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

Tinnemann, Verena LSF; Streubühr, Carla GND; Hafke, Bernd GND; Kalus, Annika GND; Hanisch-Blicharski, Anja GND; Ligges, Manuel GND; Zhou, Ping LSF; von der Linde, Dietrich LSF; Bovensiepen, Uwe GND; Horn-von Hoegen, Michael GND

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.

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Tinnemann, Verena / Streubühr, Carla / Hafke, Bernd / et al: Ultrafast electron diffraction from a Bi(111) surface. Impulsive lattice excitation and Debye–Waller analysis at large momentum transfer. 2019.

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