Electronic relaxation dynamics in large anionic water clusters: (H2O)n(-) and (D2O)n(-) (n = 25-200).

Electronic relaxation dynamics subsequent to s --> p excitation of the excess electron in large anionic water clusters, (H(2)O)(n)(-) and (D(2)O)(n)(-) with 25 < or = n < or = 200, were investigated using time-resolved photoelectron imaging. Experimental improvements have enabled considerably larger clusters to be probed than in previous work, and the temporal resolution of the instrument has been improved. New trends are seen in the size-dependent p-state lifetimes for clusters with n > or = 70, suggesting a significant change in the electron-water interaction for clusters in this size range. Extrapolating the results for these larger clusters to the infinite-size limit yields internal conversion lifetimes tau(IC) of 60 and 160 fs for electrons dissolved in H(2)O and D(2)O, respectively. In addition, the time-evolving spectra show evidence for solvent relaxation in the excited electronic state prior to internal conversion and in the ground state subsequent to internal conversion. Relaxation in the excited state appears to occur on a time scale similar to that of internal conversion, while ground state solvent dynamics occur on a approximately 1 ps time scale, in reasonable agreement with previous measurements on water cluster anions and electrons solvated in liquid water.