IONIZATION VIA AN ULTRASHORT HIGH-POWER KRF LASER

We present the systematic experimental results of photoelectron spectr a of Ar, Ne, and He in a strong KrF laser field. Above-threshold ioniz ation (ATI) peaks up to the 6th order were clearly observed and the po sitions of ATI peaks, including substructure, did not shift with laser intensity. These facts can be explained by the multiphoton resonance with Rydberg states. At a higher intensity, the continuum builds up bu t the ATI structure is still apparent. Eventually, even this structure is buried in the continuum, implying the tunnelling regime.