The local structure of small water clusters: imprints on the core-level photoelectron spectrum

We report on an O 1s photoelectron-spectroscopy study of small neutral water clusters produced by adiabatic expansion. The photoelectron spectra were acquired under two different experimental conditions. At intermediate resolution, the cluster signal was characterized by a very broad O 1s peak with a flat top. In the second set of measurements, resolution was significantly increased at the cost of lower count rates. The cluster signal was now partly resolved into a bimodal structure. Extensive theoretical calculations were undertaken to facilitate an interpretation of the spectrum. These results suggest that the bimodal feature may be ascribed to ionization of water molecules in different hydrogen-bonding configurations, more specifically, molecules characterized by donation of either one or both hydrogen atoms in H-bonding.