Plasmon Excitation in Compton-Scattering Experiments

X radiation (Cr $K\beta =2.\mathrm{}08$ Å) scattered by lithium, beryllium, and graphite at angles of 5°, 10°, and 15° was analyzed. Besides the known Rayleigh and Compton radiations, a new component, characteristic of the scattering substance, was observed and attributed to plasmon excitation. Curves giving the plasmon dispersion relation were drawn from which the values of the plasmon excitation energies for lithium, beryllium, and graphite were found to be 8.1, 20.7, and 6.5 eV, respectively. These results agree with the theoretical predictions of Nozieres and Pines (1959) and with previous experiments, such as fast-electron scattering (measurements of the characteristic energy loss) and optical measurements. For various reasons, such plasmon lines were not observed in many other experiments. In most cases the radiation used was too hard and the scattering angle too large; thus the conditions necessary for the appearance of the plasmons were not realized. However, there is one case (Theodossiou and Vosnidis, 1966) where the line might have been observed in lithium. A possible explanation for its nondetection is given.