  Rich valleytronics and diverse defect-induced or interlayer pre-bandgap
excitonics have been extensively studied in transition metal dichalcogenides
(TMDCs), a system with fascinating optical physics. However, more intense
high-energy absorption peaks (~ 3 eV) above the bandgaps used to be long
ignored and their underlying physical origin remains to be unveiled. Here, we
employ momentum resolved electron energy loss spectroscopy to measure the
dispersive behaviors of the valley excitons and intense higher-energy peaks at
finite momenta. Combined with accurate Bethe Salpeter equation calculations,
non-band-nesting transitions at Q valley and at midpoint of KM are found to be
responsible for the high-energy broad absorption peaks in tungsten
dichalcogenides and present spin polarizations similar to A excitons, in
contrast with the band-nesting mechanism in molybdenum dichalcogenides. Our
experiment-theory joint research will offer insights into the physical origins
and manipulation of the intense high-energy excitons in TMDCs-based
optoelectronic devices.
