Comparing ultrafast surface and bulk heating using time-resolved electron diffraction

Abstract

From measurements of the transient Debye-Waller effect in Bismuth, we determine the buildup time of the random atomic motion resulting from the electronic relaxation after short pulse laser excitation. The surface sensitive reflection high energy electron diffraction and transmission electron diffraction yield a time constant of about 12 ps and 3 ps, respectively. The different energy transfer rates indicate relatively weak coupling between bulk and surface vibrational modes.

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

“…8, 11, and 15–17) through energy transfer from the electron system to the lattice by electron phonon coupling and anharmonic coupling of the A 1g mode to acoustic phonons 18 . The vibrational excitation of the surface atoms is even slower: thermal motion of the Bi surface atoms sets in on a timescale of 12 ps and has been attributed to the weak coupling between bulk and surface phonons 16 …”

Thickness-dependent electron–lattice equilibration in laser-excited thin bismuth films

“…On the other hand, time-resolved diffraction techniques using ultrafast electron-or x-ray pulses provide direct structural sensitivity and are, therefore, able to monitor the build-up of vibrational excitations in real time. These techniques have frequently been used to investigate energy relaxation and lattice heating in laserexcited materials including Bi [14][15][16][17], the material studied in this work.…”

“…Bi exhibits a Peierls-distorted ground state structure (rhombohedral A7) which results in a strong coupling between the electronic and lattice degrees of freedom. This makes it very susceptible to electronic excitation [22], and ultrafast diffraction has been extensively used to study various aspects of the ensuing vibrational/structural dynamics: coherent optical [23][24][25] and acoustic [16,26] phonons, phonon squeezing [27], electron-lattice equilibration in thin films [14,15,17], nanoparticles [16], and at the surface [17], anisotropic structural effects [15,16,26,28], heat transfer processes [29][30][31], and ultrafast melting [14].…”

New aspects of electronic excitations at the bismuth surface: Topology, thermalization and coupling to coherent phonons

“…The hot electrons distribution then relax on a picosecond time scale (with a 4-6 ps characteristic time) because of electron-phonon coupling: the electron bath exchanges energy with the phonon bath until the lattice temperature equalizes with the electron temperature [11]. This part of the dynamics is well described by the two-temperature model [28,29] although the surface and bulk cooling may act of different timescales [30]. The fact that electron thermalization occurs on a longer time scale for low excitation fluences can be explained qualitatively using the theory of Fermi liquids [31].…”