an ultraviolet or visible laser source for two-color experiments. Sum-frequency experiments, described below, are one example of such two-laser studies.

The existing FELs provide a good guide to potential costs. The FELIX FEL in the Netherlands covers the wavelength regions from 5 to 30 µm and 16 to 110 µm using two separate beamlines and was designed to operate as a user facility. FELIX cost approximately $8.5M to build and install in an existing building. However, it is important to distinguish the cost of the machine from that of the complete user facility. The latter entailed remodeling and partially equipping five user rooms and cost an additional $3.5M. The 1994 operating costs for FELIX are budgeted at $1.75M and are expected to provide for 2000 hours of beam time; this translates to about $900/hr.

SCIENTIFIC OPPORTUNITIES

Not only have experiments using FEL radiation in the spectral region from 10 to 1000 µm been proposed, but recent results from several of the currently operational devices also give some sense of what can be achieved. Some of the areas in which experiments have been proposed or conducted are surveyed below.

Surface Science

Much has been learned about molecules and atoms adsorbed onto surfaces through the accessibility of tunable infrared sources, both coherent and incoherent, that can measure intramolecular vibrations in the infrared between 1 and 10 µm. Fourier-transform infrared and Raman scattering have extended the range of measurement of intramolecular vibrations down to about 400 cm−1 (25 µm).

There is substantial interest in extending this low-frequency limit even further, because one could then access the regime where most frustrated translational and rotational modes (i.e., modes associated with adsorbate-surface bonds) lie. Internal torsional modes of molecules are also located here, and these have never been seriously studied in chemisorption systems for solid, liquid, or polymeric surfaces. The situation is similar for measurements of the intermolecular modes in van der Waals clusters, which provide information about the structure and dynamics of these systems. If one wants to understand the molecule-surface bond and how it is modified, for example, by adsorption of other species during a chemical reaction, then this region of the spectrum is of crucial interest. Moreover, measuring the energy distributions and line shapes of these modes is important for understanding how such modes couple to and dissipate their energy into the electron or phonon continuum of the substrate.

Such experiments could be conducted either in a reflection or absorption mode or by resonant nonlinear optical studies such as sum-difference frequency generation and second harmonic generation. The latter requires synchronization of



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