Water Dissociation upon Adsorption onto Free Iron Clusters Is Size Dependent

Cationic iron clusters, produced through laser ablation and subsequently complexed with a water molecule Fe_n⁺–H₂O (n = 6–15) are mass-selectively investigated via infrared multiple photon dissociation (IR-MPD) spectroscopy in the 300–1700 cm^–1 spectral range. The experimental data are complemented by density functional theory calculations at the OPBE/TZP level for the Fe₁₃⁺–H₂O system. The observed spectra can be explained by a mixture of clusters where for a majority water is adsorbed molecularly but for a small but significant fraction also dissociation of water molecules occurs. The bands observed at frequencies 300–700 cm^–1 exhibit regular, size-dependent frequency shifts, showing that (a) dissociation takes places on all cluster sizes and (b) the interaction of water with the cluster surface is not influenced much by the particular cluster structure. The intensity evolution of the absorption bands suggests that dissociation is increasingly probable for larger cluster sizes.