the FEL output with a visible laser but offers the advantage of detecting the signal in the visible region. The strongest motivation for using these nonlinear techniques would be to characterize the molecular structure and coupling of energy from the molecule to the substrate or surface. There is also some interest in measuring surface and interfacial electronic properties in this region by nonlinear optical methods.
There are some initial results from these types of experiments using FELs, albeit at shorter wavelengths. The French group using CLIO has reported on the use of sum-frequency generation to study the Pt-methanol interface using 5-µm radiation (Peremans and Tadjeddine, 1994; Peremans et al., 1993).
Surface science experiments would use several unique properties of FEL radiation: (1) continuously tunable high-intensity radiation, which would allow probing of specific vibrational frequencies with good signal-to-noise ratio and (2) picosecond pulses, which would allow pump-probe techniques to be used to study energy transfer processes at the surface.
The study of energy transfer in molecules in the gas and liquid phase would be enhanced by the availability of pulsed sources that could access lower-frequency molecular modes. A recent report from the CLIO group discusses studies of the kinetics of d2ethane isomerization using FEL radiation (Roubin et al., 1994). The experiment used radiation of wavelengths no longer than 10 µm and was not time resolved. Therefore, it might have been carried out using ordinary lasers. However, it suggests a class of experiments to study mode-selective chemistry that would require high-powered, short-pulsed sources capable of exciting the full range of molecular vibrations, and these future experiments would require a far-infrared FEL. The Stanford FEL was used in studies of vibrational infrared photon echoes in a liquid and glass (Zimdars et al., 1993). Once again, since this particular experiment was done using 5.1-µm radiation (although here picosecond pulses were required), it could have been done with sophisticated laboratory lasers. However, an important extension of this type of work would be to investigate lower-frequency modes closer in frequency to vibrations of the surrounding medium in order to study the transfer of energy from the molecule to the medium. These longer-wavelength experiments would require far-infrared picosecond pulses and could only be done using an FEL. Although these experiments used wavelengths at the short-wavelength end of the region of interest, they are examples of the kind of chemical studies that can be performed.
For bulk, homogeneous materials the photon energies accessible to the infrared-FEL overlap essentially completely the principal excitations in condensed