|Ultrafast Spectroscopy of Warm Dense Matter|
Being neither solid, liquid, gas, nor plasma, warm dense matter (WDM) occupies a no man's land in the map of material phases. Its temperature can range between that of planetary cores (tens of thousands K) to that of stellar cores (hundreds of thousands K). Not only is it prevalent throughout the universe, it is relevant to inertial confinement fusion (ICF) and material performance under extreme conditions. However, because of its extreme temperatures and pressures, WDM tends to be drastically transient and thus difficult to study in the laboratory. Now, researchers have set up ultrafast x-ray absorption spectroscopy at the ALS to measure the electronic structure of WDMs, demonstrating that fast-changing electron temperatures of matter under extreme conditions can be determined with picosecond resolution.
One common method to create WDM conditions is the isochoric (constant-volume) heating of a solid. A femtosecond laser system at Beamline 6.0.2 delivers an intense laser pulse to heat a thin copper foil rapidly (~150 fs), maintaining the initial volume of sample. Under strong excitation, electron temperature can increase up to ~10,000 K (1 eV) while the lattice remains cold (nonequilibrium condition). Before equilibrium is reached in a few tens of picoseconds, the superheated sample maintains its original density by inertia. The matter is "warm" (compared to a "hot" plasma) but still as dense as a solid—warm dense matter. On a longer time scale (>100 ps), it expands into a plasma. Therefore, to probe a warm dense state undergoing a nonreversible process, an ultrafast technique faster than the ALS pulse duration (70 ps) and a single-shot measurement capability are essential.
ALS Beamline 6.0.2 is equipped with a pink-beam sample chamber, upstream of the spectrograph (monochromator), for broadband x-ray absorption near-edge spectroscopy (XANES). In addition, researchers installed a grazing-incidence x-ray streak camera detector at the endstation. The detector has the ability to spread out x-ray pulses for a time resolution of ~1 ps. It also has a high quantum efficiency and 6-mm imaging length in the spectral direction; therefore, it can record a broad spectrum with a single camshaft x-ray pulse. This unique combination of beamline and detector allows recording of ultrafast XANES spectra of a sample of WDM that undergoes nonreversible change in a broad-band single-shot mode.
X-ray absorption spectra around the L-edge of warm dense copper, obtained ~2 to 20 ps after a femtosecond laser pulse heated the sample, show significant differences from the ambient-condition data. The edges are strongly shifted and broadened, and the above-edge absorption is smoother and lower, indicating that the electronic structure and electron distribution were significantly altered. A series of XANES calculations based on a first-principles density functional theory (DFT) molecular-dynamic simulation, when compared with the experimental data, showed that the shifting and broadening of the absorption edge can be used as a sensitive temperature sensor in the WDM regime.
The fast-changing XANES data indicate that the electron temperature peaks (~10,000 K) with optical excitation, then drops and reaches equilibrium with the lattice (~ 5,000 K) in 10 ps. This is faster than the time scale expected from the known electron–phonon coupling constant of copper (>20 ps). The result suggests that in the WDM regime, the energy exchange rate between electron energy and atomic vibration is expected to be temperature dependent and is about 3 to 6 times faster than previously assumed.
The researchers here successfully demonstrated the ability of this technique to provide new data sets for WDM electronic structures and thermal properties that are quite different from what was previously assumed. The results will also serve as benchmarks for theoretical modeling in this regime and can be applied to further investigation of plasmas for fusion, astrophysics, planetary science, and extreme material science.
Research conducted by: B.I. Cho, K. Engelhorn, J. Feng, and P.A. Heimann (ALS); A.A. Correa, T. Ogitsu, Y. Ping, A.J. Nelson, and R.W. Lee (Lawrence Livermore National Laboratory); C.P. Weber (Santa Clara University); H.J. Lee (SLAC National Accelerator Laboratory); P.A. Ni and D. Prendergast (Lawrence Berkeley National Laboratory); and R.W. Falcone (ALS and University of California, Berkeley).
Research funding: U.S. Department of Energy (DOE) National Nuclear Security Administration - Stewardship Science Academic Alliances (NNSA - SSAA) and Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.
Publication about this research: B.I. Cho, K. Engelhorn, A.A. Correa, T. Ogitsu, C.P. Weber, H.J. Lee, J. Feng, P.A. Ni, Y. Ping, A.J. Nelson, D. Prendergast, R.W. Lee, R.W. Falcone, and P.A. Heimann, "Electronic structure ofwarm dense copper studied by ultrafast x-ray absorption spectroscopy," Phys. Rev. Lett. 106, 167601 (2011).
ALS Science Highlight #246