Resonantly Enhanced Multiphoton Ionization under XUV FEL radiation: A   case study of the role of harmonics
  We provide a detailed quantitative study of the possible role of a small admixture of harmonics on resonant two-photon ionization. The motivation comes from the occasional presence of 2nd and 3rd harmonics in FEL radiation. We obtain the dependence of ionic yields on the intensity of the fundamental, the percentage of 2nd harmonic and the detuning of the fundamental from resonance. Having examined the cases of one and two intermediate resonances, we arrive at results of general validity and global behavior, showing that even a small amount of harmonic may seem deceptively innocuous. 
Georgios M. Nikolopoulos and Peter Lambropoulos
physics.atom-ph