Thermal effects in the photoelectron spectra of Na-N(-) clusters (N=4-19)

Photoelectron spectra (PES) of sodium cluster anions NaN (4<=N<=19), measured at close to room temperature, are compared to the electronic density of states obtained from Born-Oppenheimer local-spin-density Langevin molecular-dynamics simulations. Although the shapes of the clusters follow the general predictions of jellium models, considerable deviations occur for the sizes with N<=7. Most notably, Na7- shows a triaxial deformation at elevated temperatures, caused by thermally induced quadrupole shape fluctuations. For Na12- as well as for Na13- our simulations confirm the jellium model prediction of the existence of two quasidegenerate (and thermally stable) shape isomers. The experimental PES data of these 1d midshell clusters, however, suggest that only the oblate shapes are present in the experiment. This phenomenon may originate from the formation mechanism of the clusters in the beam.