  Monolayers of transition metal dichalcogenides are promising materials for
valleytronic applications, since they possess two individually addressable
excitonic transitions at the non-equivalent $K$ and $K'$ points with different
spins, selectively excitable with light of opposite circular polarization.
Here, it is of crucial importance to understand the elementary processes
determining the lifetime of these optically injected valley excitons. In this
study, we perform microscopic calculations based on a Heisenberg equation of
motion formalism to investigate the efficiency of the intervalley coupling in
the presence (W based TMDCs) and absence (Mo based TMDCs) of energetically low
lying momentum-dark exciton states. While we predict a valley exciton lifetime
on the order of some hundreds of fs in the absence of low lying momentum-dark
states we demonstrate a strong quenching of the valley lifetime in the presence
of such states.
