Ion implantation has been demonstrated as a valuable new tool to improve the photoelectric emission of pure metals. An enhancement of the photoelectric sensitivity by a factor from 3 to 50 was measured in potassium ion-implanted tungsten irradiated by 450 fs laser pulses with a peak intensity of a few GW cm(-2). Investigation of the chemical and physical properties al the origin of this enhancement is reported in this paper. The dose and the profile of the implanted ions were controlled by Rutherford backscattering spectroscopy. The chemical structure of the surface was determined by X-ray photoelectron spectroscopy, Grazing incidence X-ray diffraction measurements have shown the presence of metallic potassium. The changes on the reflectivity R(lambda) and the ellipsometric measurements N(lambda) and K(lambda) can be interpreted both by the presence of an interface layer due to the radiation damage and by a modification of the dielectric function of W in the presence of the K+ impurity.