Observation of Two-Photon Above-Threshold Ionization of Rare Gases by xuv Harmonic Photons

We have successfully observed two-photon above-threshold ionization in rare gas atoms (Ar, Xe, and He) by the fifth harmonic (25 eV photon energy) of a KrF laser. Use of the energy-resolved photoelectron counting system together with our laser, providing strong 25 eV radiation at 40–100 Hz, enabled us to detect the very weak single-color two-photon above-threshold ionization signals. Experimental data are in good agreement with our theoretical calculations newly developed along the line of multichannel quantum defect theory.

Figure 1

Experimental setup for the two-photon ATI measurement.

Figure 2

The level diagrams of Ar, Xe, and He relevant to the two-photon ATI by 25 eV photons.

Figure 3

Photoelectron spectra of two-photon ATI in (a) Ar, (b) Xe, and (c) He.