Second harmonic observation of Cu(1 1 1) surface: In situ measurements during molecular adsorption

Second harmonic generation (SHG) spectroscopy using a tunable femtosecond laser has been demonstrated as an effective and a practical in situ monitor of surface electronic states during adsorption processes. We have successfully shown this technique to be suitable for the study of surface electronic states, not only those induced by adsorbed molecules but also those associated with clean surfaces. By observing the change in the SH signals from a Cu(1 1 1) surface during exposure to benzene or CO molecules, we discovered new resonances of the clean Cu surface that could not be ascribed to the well-known surface state (SS)-image state (IS) transition. One of these was from a surface site that is less likely to adsorb benzene, where the SH signal intensity was kept constant until the Cu surface was covered by the sub-monolayer.