Photoelectron spectroscopy of large (water)n- (n = 50-200) clusters at 4.7 eV.

The hydrated electron eaq − has long been a species of considerable interest, studied extensively by both experimental and theoretical methods. Several recent papers have focused on the relation between eaq − and its cluster analog, H2O n , exploring, for example, the connection between the dynamics of the bulk hydrated electron and those of the excess electron in water clusters. A key question underlying this discussion is whether or not the excess electron is solvated internally in the cluster, roughly approximating the bulk situation, or if it is solvated on the surface of the cluster, an issue that arose after early experimental and theoretical studies of these species. Photoelectron spectra of large anionic water clusters n 200 taken at 3.1 eV in our laboratory revealed two isomers coexisting over a wide range of cluster sizes. Based on our interpretation of the time-resolved dynamics of the two isomers and theoretical studies which predicted that a surface-solvated excess electron would have a lower vertical detachment energy VDE than an internally bound excess electron, we assigned the less tightly bound isomer isomer II to a solvent configuration in which the excess electron is bound on the surface of the water cluster. Further support for this assignment was obtained, indirectly, in studies of the dynamics following charge-transfer to solvent in I− water n clusters. The more tightly bound isomer isomer I was correspondingly assigned to an isomer in which the excess electron is internally solvated. Recent theoretical work by Turi et al., however, suggested that isomer I may also be a surface state. Their simulation recovered two isomers in the range of n=20–200. The VDE’s for the more weakly bound of their predicted isomers had VDEs that corresponded well to those experimentally determined for isomer II clusters in the size range of n =20–66 but lay between the experimental VDEs for isomers I and II for n=104. Turi et al. correlated this isomer to our isomer I and assigned it to a solvent configuration with the electron solvated on the cluster surface. They assigned the more tightly bound calculated isomer, which they claimed was not seen in our experiment, to an internally solvated electron. In agreement with the earlier work by Barnett et al., they found this theoretically predicted internal state to be more tightly bound than the experimentally determined VDEs of isomer I. This situation has motivated us to revisit the photoelectron spectroscopy of large water clusters in the range of n 200, using a significantly higher photon energy of 4.7 eV. This energy exceeds the VDEs predicted for the most tightly