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A survey of biophysical and biomedical applications of free-electron lasers ͑FELs͒ is presented. FELs are pulsed light sources, collectively operating from the microwave through the x-ray range. This accelerator-based technology spans gaps in wavelength, pulse structure, and optical power left by conventional sources. FELs are continuously tunable and can produce high-average and high-peak power. Collectively, FEL pulses range from quasicontinuous to subpicosecond, in some cases with complex superpulse structures. Any given FEL, however, has a more restricted set of operational parameters. FELs with high-peak and high-average power are enabling biophysical and biomedical investigations of infrared tissue ablation. A midinfrared FEL has been upgraded to meet the standards of a medical laser and is serving as a surgical tool in ophthalmology and human neurosurgery. The ultrashort pulses produced by infrared or ultraviolet FELs are useful for biophysical investigations, both one-color time-resolved spectroscopy and when coupled with other light sources, for two-color time-resolved spectroscopy. FELs are being used to drive soft ionization processes in mass spectrometry. Certain FELs have high repetition rates that are beneficial for some biophysical and biomedical applications, but confound research for other applications. Infrared FELs have been used as sources for inverse Compton scattering to produce a pulsed, tunable, monochromatic x-ray source for medical imaging and structural biology. FEL research and FEL applications research have allowed the specification of spin-off technologies. On the horizon is the next generation of FELs, which is aimed at producing ultrashort, tunable x rays by self-amplified spontaneous emission with potential applications in biology.

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Pronounced Fano-like resonant effects are observed in the photoemission spectra of the localized U 5f and 02p -U 5f, 6d bonding bands of U02 when photon energies near the U 5d3/2 $/2 thresholds (105 and 97.1 eV, respectively) are employed. The constant-final-state spectra are well hv described by calculated transition energies and oscillator strengths for the 5d' Sf '~5d 5f ' process;also, the decay rates of the intermediate-state multiplets are in qualitative agreement with the observed width of the spectral features. Distinctly different constant-initial-state (CIS) spectra are obtained at the intensity maximum of the U Sf photopeak and at the minimum between the Sf peak and the 0 2p bonding band. Calculated decay rates indicate that this results from variations in the branching ratio for decay of intense intermediate-state multiplets into F5&2 and F7~2 final states.Resonant effects in the 02p -U 5f, 6d bonding band show the bonding participation of U orbitals; moreover, the higher intensity of the CIS spectrum obtained near the top of the bonding band agrees with band-structure calculations showing most of the U 5f character in this region.

Surface-sensitive photoelectron measurements reveal bulk-to-surface shifts of the Sm 4f ' level which imply inhomogeneous valence mixing on the surface of Sm and Sm86. The surface valence fraction is estimated to be the same for both materials. The measurements take advantage of a large resonant enhancement of 4f electron emission due to 4d~4f photon absorption, and detailed spectra showing this phenomena are presented. It is shown that the 4d hole in the 4d -+4f absorption process stabilizes the 4f state by -4-7 eV. Exposure of Sm films to oxygen is found to eliminate, rather than increase, the emission from the surface 4f state, showing that the 4f state does not arise from oxygen contamination. Observed variations 6 6 in Sm film spectra are described, including the finding in some films of an unexplained photoemission peak 2.4 eV below the Fermi level. Sm86 also displays a broad band of Auger emission when a boron 1s core hole is created, and this is ascribed to electrons in the boron 2p bonding band. Various trends in 4d and 4f binding energies for Sm and Sm86 are pointed out and discussed.

Surface extended-x-ray-absorption fine structure (SEXAFS) has been combined with scanning tunneling microscopy (STM) to determine both the local and long-range bonding properties of the Si(001)2x l-Sb interface. Sb L3 edge SEXAFS shows that Sb dimers occupy a modified bridge site on the Si (001) surface with a Sb-Sb near-neighbor distance of 2.88 ±0.03 A. Each Sb atom of the dimer is bonded to two Si atoms with a Sb-Si bond length of 2.63 ± 0.04 A. STM resolves the dimer structure and provides the long-range periodicity of the surface. Low-energy-electron diffraction of vicinal Si(OOl) shows that the Sb dimer chains run perpendicular to the original Si dimer chains.

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