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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.

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.

Abstract. We present time-resolved RHEED from a laser excited Pb(111) surface using a pulse front tilter for the compensation of the velocity mismatch of electrons and light. The laser pulses with tilted fronts were characterized by a spatially resolving cross correlator. The response of the surface upon excitation was observed to be less than 2 ps.Ultrafast electron diffraction is a promising technique for studying the dynamics of the atomic structure on a sub-picosecond time scale. Time resolved transmission electron diffraction (TR-TED) has been used, for example, to study bulk phenomena such as ultrafast laser-induced structural phase transitions [1], cooperative rearrangement of crystalline structures [2], and dynamics of charge density waves [3]. Time resolution of less than 100 fs [4] has been achieved. Time resolved reflection high energy electron diffraction (TR-RHEED), on the other hand, has been used for the investigation of ultrafast surface dynamics, for example the observation of ultrafast heating of surfaces [5], the heat transfer in hetero structures [6] and laser-induced phase transition of the surface reconstructions [7]. However, the time resolution in these experiments was limited to tens of picoseconds because of the different velocities of electrons and light in combination with the grazing angle of incidence of the electron beam in RHEED geometry.This velocity mismatch can be overcome and the time resolution significantly improved by tilting the laser pulse front with respect to the phase fronts. Baum et al. [8] have demonstrated a configuration for pulse front tilting and achieved an improvement of the temporal resolution to less than one picosecond. Here we report the use of a 4f zero dispersion delay line capable of matching the pulse front of the laser to the electron pulse. The improvement of the temporal resolution is demonstrated by measuring the ultrafast lattice heating of Pb(111) islands on a Si(111) surface.Generally speaking, when a laser pulse encounters angular dispersion, the planes of constant amplitude (intensity) will be tilted with respect to the planes of constant phase, for example when the pulse is reflected from a diffraction grating [9]. The pulse then broadens during the free propagation after the dispersive component. The broadening is reversed and the original pulse width restored by passing the pulse through the two lenses in a 4f configuration. Our pulse tilter was designed to compensate the velocity mismatch for 30 keV electrons. Based on the results of our electron source [10] a temporal resolution of better than 700 fs is expected.In order to independently control and characterize the tilted pulse we used a cross-correlator based on second harmonic generation (SHG). The special feature of this cross-correlator is the possibility to perform spatially resolved measurements of the cross-correlation function between the This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, dis...

Abstract

The cooling process of ultrathin hetero films upon excitation with short laser pulses was studied for epitaxial Bi(111) films on Si(001) and Si(111) substrates by means of the Debye-Waller effect with ultrafast electron diffraction. From the exponential decay of the temperature, a cooling time constant was determined as a function of thickness for both substrates. For Bi/Si(111), a linear dependence between the decay constant and thickness was observed, even for 2.8 nm thin films , as predicted from the diffuse mismatch model (DMM) and the acoustic mismatch model (AMM). However, with Bi/Si(001), a significant deviation from this linear dependence was observed for film thicknesses below 5 nm.

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.