High-order harmonic generation in a partially ionized medium

The harmonic spectra generated by atoms in the presence of an ionized neighborhood are investigated. Numerical calculations in a one-dimensional model show an increase of the maximum harmonic energy radiated that leads to photon frequencies well beyond the single-atom cutoff I-p + 3.17U(p). We identify these harmonics with those generated when the atom's detached electron is captured by a neighboring ion. By means of classical considerations, we give simple laws for the new harmonic cutoff in the tunneling and the multiphoton ionization regimes. (C) 1996 Optical Society of America