PHOTOEMISSION FROM PHYSISORBED N2 - OBSERVATION OF NUCLEAR MOTION THROUGH VIOLATION OF THE SUDDEN APPROXIMATION

N2 physisorbed on top of a Xe spacer layer on Ag (111) has been studied by angle-resolved UV photoemission in the photon energy range 16.5 less-than-or-equal-to h-nu less-than-or-equal-to 45 eV. The N2 1-pi(u) valence level exhibits a vibrational fine structure which is composed of two multiplets corresponding to the molecular stretching frequencies in the ionic (nu+) and neutral (nu-0) electronic states. Nu-0 gets excited through delocalization of the photohole in the physisorbed layer. The delocalization process occurs during the escape time tau(e) of the photoelectron (violation of the sudden approximation). By increasing the photon energy, tau(e) is shortened, which is mirrored in an intensity decrease of the neutral oscillator.