CAN WE CONTROL LIFETIMES OF ELECTRONIC STATES AT SURFACES BY ADSORBATE RESONANCES

Femtosecond time-resolved two-photon photoemission is used to study th e excited electronic states of a Cu(lll) surface covered with a monola yer of physisorbed O-2 on top of one to five Xe spacer layers. The spe ctra are dominated by a state 4.0 eV above E-F whose lifetime depends strongly on the number of Xe spacer layers. The lifetime decreases wit h increasing number of spacer layers from (650 +/- 30) fs for one Xe l ayer to (90 +/- 10) fs for five layers. To explain this trend we propo se a model where the image potential states couple to a negative-ion r esonance of the O-2 molecule. (C) 1998 Elsevier Science B.V.