Doubly resonant sum-frequency generation (DR-SFG) spectroscopy of fluorescein-4-isothiocyanate isomer-I (FITC) monolayers on platinum was performed. Vibrational spectra of the monolayers for the IRwavenumber of 1750-1450 cm −1 were measured with visible probes ranging from 431 to 582 nm. Two vibrational bands at 1643 and 1610 cm −1 were observed, and their DR-SFG excitation profiles displayed different shapes. By rinsing the monolayers with an alkaline solution, the smaller wavenumber band disappeared and the larger wavenumber band gained intensity. On the basis of the spectral response to the rinsing, we concluded that the FITC molecules existed on platinum as deprotonated and protonated forms; the former corresponds to the 1643 cm −1 band and the latter to the 1610 cm −1 band. The deprotonated form was assigned to an anionic surface species, and the protonated form to a neutral surface species by comparing the DR-SFG excitation profiles with electronic absorption spectra of the protolytic forms of fluorescein in an aqueous solution (Sjöback R et al., Spectrochimica Acta A 1995; 51: L7-L21). The results demonstrate that the measurement of DR-SFG excitation profiles is a useful technique to identify chemical species of monolayers on metal surfaces.

We have constructed a multiplex infrared-ultraviolet (IR-UV) sum-frequency generation (SFG) spectrometer that has a wide tunable range (235-390 nm) of the UV probe wavelength. The tunable UV probe was obtained by doubling the signal output of an optical parametric amplifier pumped by a 400 nm picosecond pulse. 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 UV probe. The SFG spectra of p-mercaptobenzoic acid on a gold substrate was measured with 289 and 334 nm UV probes. The SFG vibrational band intensities due to the carbonyl stretch mode and a phenyl ring stretch mode measured with the 289 nm probe were approximately three times larger than those measured with the 334 nm probe. The enhancement was ascribed to an electronic resonant effect.

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