Femtosecond relaxation dynamics of image-potential states

Relaxation dynamics of the n=1 and 2 image-potential states on Ag(100) and Ag(111) are investigated using femtosecond pump-probe techniques. Transient two-photon photoemission measurements are performed using femtosecond pulses with photon energies of 2.00 and 4.00 to 4.35 eV, generated via self-phase modulation and frequency doubling. We measure the lifetime of the n=1 and 2 image states on Ag(100) to be 25±10 and 180±20 fs, respectively. The lifetimes are within the range predicted by theoretical models, and are consistent with the expected ${\mathit{n}}^3$ lifetime scaling. In contrast, measurements of the n=1 and 2 image states on Ag(111) reveal substantially shorter lifetimes, <20 fs. This compares favorably with recent theoretical models that predict shorter lifetimes due to the structure of the (111) surface and its effect on the image-potential states.