Gas-Phase Production of Hydroxylated Silicon Oxide Cluster Cations: Structure, Infrared Spectroscopy, and Astronomical Relevance

The interaction of free cationic silicon oxide clusters, SixOy+ (x = 2-5, y >= x), with dilute water vapor, was investigated in a flow tube reactor. Product mass distributions indicate cluster size-dependent dissociative water adsorption. To probe the structure and vibrational spectra of the resulting SixOyH2+ (x = 2-4) clusters, we employed infrared multiple photon dissociation spectroscopy and density functional theory calculations. The planar rhombic cluster core of the disilicon oxides (x = 2) appears to be retained upon dissociative adsorption of one H2O unit, whereas a significant structural transformation of the tri- and tetra-silicon oxides (x = 3 and 4) is induced, resulting in an increased coordination of the Si atoms and more 3D cluster structures. In an astronomical context, we discuss the potential relevance of SixOyHz+ clusters as seeds for dust nucleation and catalysts for carbon-based chemistry in diffuse or translucent interstellar clouds, where all the necessary conditions for producing these species are found. In the produced clusters, the frequency of the isolated silanol Si-OH stretching vibrational mode is considerably blue-shifted compared to that in hydroxylated bulk silica and small inorganic compounds. This mode has a characteristic frequency range between 1200 cm(-1) (8.3 mu m) and 1090 cm(-1) (9.2 mu m) and is associated with the anomalously small Si-OH bond lengths in these ionised species. In infrared observations such high frequency Si-O stretching modes are usually associated with a pure bulk silica component of silicate cosmic dust. The presence of SixOyH2+ clusters in low silica astrophysical environments could thus potentially be detected via their signature Si-O band using the James Webb space telescope.