Structure and Dynamics of Molecular Monolayers Probed by Infrared- Visible Sum Generation.

This survey of the new nonlinear spectroscopic technique of infrared-visible sum generation is to be published in the Proceedings of the Electrochemical Society Symposium on Nonlinear Optics and Ultrafast Phenomena. The sum generation technique probes molecular structure and orientation at metal and semiconductor surfaces with monolayer sensitivity. The vibrational spectra of molecules at the interface are obtained by mixing visible and infrared light pulses to generate a reflected sum frequency signal whose magnitude shows resonances when the infrared is tuned to molecular vibrations. The spectral selection rules (vibrational modes must be both infrared and Raman active to be observed) and a unique sensitivity of the spectroscopy to molecular orientation are presented. Sum spectroscopy is particularly interesting as a dynamic probe - monolayer vibrational spectra may be obtained with picosecond temporal resolution - and allows measurements of the lifetimes of vibrational excited state lifetimes at surfaces. The first time-resolved measurements of molecular vibrational relaxation at a bulk metal surface, carried out using transient sum generation spectroscopy, are discussed.