Inelastic X-ray Scattering Study on Plasmon Dispersion in Liquid Cs

Abstract

We study the plasmon dispersion of liquid Cs using inelastic X-ray scattering techniques in the range of momentum transfer, 0.32 ≤ q ≤ 0.55 Å−1. While the previous electron energy loss spectroscopy experiments reported that the plasmon energy in solid Cs decreases with increasing q below q = 0.5 Å−1, we find that the plasmon energy in liquid Cs tends to increase with q in the range 0.32 ≤ q ≤ 0.55 Å−1. We determine the slope of the dispersion normalized by the plasmon energy at q = 0, by fitting the linear function around this q range. The slope for liquid Cs is 0.10 ± 0.12 Å, whereas that for solid Cs is −0.11 ± 0.07 Å. We qualitatively interpret that the observed variation in the dispersion relation upon melting is attributable to the reduction of the band structure effect (the effect of the s–d transitions). In addition, the plasmon dispersion of Cs is compared with that of Rb for both the solid and the liquid state. Although the dispersion curves of Rb and Cs are in appearance qualitatively different from each other, common characteristics are seen for these elements when we take difference between the experimental results and the electron gas prediction. We interpret that this behavior reflects the s–d transitions, which have been theoretically shown to cause the deviation of the plasmon dispersions from the electron gas prediction commonly in Rb and Cs.