FEMTOSECOND DYNAMICS OF ELECTRONIC EXCITATIONS OF ADSORBATES STUDIED BY 2-PHOTON PHOTOEMISSION PULSE CORRELATION - CO CU(111)/

An equal pulse correlation technique based on angle-resolved two-photo n photoemission is employed to investigate the lifetime of electronic excitations of an adsorbate on a single crystal metal surface. Photoem ission 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 a t the surface the width and shape of 65 fs laser pulses at a photon en ergy of h nu = 3.4 eV. The 2PPE correlation technique allows the estab lishment of an upper limit of tau < 20 fs for the lifetime of the 2 pi resonance of CO adsorbed on Cu(111), whereas from the spectral width of the 2 pi level a lower limit of approximate to 1 fs is estimated.