Christoph T. Hebeisen

The Formation of Warm Dense Matter : Experimental Evidence for Electronic Bond Hardening in Gold

Under strong optical excitation conditions, it is possible to create highly nonequilibrium states of matter. The nuclear response is determined by the rate of energy transfer from the excited electrons to the nuclei and the instantaneous effect of change in electron distribution on the interatomic potential energy landscape. We used femtosecond electron diffraction to follow the structural evolution of strongly excited gold under these transient electronic conditions. Generally, materials become softer with excitation. In contrast, the rate of disordering of the gold lattice is found to be retarded at excitation levels up to 2.85 megajoules per kilogram with respect to the degree of lattice heating, which is indicative of increased lattice stability at high effective electronic temperatures, a predicted effect that illustrates the strong correlation between electronic structure and lattice bonding.

Electronic acceleration of atomic motions and disordering in bismuth.

The development of X-ray and electron diffraction methods with ultrahigh time resolution has made it possible to map directly, at the atomic level, structural changes in solids induced by laser excitation. This has resulted in unprecedented insights into the lattice dynamics of solids undergoing phase transitions. In aluminium, for example, femtosecond optical excitation hardly affects the potential energy surface of the lattice; instead, melting of the material is governed by the transfer of thermal energy between the excited electrons and the initially cold lattice. In semiconductors, in contrast, exciting ∼10 per cent of the valence electrons results in non-thermal lattice collapse owing to the antibonding character of the conduction band. These different material responses raise the intriguing question of how Peierls-distorted systems such as bismuth will respond to strong excitations. The evolution of the atomic configuration of bismuth upon excitation of its A1g lattice mode, which involves damped oscillations of atoms along the direction of the Peierls distortion of the crystal, has been probed, but the actual melting of the material has not yet been investigated. Here we present a femtosecond electron diffraction study of the structural changes in crystalline bismuth as it undergoes laser-induced melting. We find that the dynamics of the phase transition depend strongly on the excitation intensity, with melting occurring within 190 fs (that is, within half a period of the unperturbed A1g lattice mode) at the highest excitation. We attribute the surprising speed of the melting process to laser-induced changes in the potential energy surface of the lattice, which result in strong acceleration of the atoms along the longitudinal direction of the lattice and efficient coupling of this motion to an unstable transverse vibrational mode. That is, the atomic motions in crystalline bismuth can be electronically accelerated so that the solid-to-liquid phase transition occurs on a sub-vibrational timescale.

Femtosecond electron diffraction: ‘making the molecular movie’

Femtosecond electron diffraction (FED) has the potential to directly observe transition state processes. The relevant motions for this barrier-crossing event occur on the hundred femtosecond time-scale. Recent advances in the development of high-flux electron pulse sources with the required time resolution and sensitivity to capture barrier-crossing processes are described in the context of attaining atomic level details of such structural dynamics—seeing chemical events as they occur. Initial work focused on the ordered-to-disordered phase transition of Al under strong driving conditions for which melting takes on nm or molecular scale dimensions. This work has been extended to Au, which clearly shows a separation in time-scales for lattice heating and melting. It also demonstrates that superheated face-centred cubic (FCC) metals melt through thermal mechanisms involving homogeneous nucleation to propagate the disordering process. A new concept exploiting electron–electron correlation is introduced for pulse characterization and determination of t=0 to within 100 fs as well as for spatial manipulation of the electron beam. Laser-based methods are shown to provide further improvements in time resolution with respect to pulse characterization, absolute t=0 determination, and the potential for electron acceleration to energies optimal for time-resolved diffraction.

Grating enhanced ponderomotive scattering for visualization and full characterization of femtosecond electron pulses.

Real time views of atomic motion can be achieved using electron pulses as structural probes. The requisite time resolution requires knowledge of both the electron pulse duration and the exact timing of the excitation pulse and the electron probe to within 10 - 100 fs accuracy. By using an intensity grating to enhance the pondermotive force, we are now able to fully characterize electron pulses and to confirm many body simulations with laser pulse energies on the microjoule level. This development solves one of the last barriers to the highest possible time resolution for electron probes.

Femtosecond electron pulse characterization using laser ponderomotive scattering

We demonstrate a method for the measurement of the instantaneous duration of femtosecond electron pulses using the ponderomotive force of an intense ultrashort laser pulse. An analysis procedure for the extraction of the electron pulse duration from the transient change of the transverse electron beam profile is proposed. The durations of the electron pulses generated in our setup were determined to be 410+/-30 fs.

Characterization of ultrashort electron pulses by electron-laser pulse cross correlation.

An all-optical method to determine the duration of ultrashort electron pulses is presented. This technique makes use of the laser pulse ponderomotive potential to effectively sample the temporal envelope of the electron pulse by sequentially scattering different sections of the pulse out of the main beam. Using laser pulse parameters that are easily accessible with modern tabletop chirped-pulse amplification laser sources, it is possible to measure the instantaneous duration of electron pulses shorter than 100 fs in the energy range that is most useful for electron diffraction studies, 10-300 keV.

Femtosecond electron diffraction: an atomic perspective of condensed phase dynamics

Femtosecond electron diffraction (FED) is a new technique within the still-developing field of ultrafast diffraction. This paper presents an outline of the basic features of FED, including a brief history of its development in terms of the technical challenges of working with femtosecond electron pulses and the ultrathin samples required. Application of FED to melting in aluminium and gold excited by intense femtosecond laser pulses will be discussed. The interplay of experiment and theory will be explored, particularly with respect to molecular dynamics simulations of the same processes we experimentally observe. Homogeneous nucleation emerges as an important melting mechanism under the strongly-driving conditions that we employ. Future applications of FED will be discussed in terms of progress to date.

Experimental basics for femtosecond electron diffraction studies

This paper provides a practical introduction to the current practice of femtosecond electron diffraction. We emphasize a general implementation suitable for a wide class of photoactive samples, including nonreversible samples. High density femtosecond electron pulses are required and the consequences for diffractometer design and sample preparation and handling are discussed. Finally, the real space structural analysis of strongly-driven melting in metals possible with our implementation is outlined.

Atomic View of the Photoinduced Collapse of Gold and Bismuth

Two different mechanisms of photoinduced melting were studied by femtosecond electron diffraction. The structural response of gold indicates an electronically-induced increase of the melting temperature. Bismuth was found to disorder within one vibrational period.

Femtosecond Electron Diffraction Study on the Melting Dynamics of Gold

The melting process in gold was resolved using femtosecond electron diffraction. The results support a thermally-driven melting mechanism with homogeneous nucleation, which is in qualitative agreement with previous work on aluminum.