Femtosecond spectroscopy of the hydrated electron: Novel features in the infrared

Transient pump-probe spectroscopy of equilibrated solvated electrons in neat H₂O and D₂O is performed in an extended spectral range from the visible to the mid-infrared with polarization resolution and 100 fs pulses. After excitation in the red wing of the absorption band at 820 nm promoting the electrons from the 1s ground state to the first excited level 2p, a short-lived large absorption in the mid-infrared is observed suggesting strong coupling to hydroxylic groups - similar to observations in the photoionization of water - as well as probe transitions to higher lying bound and continuum states. The data support an initial adiabatic transition to a modified excited state p′ via a rearrangement of the solvent shell before internal conversion. The relaxation of p′ proceeds via a further intermediate assigned to a hot ground level s″ with time constants of 245 fs and 0.8 ps in H₂O as already established in the literature, and similar numbers for D₂O.