Time-resolved structural studies of the low-index faces of lead.

Time-resolved reflection high-energy electron diffraction has been developed to study the structure of surfaces. In this technique, an ultrafast laser pulse is split into two components. The first component interacts with the surface and serves as a transient heating source. The second component strikes the cathode of a photoactivated electron gun, photoelectrically producing pulses of electrons. The electron pulses are accelerated to energies on the order of 15 keV and are incident at a glancing angle to the studied surface, probing the first few atomic layers. The heating laser pulse passes through an optical delay line which provides the means by which the heating pulse and electron probe pulse are temporally synchronized on the surface. Analysis of the resulting time-resolved diffraction patterns provides information about the temperature and structure of the probed layer. The technique of time-resolved reflection high-energy electron diffraction, with 200-ps resolution, is used to study the structural behavior of the low-index faces of Pb. The surfaces are subjected to heating and cooling rates on the order of 1011 Us. A pronounced orientation-dependence of the structural dynamics is observed. The open Pb(ll0) surface is seen to reversibly disorder below the bulk melting temperature, Tm = 600.7 K. In contrast to Pb(1 lo), the close-packed Pb(ll1) surface sustains superheating to Tm + 120 K. Pb(100), a surface with atomic packing intermediate to that of Pb(ll0) and Pb(l l l ) , shows evidence of residual order at temperatures above the bulk melting point.