  Several theoretical predictions have claimed that the neutral exciton of
TMDCs splits into a transversal and longitudinal exciton branch, with the
longitudinal one, which is the upper branch, exhibiting an extraordinary strong
dispersion in the meV range within the light cone. Historically, this was
linked for semiconductor quantum wells to strong far-field optical dipole
coupling, or strong electronic long-range exchange interactions, describing two
sides of the same coin. Recently, experiments utilizing Fourier-space
spectroscopy have shown that the exciton (exciton-polariton) dispersion can
indeed be measured for high-quality hexagonal-BN-encapsulated WSe2 monolayer
samples and can confirm the energy scale. Here, the exciton fine-structure's
pseudo-spin and the valley polarization are investigated as a function of the
centre-of-mass-momentum and excitation-laser detuning. For quasi-resonant
excitation, a strong dispersion featuring a pronounced momentum-dependent
helicity is observed. By increasing the excitation energy step-wise towards and
then above the electronic band gap, the dispersion and the helicity
systematically decrease due to contributions of incoherent excitons and
emission from plasma. The decline of the helicity with centre-of-mass momentum
can be phenomenologically modelled by the Maialle-Silva-Sham mechanism using
the exciton splitting as the source of an effective magnetic field.
