Physicists at the Free-electron LASer in Hamburg (FLASH) have performed a photoelectric-effect experiment at an extreme-ultraviolet wavelength, 13 nm, and ultra-high photon intensities. In the process, they removed electrons from xenon atoms, sometimes 21 of them. The photoelectric effect---in which ultraviolet or
extreme-ultraviolet light impinging on a metal surface kicks electrons out---was used by Albert Einstein to argue in favor of the idea of light existing in quantized form, what we now call photons.
The explanation, winning Einstein the Nobel Prize in 1921, is a landmark in early quantum theory since it suggested that the light at a fixed wavelength consists of photons with fixed (quantized) energy. In the Hamburg experiment, radiation from the free electron laser (FEL) is brought to a focus (3 microns wide and about 350 microns long) within a cell containing xenon gas. The irradiance of the laser beam, the amount of power per unit area, was 10^16 W/cm^2, a record for extreme-ultraviolet light.
The light ejects electrons from the xenon, and the resultant ions are detected. In this case, charged ions with as many as 21 electrons removed were observed. This was the first time that as many as 21 electrons were removed during a photoelectric experiment, and the surprising results are not well explained by the quantization of light and photons as the light particles. (Sorokin et al., Physical Review Letters, 23 November 2007)