Femtosecond dynamics of electronic excitations of adsorbates studied by two-photon photoemission pulse correlation: CO/Cu(111)

An equal pulse correlation technique based on angle-resolved two-photon photoemission is employed to investigate the lifetime of electronic excitations of an adsorbate on a single crystal metal surface. Photoemission from an occupied surface state on Cu(111) by a non-resonant two-photon process via the sp-band gap is used to characterize directly at the surface the width and shape of 65 fs laser pulses at a photon energy of hv = 3.4 eV. The 2PPE correlation technique allows the establishment of an upper limit of τ < 20 fs for the lifetime of the 2π∗ resonance of CO adsorbed on Cu(111), whereas from the spectral width of the 2π∗ level a lower limit of ≈1 fs is estimated.