APPLICATIONS OF INFRARED FREE-ELECTRON LASERS - BASIC RESEARCH ON THE DYNAMICS OF MOLECULAR-SYSTEMS

Applications of tunable infrared (IR) picosecond (ps) pulses generated by free-electron lasers (FEL's) are considered from several perspectives. First, general considerations are presented that discuss characteristics that an IR ps FEL must have to be successfully used as an optical source in order to perform complex ultrafast experiments commonly undertaken with conventional ps lasers. The features that FEL's can have that will make them superior to conventional sources for the investigation of complex molecular systems in condensed and gas phases are pointed out, and the difficulties of combining an FEL with an intricate experimental apparatus are brought forward. Next, ps photon echo experiments on a near-IR dye molecule in a low-temperature glass, performed using the Stanford superconducting linear accelerator-pumped FEL, are discussed to illustrate the ability of FEL's to act as light sources for sophisticated applications. The methods employed and the problems encountered in the echo experiments are described. Finally, the applications of IR ps FEL's to the study of molecular vibrational dynamics are discussed, including detailed numerical feasibility calculations. It is shown how to determine the extent of vibrational excitation for a given system, given readily determined molecular and FEL parameters; substantial excited vibrational populations can be generated in the examples considered. Once a vibration is excited, a variety of methods of probing the subsequent dynamics are discussed. These include absorption probing (pump-probe experiment), probing by anti-Stokes Raman scattering, and probing by hyper-Raman scattering. A class of experiments referred to as nanoheating, i.e., selective heating on a nanometer length scale, is described. Biomedical and biophysical applications of nanoheating are suggested.