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, a clear distinction between the isotropic thermionic emission and the anisotropie 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 reined iecïretical 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 anisotropie, 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.