  The electron valley and spin degree of freedom in monolayer transition-metal
dichalcogenides can be manipulated in optical and transport measurements
performed in magnetic fields. The key parameter for determining the Zeeman
splitting, namely the separate contribution of the electron and hole g-factor,
is inaccessible in most measurements. Here we present an original method that
gives access to the respective contribution of the conduction and valence band
to the measured Zeeman splitting. It exploits the optical selection rules of
exciton complexes, in particular the ones involving inter-valley phonons,
avoiding strong renormalization effects that compromise single particle
g-factor determination in transport experiments. These studies yield a direct
determination of single band g factors. We measure gc1= 0.86, gc2=3.84 for the
bottom (top) conduction bands and gv=6.1 for the valence band of monolayer
WSe2. These measurements are helpful for quantitative interpretation of optical
and transport measurements performed in magnetic fields. In addition the
measured g-factors are valuable input parameters for optimizing band structure
calculations of these 2D materials.
