A multiplex infrared-visible sum-frequency spectrometer with wavelength tunability of the visible probe

We present a multiplex infrared-visible sum-frequency (SF) spectrometer with wavelength tunability of the visible probe. The spectrometer is based on a two-channel optical parametric amplifier producing a broad-bandwidth femtosecond infrared pulse and a narrow-bandwidth picosecond visible pulse (470–800 and 401 nm). A SF spectrum over a 400 cm−1 spectral region was obtainable by a single measurement. SF spectra of an octadecanethiol monolayer on a gold substrate were measured with visible probes of different wavelengths from 401 to 633 nm. The spectral shape showed remarkable dependence on the visible probe wavelength. The dependence was ascribed to resonance to a surface electronic state of the substrate existing in the visible region.