Ultrafast Librational Dynamics of the Hydrated Electron

Photon echo and resonant transient grating experiments on the equilibrated aqueous electron are reported, using 15 fs excitation pulses. Both experiments exhibit clear signatures of wave packet dynamics, attributed to an underdamped solvent motion of about $850\mathrm{cm}{}^{-1\mathrm{}}$. Experiments in $D_2\mathrm{O}$ expose the librational nature of this mode. The overall agreement between experiment and quantum-molecular dynamics simulations is good, except for the initial oscillatory dynamics, which have not been predicted.