On the origin of the different velocity peaks of particles sputtered from surfaces by laser pulses or charged-particle beams

The particles emitted from surfaces irradiated by either laser pulses or charged-particle beams are often analyzed by time-of-flight (TOF) spectroscopy. This technique gives, in general, details on the velocity (thence energy) of the emitted particles, parameters which are of fundamental importance in establishing the mechanisms responsible for the particle emission. However, when experiments are performed in an ambient gas, primary effects may be partially obscured by collisions between the emitted and the ambient particles. Here we will critically analyze certain TOF spectra obtained when particles are sputtered from a surface. To this end we first discuss a variety of velocity laws in connection with the processes which create the different sputtered fluxes. We then compare results which led to two or more velocity peaks, considering in so doing electrons, ions, and laser pulses, as well as the absence or presence of ambient gas. In other words, we try to establish simple criteria to distinguish primary emission mechanisms on the basis of TOF signals.