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Register a new userResonant Bound-Free Contributions to Thomson Scattering of X-rays by Warm Dense Matter
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Recent calculations [Nilsen et al. arXiv:1212.5972] predict that contributions to the scattered photon spectrum from 3s and 3p bound states in chromium (Z=24) at metallic density and T=12 eV resonate below the respective bound-state thresholds. These resonances are shown to be closely related to continuum lowering, where 3d bound states in the free atom dissolve into a resonant l=2 partial wave in the continuum. The resulting d-state resonance dominates contributions to the bound-free dynamic structure function, leading to the predicted resonances in the scattered X-ray spectrum. Similar resonant features are shown to occur in all elements in the periodic table between Ca and Mn (20 <= Z <= 25).
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A recent and unexpected discrepancy between textit{ab initio} simulations and the interpretation of a laser shock experiment on aluminum, probed by X-ray Thomson scattering (XRTS), is addressed. The ion-ion structure factor deduced from the XRTS elastic peak (ion feature) is only compatible with a strongly coupled out-of-equilibrium state. Orbital free molecular dynamics simulations with ions colder than the electrons are employed to interpret the experiment. The relevance of decoupled temperatures for ions and electrons is discussed. The possibility that it mimics a transient, or metastable, out-of-equilibrium state after melting is also suggested.
The influence of finite relaxation times on Thomson scattering from warm-dense plasmas is examined within the framework of the average-atom approximation. Presently most calculations use the collision-free Lindhard dielectric function to evaluate the free-electron contribution to the Thomson cross section. In this work, we use the Mermin dielectric function, which includes relaxation time explicitly. The relaxation time is evaluated by treating the average atom as an impurity in a uniform electron gas and depends critically on the transport cross section. The calculated relaxation rates agree well with values inferred from the Ziman formula for the static conductivity and also with rates inferred from a fit to the frequency-dependent conductivity. Transport cross sections determined by the phase-shift analysis in the average-atom potential are compared with those evaluated in the commonly used Born approximation. The Born approximation converges to the exact cross sections at high energies; however, differences that occur at low energies lead to corresponding differences in relaxation rates. The relative importance of including relaxation time when modeling X-ray Thomson scattering spectra is examined by comparing calculations of the free-electron dynamic structure function for Thomson scattering using Lindhard and Mermin dielectric functions. Applications are given to warm-dense Be plasmas, with temperatures ranging from 2 to 32~eV and densities ranging from 2 to 64~g/cc.
A scheme for analyzing Thomson scattering of x-rays by warm dense matter, based on the average-atom model, is developed. Emphasis is given to x-ray scattering by bound electrons. Contributions to the scattered x-ray spectrum from elastic scattering by electrons moving with the ions and from inelastic scattering by free and bound electrons are evaluated using parameters (chemical potential, average ionic charge, free electron density, bound and continuum wave functions, and occupation numbers) taken from the average-atom model. The resulting scheme provides a relatively simple diagnostic for use in connection with x-ray scattering measurements. Applications are given to dense hydrogen, beryllium, aluminum, titanium, and tin plasmas. At high momentum transfer, contributions from inelastic scattering by bound electrons are dominant features of the scattered x-ray spectrum for aluminum, titanium, and tin.
The development of high-brightness free-electron lasers (FEL) has revolutionised our ability to create and study matter in the high-energy-density (HED) regime. Current diagnostic techniques have been very successful in yielding information on fundamental thermodynamic plasma properties, but provide only limited or indirect information on the detailed quantum structure of these systems, and on how it is affected by ionization dynamics. Here we show how the electronic structure of solid-density nickel, heated to temperatures of 10s of eV on femtosecond timescales, can be studied by resonant (Raman) inelastic x-ray scattering (RIXS) using the Linac Coherent Light Source FEL. We present single-shot measurements of the valence density of states in the x-ray-heated transient system, and extract simultaneously electron temperatures, ionization, and ionization potential energies. The RIXS spectrum provides a wealth of information on the valence structure of the HED system that goes beyond what can be extracted from x-ray absorption or emission spectroscopy alone.
Exploring and understanding ultrafast processes at the atomic level is a scientific challenge. Femtosecond X-ray Absorption Spectroscopy (XAS) is an essential experimental probing technic, as it can simultaneously reveal both electronic and atomic structures, and thus unravel their non-equilibrium dynamic interplay which is at the origin of most of the ultrafast mechanisms. However, despite considerable efforts, there is still no femtosecond X-ray source suitable for routine experiments. Here we show that betatron radiation from relativistic laser-plasma interaction combines ideal features for femtosecond XAS. It has been used to investigate the non-equilibrium transition of a copper sample brought at extreme conditions of temperature and pressure by a femtosecond laser pulse. We measured a rise time of the electron temperature below 100 fs. This first experiment demonstrates the great potential of the betatron source and paves the way to a new class of ultrafast experiments.
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