  Moir\'{e} twist angle underpins the interlayer interaction of excitons in
twisted van der Waals hetero- and homo-structures. The influence of twist angle
on the excitonic absorption of twisted bilayer tungsten diselenide (WSe$_{2}$)
has been investigated using electron energy-loss spectroscopy.
Atomic-resolution imaging by scanning transmission electron microscopy was used
to determine key structural parameters, including the nanoscale measurement of
the relative twist angle and stacking order. Detailed spectral analysis
revealed a pronounced blueshift in the high-energy excitonic peak C with
increasing twist angle, up to 200 meV when compared to the AA$^{\prime}$
stacking. The experimental findings have been discussed relative to
first-principle calculations of the dielectric response of the AA$^{\prime}$
stacked bilayer WSe$_{2}$ as compared to monolayer WSe$_{2}$ by employing the
\textit{GW} plus Bethe-Salpeter equation (BSE) approaches, resolving the origin
of higher energy spectral features from ensembles of excitonic transitions, and
thus any discrepancies between previous calculations. Furthermore, the
electronic structure of moir\'{e} supercells spanning twist angles of
$\sim$9.5-46.5$^{\circ}$ calculated by density functional theory (DFT) were
unfolded, showing an uplifting of the conduction band minimum near the $Q$
point and minimal change in the upper valence band concurrently. The combined
experiment/theory investigation provides valuable insight into the physical
origins of high-energy absorption resonances in twisted bilayers, which enables
to track the evolution of interlayer coupling from tuning of the exciton C
transitions by absorption spectroscopy.
