obtained by these schemes is limited for each gas and laser combination and is not continuously tunable.
Molecular gas lasers using gases such as HCN, CH3F, or methanol provide a number of discrete wavelengths in the wavelength region beyond 30 µm and extending to longer than 700 µm. Most of these lasers are optically pumped by pulsed CO2 lasers and emit pulses in the microsecond time domain; some, such as HCN, can be excited by a gas discharge. They have not found extensive use as sources for far-infrared spectroscopy because of the limited line tuning that is possible.
In summary, there is a lack of intense, tunable sources in the far-infrared region. In particular, there are no tunable sources producing the intense picosecond pulses needed for some of the potential experiments discussed below.
The FELs that are currently operational or under construction are summarized in Chapter 2. The Free Electron Laser for Infrared Experiments (FELIX) located in the Netherlands is of interest as an example of an operational user facility, while the Collaboration for an Infrared Laser at Orsay (CLIO) in France is a user facility that has begun to produce a number of scientific results.
In contrast to FELs proposed for the ultraviolet and x-ray regions, the development of FELs for the far-infrared wavelength region poses a relatively low technological risk. A number of operational systems, as well as systems nearing operation, have provided experience with the technology and are a guide to the cost of future machines.
It is important to note that the electron-beam energies required for operation of an FEL in the far infrared are substantially lower than those needed for operation at 2 to 5 µm, and this has a number of favorable consequences. Typical near-infrared FELs use beam energies of the order of 40 MeV. The beam energy for a longer-wavelength machine is in the 10- to 15-MeV range. Lowering the beam energy significantly below 15 MeV can reduce the shielding requirements, and thus the cost and the space requirements. It is a goal of some current projects to develop a low-cost far-infrared FEL that could be supported as a departmental or individual investigator machine.
Some technology needs for further development of FIR FELs can be specified now. Stability of the RF power source has been shown to be a problem in some machines and can lead to undesirable and uncontrollable frequency drifting. The use of photocathodes activated by a pulsed laser (e.g., the 355-nm third harmonic of a Nd:YLF [neodymium yttrium lithium fluoride] laser) or the use of RF subharmonic bunching allows the time between micropulses to be made long enough, typically a few nanoseconds, for pump-probe measurements. The Nd:YLF laser harmonics can also be used to pump a tunable dye laser or may themselves serve as