Electronic relaxation dynamics of water cluster anions

The electronic relaxation dynamics of water cluster anions, (H ₂O)_n^-, have been studied with time-resolved photoelectron imaging. In this investigation, the excess electron was excited through the p←s transition with an ultrafast laser pulse, with subsequent electronic evolution monitored by photodetachment. All excited-state lifetimes exhibit a significant isotope effect (τ_D2O/τ_H2O ∼2). Additionally, marked dynamical differences are found for two classes of water cluster anions, isomers I and II, previously assigned as clusters with internally solvated and surface-bound electrons, respectively. Isomer I clusters with n ≥ 25 decay exclusively by internal conversion, with relaxation times that extrapolate linearly with 1/n toward an internal conversion lifetime of 50 fs in bulk water. Smaller isomer I clusters (13 ≤ n ≤ 25) decay through a combination of excited-state autodetachment and internal conversion. The relaxation of isomer II clusters shows no significant size dependence over the range of n = 60-100, with autodetachment an important decay channel following excitation of these clusters. Photoelectron angular distributions (PADs) were measured for isomer I and isomer II clusters. The large differences in dynamical trends, relaxation mechanisms, and PADs between large isomer I and isomer II clusters are consistent with their assignment to very different electron binding motifs.