Femtosecond X Rays Generated at Beam Test Facility
A research group using the Center for Beam Physics' Beam Test Facility at the ALS has produced pulses of x rays with a wavelength of 0.4 angstroms and a pulse length of just 300 femtoseconds (1 fs = 10**-15 second). The method they used holds promise for the exploration of chemical reactions, phase transitions, surface processes, and other interactions ultimately governed by the motion of atoms on the time scale of a single vibrational period, about 100 femtoseconds.
The figure above illustrates the method used to generate the x-ray pulses. Relativistic (50 MeV) electron pulses from the ALS linear accelerator and 100-fs pulses from an infrared laser cross at a 90-degree angle. In this arrangement, some infrared photons interact with electrons and are scattered. The scattered photons have x-ray energies and travel in the same direction as the electron beam; those closest to the center of the scattered pulse have the highest energies. The duration of the scattered pulse is determined by the transit time of the infrared pulse through the electron beam, and this transit time is reduced by focusing both beams so that their area of interaction has a diameter of only 90 microns.
The image above, taken 80 cm downstream of the point of intersection, uses false color to show an x-ray pulse's intensity profile. The scattered pulse contains information on the size and divergence of the electron bunch that helped create it. Thus, this scattering arrangement can act as a microprobe of 100-fs slices of the electron beam, a probe that will become increasingly important as accelerators with sub-picosecond bunch lengths are produced.
Femtosecond x-ray work was conducted by W.P. Leemans (co-principal investigator), P. Volfbeyn, M. Zolotorev, K.J. Kim, and S. Chattopadhyay (Berkeley Lab's Center for Beam Physics); R.W. Schoenlein (co-principal investigator) and T.E. Glover (Berkeley Lab's Materials Sciences Division, or MSD); A.H. Chin (University of California at Berkeley); C.V. Shank (MSD and UC Berkeley); and P. Balling (University of Aarhus, Denmark).
Funding: Office of Basic Energy Sciences of the U.S. Department of Energy, National Science Foundation.
Publications about this experiment:
R.W. Schoenlein, W.P. Leemans, et al., Science 274 (1996), p. 236.
W.P. Leemans, R.W. Schoenlein, et al., Phys. Rev. Lett. 77 (1996), p. 4182.
W.P. Leemans, R.W. Schoenlein, et al., IEEE Journal of Quantum Electronics 33 (1997), pp. 1925-1934.
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