Development of a time-resolved multiplex surface sum-frequency spetrometer

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We have developed a time-resolved multiplex infrared-visible sum-frequency spectrometer with wavelength tunability of the visible probe.An overview of the spectrometer :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). The repetition rate of the laser system is 1-kHz. An SF spectrum over a 400 cm^<-1> spectral region with 8-13 cm^<-1> resolutions is obtainable by a single measurement. UV pulse at 267 nm, a third harmonic of 800 nm femtosecond output, is also available to photoexcite a sample for pump-probe type SFG measurements.Applications of the speterometer :(1)Alkanethiolate monolayer on AuSum-frequency spectra were measured on a self-assembled monolayer of octadecanethiolate on a Au substrate for the CH stretching region with 400,477,535,and 633 nm visible probe. Vibrational band shapes in VRSFG spectra were drastically changed with wavelength of the visible laser. We ascribed this spectral change to the change in the phase of vibrational nonresonant signal from Au substrate with wavelength of visible probe.(2)All-trans retinoate ion monolayer on TiO_2(110)An all-trans retinoate ion monolayer was assessed by sum-frequency spectroscopy with 480-nm visible probe. Two strong vibrational SFG bands were observed at 1580 and 1405 cm^<-1>. The strong intensities of the bands were due to electronic resonance of the SFG signal. The 1580 cm^<-1> band was assigned to the predominantly in-phase stretching vibration of the conjugated C=C double bonds of retinyl chromophore, and the 1405 cm^<-1> band to the symmetric -COO stretching vibration of carboxylate ions attached to TiO_2 (110) with the two oxygen atoms on bridge-bonded fivefold coordinated Ti atoms.