Second-harmonic generation from Na-covered Cu(110): intensity enhancement by inter-surface-state transitions?

Deposition of Na at submonolayer coverage onto Cu(110) at 100 K causes a dramatic increase of second-harmonic intensity generated at the lambda = 1064 nm fundamental wavelength. Around 0.6 ML the resonance-like enhancement is about three orders of magnitude compared with clean Cu(110). We report on a detailed analysis of the emitted intensities and suggest a model where the intensity resonance around 0.6 ML coverage is driven by transitions between filled and empty electronic surface states. Deposition of Na at 300 K results in a reconstructed surface. Now the intensity resonance observed in SHG is localized at a coverage around 0.3 ML. No clear model emerges for the interpretation. In conclusion, the results indicate a very detailed influence of the adsorption geometry and thus do not support models based on atomic-like intra-adsorbate transitions of alkali-derived orbitals. (C) 2000 Elsevier Science B.V. All rights reserved.