Bond breaking at surfaces: Electrons or phonons?

The desorption of atoms and molecules is one of the most fundamental of all surface processes. A distinction is usually made between ‘‘thermal’’ desorption and desorption induced by energetic sources of excitation, such as incident photons or electrons, that induce electronic transitions. The paper by Trenhaile and co-workers in this issue shows the surprising result that the ‘‘thermal’’ desorption of Br from the Si(001) surface is in fact an electronic process in which the random statistical motion of phonons occasionally converges to excite an electron to an antibonding Si–Br r* state from which desorption of Br occurs [1]. Moreover, it is the entropy associated with the assembly of the phonons that is key to understanding how the desorption occurs. The results are important because they suggest that electronic transitions may have an important, but previously unrecognized, role in ‘‘thermal’’ desorption processes at surfaces. Bond breaking at surfaces, like its gas-phase counterpart, has long been understood based on two distinct mechanisms. In the ‘‘electronic’’ mechanism, some source of energy induces a transition from a bound electronic state to an unbound state; one example would be a transition from the Si–Br r state to a Si–Br r* antibonding state as illustrated in Fig. 1a. The second, ‘‘thermal’’ mechanism involves putting enough energy into a specific vibrational mode, such as an Si–Br stretching vibration, that a chemical bond literally breaks and desorption occurs; this process is illustrated in Fig. 1b. These mechanisms are usually distinct because the time scale for physical motion of an atom is quite long compared with the time needed for electrons to change their spatial distribution; this leads to the Franck–Condon approximation, which states that electronic transitions are usually not accompanied by a change in bond lengths and can usually be depicted as ‘‘vertical’’ transitions like that shown by the solid arrow in Fig. 1a.