Temperature dependence of surface state lifetimes, dephasing rates and binding energies on Cu(111) studied with time-resolved photoemission

The ultrafast electron dynamics of surface states on Cu(lll) is investigated as a function of temperature between 25 K and 460 K employing time-resolved two-photon photoemission (2PPE) spectroscopy. Analysis of the thermally-induced energy shift of the unoccupied n = 1 image potential state based on a multiple reflection model allows a precise determination of the position of the upper edge (L6+) Of the sp-gap in the (111)-direction (EL6+(T) - E-F = 4.15 eV -(0.26 meV/K)T). We find that the lifetime of the n = 1 image state decreases from 22 +/- 3 fs at 25 K to 14 +/- 3 fs at 350 K. This is attributed to the increasing penetration depth of the image state wave function into the bulk at higher temperatures, where the image state crosses the band edge. The phonon contribution to the electronic dephasing of the n = 0 surface state and the n = 1 image state on Cu(lll) is determined from their temperature-dependent linewidths using three-level optical Bloch equations and is found to correlate with their wave function overlap with bulk states. (C) 1998 Elsevier Science B.V.