Study of the decay of refractory clusters by photoelectron imaging spectroscopy

Single-photon photodetachment of mass-selected tungsten and carbon cluster anions has been studied by photoelectron imaging spectroscopy. Velocity map imaging allows us to measure simultaneously the kinetic energy spectrum an d the angular distribution of photoelectrons. This provides a clear distinc tion between the two major decay mechanisms: the isotropic thermionic emiss ion and the anisotropic direct photoemission. A careful study of threshold electrons shows that the thermal distribution cannot be described, even qua litatively, by a simple exponentially decreasing Boltzmann function. Based on qualitative arguments and on previous theoretical works describing therm ionic emission in small metallic spheres, the kinetic energy distribution o f electrons corresponding to thermionic emission in tungsten cluster anions is found to vary as p(epsilon) proportional to epsilon(1/2) exp(-epsilon/k (B)T), in contrast with the bulk-like emission profile p(epsilon) proportio nal to epsilon exp(-epsilon/k(B)T). On the contrary, in negative carbon clu sters that do not exhibit spherical symmetry at small size, this simple mod el cannot describe the thermal electron spectrum. Moreover, in tungsten clu sters, the asymmetry parameter beta of the most intense band observed in di rect photoemission decreases monotonically with size. This probably indicat es the loss of coherence induced by electron-electron collisions occurring in large systems prior to electron-phonon coupling.