In general, the plasma frequency is related to carrier density. When the effective mass of electrons is constant, a larger value corresponds to the higher carrier density. HEA had the second-highest free electron density after Au, Ag, and Cu and the highest bound electron density among the six metals. Abundant free electron density is necessary to induce a considerable plasmon resonance, and the results suggest that HEA alloys are more "plasmonic" as compared with Pd and Pt. The increased density of bound electrons indicates that some abundant free electrons in Au, Ag, and Cu were converted to the bound electrons. This agrees with the XPS measurements, which show that almost all the binding energies are shifted to the lower values, suggesting that the electrons are accepted.