Hydrated electron dynamics: from clusters to bulk.

The electronic relaxation dynamics of size-selected (H2O)n-/(D2O)n[25 </= n </= 50] clusters have been studied with time-resolved photoelectron imaging. The excess electron (ec-) was excited through the ec-(p)<--ec-(s) transition with an ultrafast laser pulse, with subsequent evolution of the excited state monitored with photodetachment and photoelectron imaging. All clusters exhibited p-state population decay with concomitant s-state repopulation (internal conversion) on time scales ranging from 180 to 130 femtoseconds for (H2O)n- and 400 to 225 femtoseconds for (D2O)n-; the lifetimes decrease with increasing cluster sizes. Our results support the "nonadiabatic relaxation" mechanism for the bulk hydrated electron (eaq-), which invokes a 50-femtosecond eaq-(p)-->eaq-(s(dagger)) internal conversion lifetime.