  Interfacial charge separation and recombination at heterojunctions of
monolayer transition metal dichalcogenides (TMDCs) are of interest to two
dimensional optoelectronic technologies. These processes can involve large
changes in parallel momentum vector due to the confinement of electrons and
holes to the K-valleys in each layer. Since these high-momentum valleys are
usually not aligned across the interface of two TMDC monolayers, how parallel
momentum is conserved in the charge separation or recombination process becomes
a key question. Here we probe this question using the model system of a type-II
heterojunction formed by MoS2 and WSe2 monolayers and the experimental
technical of femtosecond pump-probe spectroscopy. Upon photo-excitation
specifically of WSe2 at the heterojunction, we observe ultrafast (<40 fs)
electron transfer from WSe2 to MoS2, independent of the angular alignment and,
thus, momentum mismatch between the two TMDCs. The resulting interlayer charge
transfer exciton decays via nonradiatively recombination, with rates varying by
up to three-orders of magnitude from sample to sample, but with no correlation
with inter-layer angular alignment. We suggest that the initial interfacial
charge separation and the subsequent interfacial charge recombination processes
circumvent momentum mismatch via excess electronic energy and via
defect-mediated recombination, respectively.
