Plasmon behavior of Zn from electron-energy-loss spectroscopy

The momentum-dependent dielectric function of Zn was measured using electron-energy-loss spectroscopy (EELS) in transmission. The loss function in the low-energy region is found to be a superposition of a sharp absorption feature due to the excitation of electrons from the $3d$ level at a binding energy of 9.4 eV and a broad not well pronounced feature at about 14 eV, which is attributed to the collective plasmon excitation of the free charge carriers. This latter feature becomes more pronounced with increasing momentum transfer due to a strong decrease of the core-level oscillator strength. The decrease of oscillator strength leads to an initially vanishing dispersion of the plasmon, and to a reduction of the plasmon line width with increasing momentum transfer. This behavior can qualitatively be understood in the framework of a Drude-Lorentz approach.