Dynamics of O2 photodesorption from metal clusters: A significant difference from bulk behaviour

Photodesorption of O2 from size-selected Ag n O 2 - cluster anions with n = 2, 3, 4 and 8 was studied using time-resolved photoelectron spectroscopy (TR-PES). The spectra indicate that relaxations of photo-excited Ag n O 2 - clusters with n = even numbers accompany ultrafast direct O2 photodesorption. For the odd-numbered cluster Ag 3 O 2 - , in contrast, a long-lived excited state is observed, since O2 might be dissociatively chemisorbed, suppressing direct photodesorption of oxygen. Both, direct desorption and long-lived excited states, have not been observed from adsorbate covered metal surfaces, suggesting unique photochemical properties of such small clusters.