Resonant surface second harmonic generation from An films on NaCl(100)

We have found that the resonance energy of optical second-harmonic (SH) generation from An films on NaCl(100) depends strongly on the film thickness. The resonance peak of the SH intensity was found at 2 (h) over bar omega = 3.2 eV for the island film of mean thickness 0.4 mn, while it appeared at 2 (h) over bar omega = 2.4 eV for the continuous film of thickness 40 nm. We suggest that the SH intensity peak observed at 2 (h) over bar omega = 3.2 eV originates from a resonant transition from occupied to unoccupied electronic levels of the (100) surface on top of the An islands. The SH intensity peak observed at 2 (h) over bar omega = 2.4 eV for the film thickness of 40 nm originates from the roughness-induced linear dielectric property of the film.