  We investigate the dispersion of the charge carrier plasmon in the three
prototypical charge-density wave bearing transition-metal dichalcogenides
2H-TaSe2, 2H-TaS2 and 2H-NbSe2 employing electron energy-loss spectroscopy. For
all three compounds the plasmon dispersion is found to be negative for small
momentum transfers. This is in contrast to the generic behavior observed in
simple metals as well as the related system 2H-NbS2, which does not exhibit
charge order. We present a semiclassical Ginzburg-Landau model which accounts
for these observations, and argue that the vicinity to a charge ordered state
is thus reflected in the properties of the collective excitations.
