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 ch arged-particle beams an 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 i n establishing the mechanisms responsible for the particle emission. Howeve r. when experiments are performed in an ambient gas, primary effects: may b e partially obscured by collisions between the emitted and the ambient part icles. Here we will critically analyze certain TOF spectra obtained when pa rticles are sputtered from a surface. To this end we first discuss a variet y of velocity laws in connection with the processes which create the differ ent sputtered fluxes. We then compare results which led to two or more velo city peaks, considering in so doing electrons, ions, and laser pulses, as w ell as the absence or presence of ambient gas. In other words, we try to es tablish simple criteria to distinguish primary emission mechanisms on the b asis of TOF signals. (C) 1999 Elsevier Science B.V. All rights reserved.