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

V Tinnemann; C Streubühr; B Hafke; A Kalus; A Hanisch-Blicharski; M Ligges; P Zhou; D von der Linde; U Bovensiepen; M Horn-von Hoegen

doi:10.1063/1.5093637

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

Struct Dyn. 2019 May 1;6(3):035101. doi: 10.1063/1.5093637. eCollection 2019 May.

Authors

Affiliation

¹ Department of Physics and Center for Nanointegration (CeNIDE), University of Duisburg-Essen, 47048 Duisburg, Germany.

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 τ _int of decay of diffraction spot intensity depend on the momentum transfer Δk and range from 5 to 12 ps. This large variation of τ _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 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 τ_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.