First experimental programs with the free electron laser

Free electron lasers (FEL's) are not new instruments: for a long time, they have produced high-intensity photon beams, primarily in the infrared. They can be, therefore, excellent complementary facilities to synchrotron radiation, whose primary spectral domains are the ultraviolet and the xrays. The use of free electron lasers for experiments, however, has been quite limited; this situation is rapidly changing, with programs being put in operation at Santa Barbara, at Vanderbilt University in Nashville, at the LURE laboratory in Orsay, at the FOM in Holland and elsewhere. We present here our practical experience with one of the first programs using the Vanderbilt FEL - which also constitutes at present one of the largest materials research programs with an FEL. Concrete results will be presented in two areas: two-photon absorption and internal photoemission measurements of interface energy barriers (the so-called FELIPE technique), The discussion will analyze the specific problems and requirements of this class of experiments. The main point, however, is the practical evidence that the FEL can sustain long-term research programs very much like synchrotron facilities. The future possibilities are also briefly discussed, in particular as far as the possible extension of FEL's to the x-rays is concerned.