Because of the manner in which x-rays interact with matter, both the scientific opportunities and technical feasibility of the experiments improve as one goes to harder x-rays. For example, soft x-ray optics such as multilayer coatings do not compete with Bragg reflection from crystal lattices. High absorption coefficients in the soft x-ray region also lead to more energy deposition per unit volume, so that there is a greater potential for heating or damage to both optics and samples. In a workshop held at SLAC in October 1992 to explore the scientific opportunities of a 40-Å FEL, a general conclusion was that more exciting science would be possible if one could get to a few angstroms or less. The conclusions of this workshop prompted a second workshop held in February 1994 to discuss the possibilities of a hard x-ray FEL operating in the 4.5-Å and 1.5-Å range.

Time-Correlation Spectroscopy

Lasers in the infrared and visible wavelength regimes have allowed the study of collective modes, dynamical critical fluctuations near phase transitions, the evolution of conformations and structures, and a host of other phenomena at a distance scale of 100 nm or greater. The variety of methods such as frequency and time-domain spectroscopies and light-scattering techniques that have been developed in the optical domain may be applicable in the x-ray region because of the large increase in the spatial and frequency coherence of planned and proposed x-ray sources such as the APS or an x-ray FEL.

For example, a coherently illuminated sample gives rise to a random (speckle) interference pattern, and the time dependence of such a pattern at a particular point in the far field tells about the motion of the scattering sites. Time-resolved x-ray speckle interferometry may allow one to use this technique to probe distances below the 100-nm scale. The motion of defects, the diffusion of particles in liquids, critical fluctuations, liquid crystals, and charge-density waves are examples of phenomena that could be examined with increased spatial and temporal resolution.

Millisecond time-correlation experiments are now planned on the APS. These first experiments will use model systems in which the speckle effects are expected to be large. In order to study interesting physical systems with shorter correlation lengths and faster time scales, much higher spatially coherent fluxes will be needed. Ultimately, the proposed intensity of the SLAC FEL could allow a single-shot speckle interferogram to be recorded. If fast beam switching and pulse-delay methods could be developed, the observation of atomic-scale dynamics might be possible.

Other X-ray Spectroscopies

In addition to time-correlation studies, other forms of x-ray scattering would be made possible with the increased brightness of an x-ray FEL. For example, the magnetic scattering of x-rays at an absorption edge would allow the study of the



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