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

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/cm 2 incident pump fluence. The observed time constants τ int of decay of diffraction spot intensity depend on the momentum transfer Δ k 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 Δ I > 0.2. An analysis of more than 20 diffraction spots with a large variation in Δ k 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.

Manipulation of Stacking Order in Td-WTe2 by Ultrafast Optical Excitation

“…The relation between ln(I/I 0 ) and Q 2 for a00 at 2.5 mJ/cm 2 pump fluence is shown in Figure 5 d. ln(I/I 0 ) is linear with Q 2 which is consistent with what is expected from the Debye–Waller effect. 36 …”

Exchange-Striction Driven Ultrafast Nonthermal Lattice Dynamics in NiO

“…Variants of FED have recently been used to study lattice dynamics in several other systems, including metals [38,39], semiconductors [34,40], heterostructures [41,42], and systems involving structural phase transitions [43,44]. We present an approach to characterize the transient lattice state, by converting Bragg reflection intensities into units of Kelvin, producing a series of temperatures associated with each scattering vector of the probe electrons (Q).…”

Decelerated lattice excitation and absence of bulk phonon modes at surfaces: Ultra-fast electron diffraction from Bi(111) surface upon fs-laser excitation

“…All of this could be explained by the inherent nonlinearity of the exponential function: for large momentum transfers, the spot intensity decreased more than 90%, which then is associated with an apparently faster excitation. The consideration of this effect—for all spots—resulted in a consistent time constant of 12 ps for the vibrational excitation of the surface atoms 36 . Thus, the excitation of the surface phonons is slow compared to the excitation of the bulk phonon system, which occurs within 4 ps or less 11,19 .…”