Structural transitions in benzene–argon clusters: Size and temperature effects

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Abstract
The resonant two‐photon ionizationspectra of the first benzene–Ar n (n≤8) clusters are interpreted in the light of a model calculation, including local energy minima determinations and Monte Carlo simulations. Based on spectral shift information, two types of structures are distinguished: the argon cluster either only solvates one side of the substrate molecule or covers simultaneously both sides. The ‘‘wettest’’ structures are assigned to sandwich‐type forms for n≤4 and to clamshell‐type forms beyond. Such a 2D–3D transition in the n=5–8 size range is shown to be specific to small substrate size. In the one‐sided forms, the argon cluster geometry is strongly influenced by the presence of the Bz molecule since it adopts a quasiplanar geometry, more or less tightly bound to the substrate depending on its size: the small argon clusters exhibit two types of complexation sites (c and s forms) which are simultaneously visited (surface decoupling) even at low temperature (∼7 K). As the size increases, the c form becomes prevalent and the surface decoupling becomes inhibited. At n=8 a 2D–3D transition is observed: beyond the n=7 species, characterized by a caplike close‐packed structure, the one‐sided conformers disappear in favor of bridged forms. Additional results on the ionization behavior of the one‐sided species show regular variation of the ionization potential with the cluster size.

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