Angular distributions in photoelectron spectroscopy of small tungsten clusters: competition between direct and thermionic emission

Single-photon photodetachment of mass-selected W-n(-) clusters has been stu died by velocity map imaging. Photoelectron imaging allows us to measure si multaneously the kinetic energy spectrum and the angular distribution of ph otoelectrons, providing a clear distinction between the isotropic thermioni c emission and the anisotropic direct photoemission. A careful study of thr eshold electrons shows that the thermal distribution p(epsilon) cannot be d escribed, even qualitatively, by a simple exponentially decreasing Boltzman n function, as is usually assumed. On the contrary, our results are in exce llent agreement with more refined theoretical models. Our results indicate that the transition towards a bulk-like statistical behavior of the interna l energy redistribution occurs in very small systems, because of the high d ensity of states in metal clusters. The asymmetry parameter beta of the mos t intense band observed in direct photoemission for each cluster decreases monotonically with size; the direct photoemission of small systems is stron gly anisotropic, and becomes isotropic as the size of the system increases. This probably indicates the loss of coherence induced by electron-electron collisions occurring in large systems prior to electron-phonon coupling.