Chemistry

Surfaces That Shake Faster

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Science  04 Jun 2004:
Vol. 304, Issue 5676, pp. 1417
DOI: 10.1126/science.304.5676.1417a

The atoms at the surface of a solid are normally expected to have enhanced vibrational amplitudes, especially in the components normal to the surface, because fewer bonds constrain their movement in that direction. Typically, the increase is about 40 to 50% of the bulk value, but Van Hove notes that three surfaces—α-Al2O3(0001), α-Ga(010), and reconstructed Si(111)-(2×1)—have root mean square displacements double that of the bulk value, and at 90 K for water ice (0001), the increase may be as much as triple the bulk value. The author suggests that for these surfaces, the “back bonds” that hold the outermost layer in place are almost perpendicular to the surface plane, and that the vibrations are primarily soft bending modes rather than stretching modes. Such enhanced vibrations could play a role in surface pre-melting and catalysis. — PDS

J. Chem. Phys.B 10.1021/jp040047x (2004).

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