Electron energy losses at the surface of Ga, NiGa and CoGa have been s
tudied by X-ray photoemission spectroscopy (XPS). These features are i
nterpreted as being due to surface plasmons. For Ga metal the experime
ntal result is in good agreement with the surface plasmon energy obtai
ned by a free-electron model. At the surface of Coca and NiCa alloys,
the corresponding energy is about 1 eV higher than that of pure Ga met
al. This result is discussed in the framework of a free-electron model
as well.