Not-So-Thermal Desorption

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Science  29 Apr 2005:
Vol. 308, Issue 5722, pp. 605
DOI: 10.1126/science.308.5722.605a

The desorption of atoms or molecules from surfaces is thought to proceed through one of two mechanisms. Heating of the surface usually results in thermal desorption, in which the bonds holding the adsorbed species are put into such high vibrational states that they break. In electron- or photon-stimulated desorption, excitation of the adsorbate into an antibonding electronic state leads to desorption.

Trenhaile et al. followed the desorption of Br from the Si(100)-(2×1) surface at 620 to 775 K via scanning tunneling microscopy. Their analysis shows that this process does not proceed through vibrational excitation but by electron capture into long-lived states that then populate an antibonding σ* state that then ejects the Br atom. The excitation energy for desorption changes with the Fermi level for different silicon doping levels. Entropy can actually help drive this process, in which 10 to 20 optical phonons come together to push the electron over its barrier. — PDS

Surf. Sci. 10.1016/j.susc.2005.3.053 (2005).

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