Ultrafast electron diffraction from a Bi(111) surface: Impulsive lattice excitation and Debye-Waller analysis at large momentum transfer.

The lattice response of a Bi(111) surface upon impulsive femtosecond laser excitation is studied with time-resolved reflection high-energy electron diffraction. We employ a Debye-Waller analysis at large momentum transfer of 9.3 angstrom(-1) <= Delta k <= 21.8 angstrom(-1) in order to study the lattice excitation dynamics of the Bi surface under conditions of weak optical excitation up to 2 mJ/cm(2) incident pump fluence. The observed time constants tau(int) of decay of diffraction spot intensity depend on the momentum transfer Delta k and range from 5 to 12 ps. This large variation of tau(int) is caused by the nonlinearity of the exponential function in the Debye-Waller factor and has to be taken into account for an intensity drop Delta I > 0.2. An analysis of more than 20 diffraction spots with a large variation in Delta k gave a consistent value for the time constant tau(T) of vibrational excitation of the surface lattice of 12 +/- 1 ps independent on the excitation density. We found no evidence for a deviation from an isotropic Debye-Waller effect and conclude that the primary laser excitation leads to thermal lattice excitation, i.e., heating of the Bi surface.