Lifetime of excited electronic states at surfaces:: CO-(2π*) resonance on Cu(111) and Cu(100) surfaces

A model study of the (2 pi*) resonance in the CO/Cu(111) and CO/Cu(100) systems is presented. Although the CO-(2 pi*) resonance is located inside the surface projected band gap in both systems, it is found to be very short-lived. These results are at variance with the apparently similar alkali/Cu systems where a strong reduction of the resonance decay by one-electron transfer to the substrate as compared to a free-electron metal surface is observed and attributed to the presence of the surface projected band gap. The differences between the two systems are analysed, allowing a discussion of the situations where a projected band gap induced stabilisation can be expected. The present results are compared with recent experimental results obtained by time resolved 2-photon photo emission. (C) 2001 Published by Elsevier Science B.V.