Can we control lifetimes of electronic states at surfaces by adsorbate resonances?

Femtosecond time-resolved two-photon photoemission is used to study the excited electronic states of a Cu(111) surface covered with a monolayer of physisorbed O2 on top of one to five Xe spacer layers. The spectra are dominated by a state 4.0 eV above EF whose lifetime depends strongly on the number of Xe spacer layers. The lifetime decreases with increasing number of spacer layers from (650±30) fs for one Xe layer to (90±10) fs for five layers. To explain this trend we propose a model where the image potential states couple to a negative-ion resonance of the O2 molecule.