Non-resonant inelastic X-ray scattering of core electrons is a prominent tool for studying site-selective, i.e. momentum-transfer-dependent, shallow absorption edges of liquids and samples under extreme conditions. A bottleneck of the analysis of such spectra is the appropriate subtraction of the underlying background owing to valence and core electron excitations. This background exhibits a strong momentum-transfer dependence ranging from plasmon and particle-hole pair excitations to Compton scattering of core and valence electrons. In this work an algorithm to extract the absorption edges of interest from the superimposed background for a wide range of momentum transfers is presented and discussed for two examples, silicon and the compound silicondioxide.

Giant dipole resonances are collective phenomena which can be found in systems ranging from atoms through clusters to solids. In atomic and solid state physics such excitations are usually studied by soft X-ray absorption, photoelectron and electron energy-loss spectroscopies. With the advent of thirdgeneration synchrotron radiation sources, nonresonant inelastic X-ray scattering became a prominent tool to study truly bulk sensitive shallow absorption edges with high energy photons. The method is not limited to measuring dipole transitions but allows the study of final states of different symmetry due to monopole and quadrupole transitions employing its momentum-transfer dependence. In this paper the potential of nonresonant inelastic X-ray scattering to probe low-energy excitations is emphasized with the focus on symmetry selectivity, study of liquids and high pressure applications. As an example, nonresonant inelastic X-ray scattering spectra of the barium 4d-f giant dipole resonance in barite are discussed.

Double plasmons are unique fingerprints of dynamical correlations in the model of a free-electron gas beyond the random phase approximation. A combined experimental and theoretical study of double-plasmon excitations in three simple metals Na, Mg, and Al is presented. The intensities, spectral shapes, and dispersions of these excitations are analyzed as a function of momentum. The measured double-plasmon intensity is found to increase with a decreasing electron density, which is in good agreement with the expectations of strength of the correlation effects as a function of the electron gas density. The overall quantitative agreement between the experimental and the theoretical results is very good, while the remaining discrepancies may be due to higher order correlation effects and band-structure effects.

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X-ray photon-correlation spectroscopy is used to measure the dynamic structure factor f(q,tau) of gold particles moving on the surface of thin polymer films. Above the glass transition of the polymer the peculiar form f(q,tau) approximately exp[-(Gamma tau)(alpha)] is found with 0.7 < alpha < 1.5, depending on sample age and temperature. The relaxation rates Gamma scale linearly with q, excluding a simple Brownian diffusive motion. This type of behavior, already observed in aging bulk soft matter systems, is explained by a power law distribution of particle velocities due to ballistic motion.