Optical second harmonic spectroscopy of rutile TiO2(110) in air

The reflected optical second harmonic (SH) intensity from rutile TiO2(110) was measured as a function of the fundamental photon energy, A sharp rise in the SH intensity was observed > 2 (h) over bar omega 3.4 eV. The sum frequency intensity also shows a rise at (h) over bar omega(1)+(h) over bar omega(2) = 3.4 eV. Thus the observed sharp rise of the SH intensity is due to a resonance with an electronic excitation at 3.4 eV, This energy is different from the two-photon resonance energy 2 (h) over bar omega = 3.02 eV of the bulk band gap of TiO2. Thus the origin of this resonance is attributed to surface electronic states. It was also found that the SH intensity pattern as a function of the azimuthal angle changes drastically when the fundamental photon energy is scanned across this onset energy. It is suggested that this is due to the variation of the share of the wavefunction of the excited electronic states at this energy. (C) 1998 Elsevier Science B.V. All rights reserved.