Over the past few decades, scientists have focused their attention on the development of concepts and designs, leading to demonstrations, of unique x-ray sources to perform femtosecond and attosecond science. The rewards of such an effort in the x-ray wavelength range will revolutionize the subfields of atomic physics, molecular physics, biology, condensed matter physics and material science. A brief review of this subject and its impact on emerging areas of science will be presented.
The storage-ring-based synchrotron radiation sources are today's workhorses in providing both time-averaged and time-resolved structural and chemical information with subnanosecond to subsecond resolution using x-ray imaging, spectroscopy and scattering techniques. On the other hand, many phenomena are ultrafast with characteristic periods of a few femtoseconds to tens of picoseconds. These include electronic motions around a nucleus in an atom, atomic and molecular vibrational motions in matter, spin dynamics, chemical and biological reactions, and phase transitions in response to photoexcitation. Probing such phenomena using photon-excited pump-probe experiments will require both optical and x-ray sources with comparable resolution. In the future, sources based on atypical concepts in storage-rings, table-top plasma sources, laser-based high harmonic generation (HHG) sources, linac-based sources, such as energy-recovery linacs (ERLs) and x-ray free-electron lasers (FELs), will likely meet these demands.