Thermionic emission from small clusters: direct observation of the kinetic energy distribution of the electrons

Photoelectron spectra of small mass-separated Wn− clusters cooled cooled in a seeding gas are recorded with a low laser flux that ensures single-photon events. The spectra exhibit (i) sharp features due to direct photoemission and (ii) a broad emission signal whose intensity decreases with increasing kinetic energy. The latter is caused by delayed electron emission, a process similar to bulk ‘thermionic emission’. The kinetic energy distribution of these electrons can be approximated by a Boltzmann function with a temperature corresponding to the photon energy divided by the number of vibrational degrees of freedom of the cluster.