Photoelectron spectroscopy of large water cluster anions and related systems
Final Report Abstract
We have studied the nature of the solvated electron in water cluster anions in a very broad size range at low temperatures and as a function of temperature. The results were evaluated in detail, carefully testing different ways to fit the spectra obtained. Additionally exploratory studies on the application of hole-burning techniques to determine water cluster isomerization rates and the production on methanol cluster anions were performed. The main findings are: 1. While for clusters up to sizes of about n=60 several isomer classes (with different electron binding energies) can be clearly distinguished, for larger sizes (measured up to size n = 1100) a single isomer class prevails, which presumably consists of clusters with an internally solvated electron. 2. The large size range allows obtaining a reliable extrapolation of the vertical electron binding energy to the bulk. A value of 3.6 +/- 0.02 eV is obtained, which is significantly larger than the generally assumed value for bulk water (3.3 eV). 3. The size dependence of the electron binding energy for large clusters is in very good agreement with classical dielectric model prediction, if parameters of liquid water are used. This indicates that the binding energies obtained for (cold) clusters can be used to determine the binding energy of room temperature bulk water, meaning that the extrapolated value given above should be the correct one. It is interesting that water behaves so bulk-like, while other cluster systems (like ammonia) have been shown to exhibit rather different dielectric properties than the bulk. 4. Some clusters (close to the structurally magic cluster size N=50) have been shown to exhibit a rather distinct temperature dependence of the electron binding energies. With the onset of melting at about 100 K the binding energies of the two dominant isomer classes shift to lower binding energies, and shift back at higher temperatures. This behaviour hints at melting related changes of both the shape and the dielectric properties of the clusters, but is quite far from being fully understood. 5. The photoelectron spectra of well annealed large water cluster anions exhibit a single peak which is close to identical to the peak in the absorption spectrum of solvated electrons in bulk water. This hints at very similar structural relaxation processes upon electron excitation or emission. In summary, this study on the properties of water cluster anions has successfully clarified a number of open questions concerning this fascinatingly complex system, and represents an important step towards a comprehensive understanding of the nature of the solvated electron.
