Electronic resonant multiplex SFG spectroscopy with tunable UV probe

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We have constructed a multiplex IR-UV SFG spectrometer that has a wide tunable range (235-400 nm) of the UV probe wavelength. A prism monochromator was used as a tunable sharp-cut bandpass filter to reduce stray light due to the scattering of the UV probe so that any wavelength within the tunable range can be chosen as that of the visible probe.The IR-UV SFG spectrometer : The probe light source is based on a Ti : sapphire amplifier operated at 1 kHz. A femtosecond broad-bandwidth (200 cm^<-1>) IR pulse and a picosecond narrow-bandwidth (10 cm^<-1>) UV pulse are obtained as the probe lights by converting the fundamental output of the amplifier with nonlinear optical techniques. The wavelength of the UV pulse is tunable in the range of 235-400 nm. A grating polychromator (f=55 cm, 4320 grooves/mm) combined with a prism monochromator (f=20 cm) is used as the spectrograph for analyzing broad-bandwidth SFG signal. The prism monochromator acts as a tunable sharp-cut bandpass filter to reduce stray light due to the scattering of the UV probe.Test measurements : Test measurements were conducted on a p-mercaptobenzoic acid (HOOC-C_6H_4-SH) monolayer on a gold substrate. SFG spectra of p-mercaptobenzoic acid on a gold substrate were measured with 289 nm and 334 nm UV probes. Two vibrational bands were identified on the spectrum. These bands were assigned to phenyl ring stretch (〜1590 cm^<-1>), COOH stretch (〜1746 cm^<-1>). The SFG intensity of the stretch mode with 289 nm was three times larger than that with 334 nm probe. The enhancement was ascribed to due to electronic resonant effect with a strong absorption of the molecule located at 275 nm.