Photoelectron imaging spectroscopy of small tungsten clusters: Direct observation of thermionic emission

Single-photon photodetachment of mass-selected Wₙ^- clusters has been studied by photoelectron imaging spectroscopy. Velocity map imaging allows us to measure simultaneously the kinetic-energy spectrum and the angular distribution of photoelectrons. This provides a clear distinction between the two major decay mechanisms: isotropic thermionic emission and anisotropic direct photoemission. A careful study of threshold electrons shows that the thermal distribution cannot be described by a bulklike formula or a simple exponentially decreasing Boltzmann function. On the contrary, our results are in excellent agreement with more refined theoretical models taking into account the spherical symmetry of the cluster. The kinetic-energy distribution of thermal electrons corresponding to thermionic emission is found to vary as p(ε)∝ε1/2exp(-ε/kBT). Our results indicate that a transition toward a bulklike statistical behavior of the internal-energy redistribution occurs in very small systems owing to the high density of states in metal clusters. Moreover, the angular distribution of direct photoelectrons is obtained and the evolution as a function of size is discussed. The asymmetry parameter {β} of the most intense band observed in direct photoemission for each cluster decreases monotonically with size: the direct photoemission of smaller systems is strongly anisotropic, becoming isotropic as the size of the system increases. This probably indicates the loss of coherence induced by electron-electron collisions occurring in large systems.