Monolayer vibrational spectroscopy by infrared-visible sum generation at metal and semiconductor surfaces.

Infrared-visible sum generation spectroscopy, a potentially valuable probe of molecular vibrations at interfaces, has been recently demonstrated on glass and water substrates. We report sum generation spectra of molecular monolayers on chemically important metal and semiconductor surfaces, and compare them to infrared absorption spectra for each sample. Vibrational resonances are observed in the sum spectra, even in the presence of strong background sum generation from the substrates. The spectra obey electric dipole selection rules: vibrational modes must be both Raman and infrared active to show sum frequency resonances.