Momentum-resolved lifetimes of image-potential states on Ag(001)

Binding energies and decay rates of image potential states at an Ag(001) surface have been investigated with time-resolved two-photon photoemission. For the first four image potential states the binding energies are determined to be 561, 170, 72 and 37 meV with respect to the vacuum level. Lifetimes of the first three states are extracted to be 57, 165 and 380 fs at k_∥ = 0. With increasing parallel momentum k_∥ the lifetime of the n = 1 state decreases such that the inverse lifetime scales proportional to the kinetic energy within the parallel motion with 34 meV/eV. As light source for the pump-probe photoemission experiments a novel all fiber based femtosecond laser system with a repetition rate of 1.5 MHz has been used.