High-resolution sum-frequency generation from alkylsiloxane monolayers

During the last decade, infrared optical sum-frequency generation (SFG) has been demonstrated to be a valuable technique for acquiring vibrational spectra of molecules at interfaces. However, the instrumentation that most frequently has been employed, which is based upon picosecond-scale pulse-width lasers, is fundamentally limited in the spectral resolution that can be achieved (typically ~6 cm). We have optimized an older technology, based on a nanosecond-scale pulse-width laser, which enables the infrared frequency to be specified to within 0.2 cm. Exploiting this advantage, we have observed previously unrecognized features in the SFG spectrum of self-assembled monolayers. We demonstrate this through experiments performed on rubbed octadecylsiloxane (ODS) on glass, a system of considerable importance in liquid-crystal-display technology.