Parallel excitation pathways in ultrafast interferometric pump-probe correlation measurements of hot-electron lifetimes in metals

Hot electron (E - E-Fermi = 0.75 to 1.55 eV) lifetimes for cesiated Cu(100) and Cu(lll) surfaces are measured via interferometric time-resolved two-ph oton photoemission with a 19-fs intensity FWHM mode locked Ti:sapphire lase r at 1.55 eV, The data are analyzed using the optical Bloch equations and a laser pulse characterized in situ via surface second-harmonic generation i nterferometric autocorrelation. It is found that the retrieved hot-electron lifetimes are unphysically fast, and have a strong dependence on the tempe rature of the sample and the polarization of the laser. A simple explanatio n for the data is that the measured signal consists of contributions from t ransitions through both virtual and real intermediate states.