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The surface step density on the vicinal Pb͑111͒ and the surface vacancy density on Pb͑100͒ after laser superheating and melting are investigated using reflection high-energy electron diffraction. With ϳ100-ps laser pulses, Pb͑111͒ surface superheating does not significantly change the density of the steps and step-edge roughness. However, after laser surface melting, the average terrace width and the string length at the step edge become as large as those at room temperature. The average terrace width at 573 K changes from 38Ϯ15 to 64Ϯ19 Å after laser surface melting, while the average string length at the step edge changes from 90Ϯ14 to 250Ϯ38 Å. For Pb͑100͒, the surface vacancy density remains unchanged when the surface is superheated without melting. However, when the laser fluence is high enough to cause surface melting, the surface vacancy density increases. This increase in vacancy density is attributed to fast diffusion of atoms in the liquid film formed on Pb͑100͒ during laser melting. ͓S0163-1829͑98͒02316-9͔

The structure of the low-index surfaces of germanium near its bulk melting temperature is investigated using 100-ps time-resolved reflection high-energy electron diffraction. The surface is heated by 100-ps laser pulses while a synchronized electron beam probes the structure. Ge͑111͒ was observed to remain in its incomplete melting structure up to at least T m ϩ134Ϯ40 K when heated by a 100-ps laser pulse. Both the Ge͑100͒ and Ge͑110͒ surfaces are observed to melt near the bulk melting temperature when heated with 100-ps laser pulses. Because of the low-diffraction intensity-to-background ratio at high temperatures and because of the temperature uncertainty in the time-resolved experiments, we are unable to accurately identify the melting point of Ge͑100͒ and Ge͑110͒ when heated with a 100-ps laser pulse. The results, however, favor the lack of surface superheating of Ge͑100͒ and, to some extent, Ge͑110͒. The superheating of the incomplete melting state of Ge͑111͒ could be due to the metallization of the top germanium bilayer and its interaction with the solid underneath causing an energy barrier sufficient to allow for transient surface superheating.

The dynamics of the Ge(111)-c(2ϫ8)-(1ϫ1) phase transition is investigated by 100-ps time-resolved reflection high-energy electron diffraction. A laser pulse heats the surface while a synchronized electron pulse is used to obtain the surface diffraction pattern. Slow heating shows that the adatoms in Ge(111)-c(2ϫ8) start to disorder at ϳ510 K and are converted to a disordered adatom arrangement at 573 K. For heating with 100-ps laser pulses, the Ge(111)-c(2ϫ8) reconstructed adatom arrangement starts to disorder at 584Ϯ16 K, well above the onset temperature of ϳ510 K for the disordering of Ge(111)-c(2ϫ8) observed for slow heating.

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