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

V Tinnemann,B Hafke,A Hanisch-Blicharski,C Streubühr,P Zhou,M Ligges,T Witte,A Kalus,M Horn-Von Hoegen,D Von Der Linde,U Bovensiepen

doi:10.1063/1.5128275

V Tinnemann, B Hafke + Show 9 more

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https://doi.org/10.1063/1.5128275

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Abstract

Ultrafast reflection high-energy electron diffraction is employed to follow the lattice excitation of a Bi(111) surface upon irradiation with a femtosecond laser pulse. The thermal motion of the atoms is analyzed through the Debye–Waller effect. While the Bi bulk is heated on time scales of 2 to 4 ps, we observe that the excitation of vibrational motion of the surface atoms occurs much slower with a time constant of 12 ps. This transient nonequilibrium situation is attributed to the weak coupling between bulk and surface phonon modes which hampers the energy flow between the two subsystems. From the absence of a fast component in the transient diffraction intensity, it is in addition concluded that truncated bulk phonon modes are absent at the surface.

Highlights

The initial dynamics of atoms in solid states upon impulsive femtosecond laser excitation has recently attracted much attention because energy transfer processes between the electronic system and the lattice are of general importance and high technological relevance
While the Bi bulk is heated on time scales of 2 to 4 ps, we observe that the excitation of vibrational motion of the surface atoms occurs much slower with a time constant of 12 ps
One would expect that the surface atoms respond on two different time scales upon impulsive excitation

Summary

Introduction

The initial dynamics of atoms in solid states upon impulsive femtosecond laser excitation has recently attracted much attention because energy transfer processes between the electronic system and the lattice are of general importance and high technological relevance. With the advent of ultrafast transmission electron diffraction and X-ray diffraction techniques, the structural dynamics became accessible on the picosecond and femtosecond time scales through the transient changes in the diffraction patterns upon ultrashort laser excitation. Crystalline Bi has widely been used as a prototypical system for the study of such ultrafast energy transfer processes from the initially excited electron system to the phonon system.

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