Sodium 1s photoabsorption spectra of Na and NaF clusters deposited in raregas matrices

Na and NaF clusters were investigated by Na 1s edge excitation using synchr otron radiation provided by the Super Aco storage ring in Orsay. The Na clu ster spectra show a close resemblance to the solid Na spectrum which consis ts essentially in an absorption jump. Only for very small clusters discrete structures appearing below the absorption spectrum are visible with small oscillator strength. This contrasts with the spectrum of Na vapour which sh ow intensive lines attributed to Rydberg stares, indicating that even in sm all Na clusters the valence electrons are delocalised over the whole aggreg ates. The NaF cluster spectra present a strong decrease in the excitonic st ructures with decreasing size, approaching a metallic threshold for the sma llest ones. This, together with mass spectroscopy observations, allow us to infer the presence of non-stoichiometric clusters for the smallest aggrega tes which undergo a 'metallisation'. The discrete excitations in NaF cluste rs are described in terms of a model which considers perturbation of the io n-like Rydberg states by the environment of the absorbing cation and this a llows simulations in satisfactory agreement with the experimental data. (C) 2000 Elsevier Science B.V. All rights reserved.