The principal challenge of the emerging field of ultrafast materials dynamics hinges on our ability to characterize materials at both the time and length scales appropriate to the underlying physics. To probe the electronic and structural dynamics of molecules—in real time and with femtosecond resolution, we are using x-ray techniques, in combination with femtosecond lasers, to study the dynamics of metals during ultrafast laser irradiation.
These methods rely on the newest technological developments in x-ray sources and detection systems. Experimental protocols being developed to achieve the requisite time and spatial resolution not only bring our experiments into contact with theoretical studies but also inspire new theoretical tools. We are interpreting our experimental results using multiscale modeling techniques that account for electronic excitations and electron-ion equilibration in metals. This has seeded new theoretical ideas and has led to a continuum model of the electron fluid coupled to the ionic system.
|Figure 1. Photoelectron spectra for static versus laser-heated copper foil show how electrons respond to increasing laser heating.|
Relevance to PLS Research Themes
Advancing the fundamental scientific understanding of the dynamic behavior of materials is essential for our national-security mission. Investigation of materials properties and performance is challenging because of the diversity and rapidity of the structural changes under extreme and dynamical conditions. By combining in-situ experimental diagnostics and new computational schemes, we are focusing on the fundamental aspects of how materials respond to strong shocks and extreme non-equilibrium conditions.
Major Accomplishments in 2004
To probe ultrafast changes in the chemical and electronic structure of materials, we use the compact multipulse terawatt (COMET) x-ray laser to obtain the necessary high-photon flux (greater than 1012/pulse), monochromaticity, picosecond pulse duration, and coherence. We have obtained the first photoemission spectra of laser-heated, ultra-thin copper foils showing changes in electronic structure.
COMET is ideal for studying the electron time-of-flight photoemission process (electrons ejected from solids by radiation), as its short-pulse laser irradiation creates a non-equilibrium electron distribution at elevated temperatures, while the lattice remains at room temperature. We illuminated ultra-thin polycrystalline copper foils with 108–109 x-ray-laser photons to measure how electrons respond to increasing laser intensity (Figure 1). We observe a strong d-state photoemission that corresponds to direct transitions from d-like occupied bands (high valency) to unoccupied bands (high conductivity) above the Fermi level (Figure 2). The data also indicate the high density of filled d states that are 2 eV below the Fermi level.
|Figure 2. Schematic of photoemission process for copper illustrates a strong photoemission from direct transition from valance band to conduction band above the Fermi level.|
Many other interesting and important features evolve with increased laser heating, and at the highest temperatures, depopulation of the valence band d states also creates vacancies in the conduction band, thus allowing interband absorption below the edge, in this case, from 3d to 4p transitions in copper. Our ability to access this complex spectroscopy is fundamentally important in advancing our understanding of the dynamic non-equilibrium processes at play that precede materials disassembly.
We have also observed that the peak of copper 3d state shifts toward lower kinetic energy (higher binding energy). Depopulation of the d band is predicted to affect its binding energy in this manner. We observed no broadening of the copper 3d state upon heating, which implies a non-equilibrium distribution of occupied states, or smearing of the Fermi-Dirac electron energy distribution. Increasing the laser energy further by a factor of ten generates a strong electron signal before reaching the valence band maximum, indicating the sample is in an ionized or non-equilibrium state.
In order to model laser interaction with metals, we combine classical molecular dynamics simulations with a model for the dissipative dynamics of the electron-ion system to bring the two systems into equilibrium with each other (Figure 3). Specifically, we have formulated a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons and are applying first-principles methodology for determining the relevant material-specific characteristics. We are also extending the model to micrometer and larger length scales to predict structural information about voids and the ablated surfaces.
|Figure 3. Temperature contour plot shows simulations of laser melting of a 50-nanometer nickel film. Strong electron–phonon coupling leads to a large temperature gradient in the film. Ultrafast melting of a large fraction (~35 nm) of the film is observed.|
These experiments demonstrate the first picosecond time-resolved photoemission spectra of laser-heated, ultra-thin copper foils showing these changes in electronic structure. This study also included the first measurements of the disassembly dynamics of ultra-thin copper foil. The complementary relationship between these experiments and the newly developed, massively parallel hybrid atomistic-continuum model allows us to model the evolution of photomechanical damage in large-scale simulations of laser melting and spallation of nickel and copper thin films. In collaboration with UC Berkeley, this project provides a clear vision for ultrafast examination of extreme states of matter.
A.J. Nelson, et al., “X ray Laser-Induced Photoelectron Spectroscopy for Single-State Measurements,” Appl. Phys. Lett. 85(25), 6290 (2004).
L.V. Zhigilei, et al., “Computer Modeling of Laser Melting and Spallation of Metal Targets,” in Proc. Intl. Soc. Opt. Eng. 5448, 505 (2003).
When it becomes operational in 2009, the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center will be the world’s first x-ray, free-electron laser. LLNL researchers will use the unique quality of the LCLS to create extreme states of matter at high temperature and density, to probe the ablation/damage process for high-energy-density science, and to study accompanying structural changes.
LCLS will be an important new experimental platform to study the interaction of intense x rays with matter. The unique attributes of the LCLS have the potential to revolutionize the experimental investigation of structural dynamics by directly following the time evolution of the electron density during the course of a biological, chemical, or physical transformation.