  The terahertz (THz) response of solvated electrons in liquid water is studied
in nonlinear ultrafast pump-probe experiments. Free electrons with
concentrations from c_e = 4 to 140 \times 10-6 moles/liter are generated by
high-field THz or near-infrared multiphoton excitation. The time-resolved
change of the dielectric function as mapped by broadband THz pulses exhibits
pronounced oscillations persisting up to 30 ps. Their frequency increases with
electron concentration from 0.2 to 1.5 THz. The oscillatory response is
assigned to impulsively excited coherent polarons involving coupled electron
and water shell motions with a frequency set by the local electric field.
