  The semiconducting single-layer transition metal dichalcogenides have been
identified as ideal materials for accessing and manipulating spin- and
valley-quantum numbers due to a set of favorable optical selection rules in
these materials. Here, we apply time- and angle-resolved photoemission
spectroscopy to directly probe optically excited free carriers in the
electronic band structure of a high quality single layer of WS$_2$. We observe
that the optically generated free hole density in a single valley can be
increased by a factor of 2 using a circularly polarized optical excitation.
Moreover, we find that by varying the photon energy of the excitation we can
tune the free carrier density in a given spin-split state around the valence
band maximum of the material. The control of the photon energy and polarization
of the excitation thus permits us to selectively excite free electron-hole
pairs with a given spin and within a single valley.
