Photoelectron imaging spectroscopy of small tungsten clusters: Direct observation of thermionic emission - art. no. 023204

Single-photon photodetachment of mass-selected W-n(-) clusters has been stu died by photoelectron imaging spectroscopy. Velocity map imaging allows us to measure simultaneously the kinetic-energy spectrum and the angular distr ibution of photoelectrons. This provides a clear distinction between the tw o major decay mechanisms: isotropic thermionic emission and anisotropic dir ect photoemission. A careful study of threshold electrons shows that the th ermal distribution cannot be described by a bulklike formula or a simple ex ponentially decreasing Boltzmann function. On the contrary, our results are in excellent agreement with more refined theoretical models taking into ac count the spherical symmetry of the cluster. The kinetic-energy distributio n of thermal electrons corresponding to thermionic emission is found to var y as p(epsilon)proportional to epsilon (1/2) exp(-epsilon /k(B)T). Our resu lts indicate that a transition toward a bulklike statistical behavior of th e internal-energy redistribution occurs in very small systems owing to the high density of states in metal clusters. Moreover, the angular distributio n of direct photoelectrons is obtained and the evolution as a function of s ize is discussed. The asymmetry parameter beta of the most intense band obs erved 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 indi cates the loss of coherence induced by electron-electron collisions occurri ng in large systems.