  We predict a fast relaxation of photo-excited carriers in monolayer
transition metal dichalcogenides (TMDCs), which is mediated by the emission of
longitudinal optical (LO) phonons. By evaluating Born effective charges for
${\rm MoS_2, MoSe_2,WS_2}$, and ${\rm WSe_2}$, we find that, due to the polar
coupling of electrons with LO phonons, the relaxation times for hot electrons
and holes are of the order of a few ps.
