Determination of ionization energies of small silicon clusters with vacuum ultraviolet radiation.

In this work we report on single photon vacuum ultraviolet photoionization of small silicon clusters (n = 1-7) produced via laser ablation of Si. The adiabatic ionization energies (AIE) are extracted from experimental photoionization efficiency (PIE) curves with the help of Franck-Condon simulations, used to interpret the shape and onset of the PIE curves. The obtained AIEs are (all energies are in eV) Si (8.13 +/- 0.05), Si(2) (7.92 +/- 0.05), Si(3) (8.12 +/- 0.05), Si(4) (8.2 +/- 0.1), Si(5) (7.96 +/- 0.07), Si(6) (7.8 +/- 0.1), and Si(7) (7.8 +/- 0.1). Most of the experimental AIE values are in good agreement with density functional electronic structure calculations. To explain observed deviations between the experimental and theoretical AIEs for Si(4) and Si(6), a theoretical search of different isomers of these species is performed. Electronic structure calculations aid in the interpretation of the a(2)Pi(u) state of Si(2)(+) dimer in the PIE spectrum. Time-dependent density functional theory calculations are performed to reveal the energies of electronically excited states in the cations for a number of Si clusters.