Plasmon structure in the x-ray absorption spectra of metals

The effect of plasmon production on the x-ray photoabsorption spectra of metals has been studied in the framework of the many-body perturbation theory. The plasmon starts to contribute very weakly at the frequency $E_F+{\omega }_p^o$ ($E_F$ is the absorption-edge frequency and ${\omega }_p^o$ is the frequency of a zero-momentum plasmon). The plasmon absorption edge at this frequency is, in fact, so weak that it does not have any significant effect on the calculated spectrum and possibly cannot be detected. However, a new structure appears at a higher frequency. This structure occurs as a dip of 5%-7% at the frequency $E_F+{\omega }_p^c$ where ${\omega }_p^c$ is the highest frequency of the plasmon dispersion curve beyond which plasmon production is precluded. The physical significance of $E_F+{\omega }_p^c$ is that it is the lowest x-ray frequency at which the plasmon excitation can become a real process during x-ray photoabsorption in metals. The existence of the plasmon structure at this frequency allows an estimation of the extent of plasmon dispersion in metals.