Geometric and electronic structures of fluorine bound silicon clusters

Geometries and energies of Si_nF^- and Si_nF (n=1-7) were investigated with ab initio MO calculations; the Møller-Plesset perturbation and coupled cluster methods were used to take into account the electron correlation. The F atom is bound to the apex atom in the Si_n frame, and the electronic structures of the Si_n part in Si_nF^- and Si_nF are similar to those of the corresponding Si_n and Si⁺ _n, respectively. The calculated electron affinities (EAs) of Si_nF are in agreement with the experimental values. The size dependence of EAs of Si_nF is similar to that of the ionization energies of Si_n at n≥5, whereas it is different at n≤4. In the HOMO of Si_nF (n≤4), the weak antibonding nature between F and Si greatly makes the Si_nF^- anions unstable, resulting in small EA. The vibrationally resolved photoelectron spectra (PES) of Si_nF^- (n=1-3) were measured, and were completely reproduced theoretically.