Size-dependent reactivity in open shell metal-ion polar solvent clusters: spectroscopic probes of electronic-vibration coupling, oxidation and ionization

This review considers the spectroscopy and structure of clusters formed by stepwise addition of polar solvent molecules such as NH3, H2O, and CH3OH to effective one-electron chromophores that include the singly-charged alkaline earth cations Mg+, Ca+, and Sr+; atomic sodium; and the Rydberg molecule NH4 . The absorption of photons by such species results in initial electronic excitation, followed by energy transfer to vibrational degrees of freedom, ultimately leading to dissociation. Experimental data are presented to support this electronic-to-vibrational energy transfer mechanism. Wavelength-dependent photodissociation signals for all of these species exhibit a similar size-dependence in which large spectral red shifts are observed as the first solvation shell fills. An examination of ab initio calculations on a number of related systems, as well as theoretical models for charge transfer, suggests that non-covalent interactions of solvent molecules with the single valence electron of the cluster core lead to spontaneous ionization of the core with increasing cluster size. The process is analogous to the formation of solvated electrons in condensed phases. The collective results suggest that within the broad topic of solvation and the formation of solutions, clusters can provide a linkage between the properties of the gas phase and those of condensed phases.