A new mode for desorption

Has been uncovered. The detachment of atoms and molecules from a surface is one of the fundamental processes of surface science. One of two mechanisms is generally invoked. Thermal desorption calls for the material to be heated, which can stretch and eventually break the bonds of adsorbed atoms and molecules through the action of phonons. In contrast, electronic desorption calls for an external stimulus—say, from an incident electron or photon—to induce an electronic transition of sufficient energy to promote the adsorbed atom or molecule from a bound to an unbound state. The two mechanisms operate on vastly different time scales, with electronic transitions being faster. Studying bromine adsorbed on silicon, John Weaver and his colleagues at the University of Illinois at Urbana-Champaign have found a third mode, one that has elements of both of the others. The researchers examined bromine’s desorption kinetics as a function of silicon doping and of temperature. A detailed analysis revealed the rare but crucial event of 10–20 phonons simultaneously interacting with a single electron. Rather than directly breaking a bond as in the thermal case, the phonons induce an electronic transition that promotes the adsorbate to an unbound state. Thus, the Illinois group found the surprising result that electronic desorption prevails in this system without needing any external excitation. Multiphonon processes are common during a system’s relaxation, but the Illinois work may show that they can also play an important role in surface chemistry. (B. R. Trenhaile et al. , Surface Science, in press.)