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

Abstract.

Single-photon photodetachment of mass-selected W _n ^- clusters has been studied by velocity map imaging. Photoelectron imaging allows us to measure simultaneously the kinetic energy spectrum and the angular distribution of photoelectrons, providing a clear distinction between the isotropic thermionic emission and the anisotropic direct photoemission. A careful study of threshold electrons shows that the thermal distribution p(ε) cannot be described, even qualitatively, by a simple exponentially decreasing Boltzmann function, as is usually assumed. On the contrary, our results are in excellent agreement with more refined theoretical models. Our results indicate that the transition towards a bulk-like statistical behavior of the internal energy redistribution occurs in very small systems, because of the high density of states in metal clusters. The asymmetry parameter β of the most intense band observed in direct photoemission for each cluster decreases monotonically with size; the direct photoemission of small systems is strongly 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.