Title: Search for Interstellar LiH in the Milky Way,
Abstract: We report the results of a sensitive search for the 443.952902 GHz $J=1-0$
transition of the LiH molecule toward two interstellar clouds in the Milky Way,
W49N and Sgr B2 (Main), that has been carried out using the Atacama Pathfinder
Experiment (APEX) telescope. The results obtained toward W49N place an upper
limit of $1.9 \times 10^{-11}\, (3\sigma)$ on the LiH abundance, $N({\rm
LiH})/N({\rm H}_2)$, in a foreground, diffuse molecular cloud along the
sight-line to W49N, corresponding to 0.5% of the solar system lithium
abundance. Those obtained toward Sgr B2 (Main) place an abundance limit $N({\rm
LiH})/N({\rm H}_2) < 3.6 \times 10^{-13} \,(3\sigma)$ in the dense gas within
the Sgr B2 cloud itself. These limits are considerably smaller that those
implied by the tentative detection of LiH reported previously for the $z=0.685$
absorber toward B0218+357.