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 infr ared. They can be, therefore, excellent complementary facilities to sy nchrotron radiation, whose primary spectral domains are the ultraviole t and the xrays. The use of free electron lasers for experiments, howe ver, has been quite limited; this situation is rapidly changing, with programs being put in operation at Santa Barbara, at Vanderbilt Univer sity in Nashville, at the LURE laboratory in Orsay, at the FOM in Holl and and elsewhere. We present here our practical experience with one o f 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 absorpti on and internal photoemission measurements of interface energy barrier s (the so-called FELIPE technique), The discussion will analyze the sp ecific problems and requirements of this class of experiments. The mai n point, however, is the practical evidence that the FEL can sustain l ong-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.