Extreme Ultraviolet Laser Excites Atomic Giant Resonance

Exceptional behavior of light-matter interaction in the extreme ultraviolet is demonstrated. The photoionization of different rare gases was compared at the free-electron laser in Hamburg, FLASH, by applying ion spectroscopy at the wavelength of 13.7 nm and irradiance levels of thousands of terawatts per square centimeter. In the case of xenon, the degree of nonlinear photoionization was found to be significantly higher than for neon, argon, and krypton. This target specific behavior cannot be explained by the standard theories developed for optical strong-field phenomena. We suspect that the collective giant $4d$ resonance of xenon is the driving force behind the effect that arises in this spectral range.

Figure 1

Ion time-of-flight (TOF) mass-to-charge spectra of xenon (Xe) taken at 90.5 eV photon energy and irradiance levels of (a) $2.5\times {10}^{12}\mathrm{W}{\mathrm{cm}}^{-2}$, (b) $1.7\times {10}^{15}\mathrm{W}{\mathrm{cm}}^{-2}$, and (c) $2.0\times {10}^{15}\mathrm{W}{\mathrm{cm}}^{-2}$. Signals from residual gas are also indicated.

Figure 2

Ion time-of-flight (TOF) mass-to-charge spectra of (a) krypton (Kr), (b) argon (Ar), and (c) neon (Ne), taken at 90.5 eV photon energy and irradiance levels between 1.5 and $1.8\times {10}^{15}\mathrm{W}{\mathrm{cm}}^{-2}$. The $C_n{}^{+}$ signals ($n=2$ to 5) possibly arise from carbon clusters desorbed from the carbon coated BL2 focusing mirror.