FLASH experiment: At the limits of the photoelectric effect (April 2009)

Strong-field multiple ionization

Scientists from PTB (Berlin), Ioffe Institute (St. Petersburg), Fraunhofer Institute (Jena), Univ. Hamburg and HASYLAB/DESY discovered an exceptional behavior which concerns the fundamentals of physics. The measurements were performed at FLASH at DESY (13.7 nm).

The scientists actually wanted to develop methods for the radiometric characterization of X-ray lasers. They irradiated different gases to derive the laser strength from the ionization effect. The aim: with the laser well characterized was, for example, the testing of EUV lithography mirrors. The EUV lithography (EUV stands for extreme ultraviolet) at wavelengths in the range of 13 nanometers is considered as the future technology for the production of ever smaller computer chips.

Photoionization of xenon

Photoionization of xenon (see text). (Courtesy: PTB Berlin)

With the classical photoelectric effect (a)(see picture), a single light particle (photon) of sufficient energy interacts with a single electron of the material. The process is energetically described by the Einstein equation (1905) and demonstrates the quantum structure of light. Only at very high intensities, does the multiphoton ionization occur, a process which is described in the extreme case of highly intensive ultra-short light flashes as emitted by long-wave femtosecond lasers, again, in the wave picture of light (b). Nevertheless, the suitable theoretical models fail in the short-wave X-ray regime as shown by the experiments in Hamburg in which, for the first time, soft X-ray irradiance levels of several petawatts per square centimeter were achieved by strong beam focusing. The comparative quantitative studies prove that the degree of light-matter interaction and, thereby, the nature of the X-ray light are decisively determined by the structure of the atom and correlations in, above all, inner electron shells. In the extreme case (xenon), a whole wave packet of photons seems to lead to the simultaneous emission of several inner electrons (c).

(from: PTB Press Release 24 April 2009)