Dispersion of the Volume Plasmon of Silicon (16.7 eV) at Large Wave Vectors

The dispersion of the volume plasmon of silicon at 16.7 eV has been measured with 50-keV electrons in the directions $〈100〉$ and $〈111〉$ up to the critical wave vector $q_c\simeq 1.1$ ${\mathrm{\AA }}^{-1\mathrm{}}$. It shows a strong anisotropy. Along the $〈100〉$ direction the $q$ dependence could be measured up to large $q$ vectors of ∼2.4 ${\mathrm{\AA }}^{-1\mathrm{}}$. At $q\gtrsim q_c$ the slope of the dispersion curve decreases strongly in a manner similar to what has been found earlier for Al and Be.