Abstract
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 Å−1 ≤ Δ k ≤ 21.8 Å−1 in order to study the lattice excitation dynamics of the Bi surface under conditions of weak optical excitation up to 2 mJ/cm2 incident pump fluence. The observed time constants τint of decay of diffraction spot intensity depend on the momentum transfer Δk and range from 5 to 12 ps. This large variation of τint 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 ΔI > 0.2. An analysis of more than 20 diffraction spots with a large variation in Δk gave a consistent value for the time constant τT 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.
Highlights
Bismuth is a prototypical model system for studies of laser induced energy transfer from an excited electron system to the lattice system in the time domain
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
Employing all detected diffraction spots of the reflection high-energy electron diffraction (RHEED) pattern for the analysis provides the variation of the momentum transfer Dk of diffraction, i.e., a wide range of parallel kjj and vertical k? momentum transfers are available all at once
Summary
Bismuth is a prototypical model system for studies of laser induced energy transfer from an excited electron system to the lattice system in the time domain. The charge carriers are holes at T point and electrons at L point in the Brillouin zone.[2] The almost vanishing density of states at the Fermi energy results in a low number of free carriers of 1017–1019 cmÀ3 This makes this material very sensitive to optical excitations as changes in the electron occupation affects the potential energy surface and trigger atomic motion through displacive excitation. The crystal basis consists of two Bi atoms: atom 1 on an undistorted lattice site and atom 2 at a position slightly displaced from the center along the body diagonal of the unit cell This equilibrium structure, in particular, the distance of the two atoms of the basis, can be perturbed by electronic excitation.[3,4] When the distance is changed by an ultrafast displacive excitation, the Bi atoms perform a damped oscillation along the body diagonal. This mode of coherent atomic motion represents a symmetric A1g optical phonon mode of the crystal.[5,6,7,8,9,10]
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