Time Resolved Photoelectron Imaging of Electronic Relaxation Dynamics in Anionic Clusters

Electronic relaxation dynamics are measured on a femtosecond timescale in three types of anionic clusters using time resolved photoelectron imaging. Auger relaxation timescales following interband excitation of electron-hole pairs in small Hgn− (n=9-20) are determined. Relaxation dynamics following charge transfer are investigated in I−(CH3CN)n (n=5-10). Internal conversion lifetimes of excited states of large anionic water clusters, (H2O)n− and the fully deuterated isotopolog (D2On− (n=25-200), as well as solvation dynamics in these clusters, are evaluated. A pronounced increase in the Auger lifetime of interband-excited states of Hgn − clusters with 13 or more constituent mercury atoms is revealed, indicating a shift from the van der Waals interactions typical of smaller clusters towards covalent bonding between mercury atoms in the cluster. This creates more delocalized electronic orbitals which reduce the coulomb interactions of the electron-hole pair, increasing the amount of time required for recombination and ejection of Auger electrons. Initial dynamics following charge transfer in I−(CH3CN)n clusters are associated with localization of an initially diffuse electron contained within the cluster. Later dynamics are assigned to rearrangement of the network of CH3CN molecules and ejection of neutral iodine from the cluster. Ultrafast internal conversion lifetimes of the first electronic excited state of anionic water clusters are measured at larger cluster sizes and with better time resolution than previous measurements. A marked reduction in the size dependence of the internal conversion lifetime at large sizes indicates a change in the electron water interaction for clusters larger than n≈70. Extrapolating internal conversion lifetimes of the larger clusters towards infinite cluster size predicts a condensed phase internal conversion lifetime of ~50 fs for the hydrated electron, supporting the nonadiabatic relaxation model. Solvation dynamics on both the ground and excited states are also observed