Abstract N 2 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⩽ hν⩽45 eV. The N 2 1π u valence level exhibits a vibrational fine structure which is composed of two multiplets corresponding to the molecular stretching frequencies in the ionic (ν +) and neutral (ν 0) electronic states. ν 0 gets excited through delocalization of the photohole in the physisorbed layer. The delocalization process occurs during the escape time τ e of the photoelectron (violation of the sudden approximation). By increasing the photon energy, τ e is shortened, which is mirrored in an intensity decrease of the neutral oscillator.