  Understanding the retention and recycling of hydrogen isotopes in liquid
metal plasma-facing materials such as liquid Li, Sn, and Li-Sn are of
fundamental importance in designing magnetically confined fusion reactors. We
perform first-principles molecules dynamics simulations of liquid Li-Sn slab
with inserted D atoms to provide microscopic insights into the interactions of
D with Li-Sn liquid metal. We observe evaporation of D$_2$ and LiD molecules
out of the Li-Sn slabs. With detailed analysis, we unveil a cooperative process
of forming D$_2$ molecules in liquid Li-Sn, where Li atoms act as catalytic
centers to trap a D atom before another D comes nearby to form a molecule, and
the surplus charges are transferred from D$_2$ to nearby Sn atoms. Furthermore,
we predict a temperature window in which D$_2$ molecules can escape to vacuum,
while LiD molecules cannot. The above findings deepen our understanding of
interactions between hydrogen isotopes and Li-Sn liquid metal.
