \leftarrow 2p^2\Pi$ electronic transition in the LiHe van der Waals molecule. These observations, which represent the first optical double-resonance measurements of the rovibrational structure of any alkalirare-gas molecule, have been analyzed, using standard techniques, to yield values for the $3d^2\Delta$ state well depth $D_e=430\mathrm{}\left(70\right)\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and equilibrium separation ${\mathcal{r}}_e=3.6\mathrm{}\left(1\right)\mathrm{}{a}_0$. The experimental scheme, which should allow spectroscopy of excited states in a number of excimer molecules, is also described.