Soft X-ray Scattering Instrument

PURPOSE: This instrument will enable the high brightness and timing capability of the LCLS to be applied to scattering and imaging experiments that require the use of soft x-rays.

The soft x-ray end of the LCLS spectrum, roughly 800-2000 eV, is characterized by the need for vacuum flight paths (to avoid air absorption) and by optical and detection techniques adapted for the longer wavelengths. This radiation is not appropriate for use with the LCLS instruments being designed for hard x-ray scattering applications such as nano-scale imaging and pump-probe diffraction. Nevertheless, there are scattering experiments which require the use of soft x-rays. These experiments typically require a particular x-ray energy matched to an atomic resonance energy, in order to enhance sensitivity to the magnetic or chemical state of the atom. Examples of applications include:

Coherent imaging of magnetic domain structures with precise time resolution. The magnetic states of iron, cobalt, or nickel atoms can be observed by using x-rays at the low-energy end of the LCLS range. Magnetic domain structures based on these materials are commonly used for computer data storage devices such as hard disks. Time-resolved images of these domain structures can help to increase data storage capacity and speed.

Pump-probe studies of chemical reactions involving light atoms. Sensitivity to light atoms can be enhanced by using x-rays tuned to their atomic resonances in the soft x-ray region. This is particularly important when the interesting atoms make up only a small component of the sample, as when a thin layer of atoms lies on the surface of a bulk substrate (e.g., in studies of chemical reactions on catalyst substrates).

Coherent soft x-rays form a Fourier transform holograph of the magnetic domain sample on the right. Computer reconstruction processes the holograph to reproduce an image of the sample. (Image rendered by Michael Hyde.) [See S. Eisebitt, et al., Nature 432, 885 (2004)]

INSTRUMENT CONCEPT: This instrument will require x-ray optics for focusing and monochromatization, sample handling apparatus, and detectors, all optimized for the soft x-ray range. This requirement implies that the optics, sample, and detector must all be contained in evacuated tubes and chambers. In addition, for pump-probe experiments a versatile, tunable optical laser system will be needed.

UNIQUENESS: There are many soft x-ray imaging instruments at synchrotron sources, but no source in existence offers the ultra-short pulse capability of LCLS. This distinction applies equally well to other probes of nano-scale magnetic and chemical structures, such as electron micrography and atomic-force micrography: none of these other techniques can acquire sub-picosecond images. Ultrafast optical lasers, which can collect sub-picosecond data, cannot resolve sub-micron spatial structure. The LCLS offers a unique combination of resolution in both space and time, which will make it the premier tool for the study of nano-scale dynamics.