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 n³ 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.