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), Si₂ (7.92 ± 0.05), Si₃ (8.12 ± 0.05), Si₄ (8.2 ± 0.1), Si₅ (7.96 ± 0.07), Si₆ (7.8 ± 0.1), and Si₇ (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 Si₄ and Si₆, a theoretical search of different isomers of these species is performed. Electronic structure calculations aid in the interpretation of the a²Π_u state of Si₂⁺ 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.