Surface Enhanced Photodissociation of Physisorbed Molecules

We have investigated the photon stimulated desorption of ions from physisorbed layers of CO and ${\mathrm{O}}_2$ on graphite using VUV synchrotron radiation in the range of 13 to 35 eV. In both systems we find a significant enhancement in the monolayer regime of the desorption of ${\mathrm{O}}^{-}$ ions via resonant states which correspond to direct molecular photoabsorption. This enhancement, which is accompanied by suppression of positive ion desorption, is attributed to surface electron attachment following molecular photoexcitation.