  The coupled spin and valley degrees of freedom in transition metal
dichalcogenides (TMDs) are considered a promising platform for information
processing. Here, we use a TMD heterostructure
${\text{MoS}_{2}-\text{MoSe}_{2}}$ to study optical pumping of spin/valley
polarized carriers across the interface and to elucidate the mechanisms
governing their subsequent relaxation. By applying time-resolved Kerr and
reflectivity spectroscopies, we find that the photoexcited carriers conserve
their spin for both tunneling directions across the interface. Following this,
we measure dramatically different spin/valley depolarization rates for
electrons and holes, $\sim 30\,{\text{ns}}^{-1}$ and $< 1\,{\text{ns}}^{-1}$,
respectively and show that this difference relates to the disparity in the
spin-orbit splitting in conduction and valence bands of TMDs. Our work provides
insights into the spin/valley dynamics of free carriers unaffected by complex
excitonic processes and establishes TMD heterostructures as generators of spin
currents in spin/valleytronic devices.
