Violation of the electric-dipole selection rules in indirect multiphoton excitation of image-potential states on Ag(100)

Photoemission from image potential states on Ag(100) is investigated using angle resolved multiphoton photoemission induced by 150 fs laser pulses. For the first time we demonstrate that image potential states populated by indirect transitions can be observed with light polarized parallel to the plane of incidence and light polarized normal to the plane of incidence. The latter is a process normally forbidden by the dipole transition selection rules. These findings are related to the creation of a hot-electron population whose properties largely remains to be understood.