Collective electron effects in molecular liquids

Accurate Measurements of Dielectric and Optical Functions of Liquid Water and Liquid Benzene in the VUV Region (1-100 eV) Using Small-Angle Inelastic X-ray Scattering

Using a third-generation synchrotron source (the BL12XU beamline at SPring-8), inelastic X-ray scattering (IXS) spectra of liquid water and liquid benzene were measured at energy losses of 1-100 eV with 0.24 eV resolution for small momentum transfers (q) of 0.23 and 0.32 au with ±0.06 au uncertainty for q. For both liquids, the IXS profiles at these values of q converged well after we corrected for multiple scattering, and these results confirmed the dipole approximation for q ≤ ∼0.3 au. Several dielectric and optical functions [including the optical oscillator strength distribution (OOS), the optical energy-loss function (OLF), the complex dielectric function, the complex index of refraction, and the reflectance] in the vacuum ultraviolet region were derived and tabulated from these small-angle (small q) IXS spectra. These new data were compared with previously obtained results, and this comparison demonstrated the strong reproducibility and accuracy of IXS spectroscopy. For both water and benzene, there was a notable similarity between the OOSs of the liquids and amorphous solids, and there was no evidence of plasmon excitation in the OLF. The static structure factor [S(q)] for q ≤ ∼0.3 au was also deduced and suggests that molecular models that include electron correlation effects can serve as a good approximation for the liquid S(q) values over the full range of q.

Similarity treatment of phase effects in stopping power of low energy heavy charged particles

Bragg's additivity rule is the basis for calculating stopping powers of compounds from data on their constituent elements. Major assumptions in applying it are that chemical binding and the physical state of the stopping material have negligible influence on the slowing down process. The fragmentary and, at times conflicting evidence for phase effects for low energy heavy charged particles is summarised. A detailed survey of published stopping powers has been made and the information grouped according to target medium and phase. Each group has been fitted to universal curves of Lindhard type with the object of separating phase contributions to stopping. Least square fits have been made to each set of data. From these a quantitative measure of the effect is obtained and presented as stopping power ratios (vapour/condensed phase). From the experimental evidence available, it is concluded that phase effects do occur, decreasing the stopping power in solids and liquids as compared to the corresponding vapours, and are thought to be due largely to changes in electronic excitation levels as the phase is changed, combined with a physical-state dependence of the effective ion change at low energies. Magnitudes of the stopping power ratio deviate from unity by up to approximately 25%, but have significant associated errors, pointing to need for more accurate stopping power measurements.