High-order harmonic generation at long range in intense laser pulses

The physics of high-order harmonic generation in single-electron molecular ions at long range (large internuclear distances) is examined by exact numerical solutions of the three-dimensional time-dependent Schrodinger equation. These numerical solutions show that photon energies can be produced with energies exceeding the atomic cut-off law I-p+3U(p), where I-p is the ionization energy and U-p is the ponderomotive energy. It is shown that preferential high-order harmonic generation occurs at critical distances R-c = alpha(0), (pi/2)alpha(0) and pi alpha(0), where alpha(0) is the ponderomotive radius, by collision of electrons with neighbouring ions as opposed to recollision with the parent ion. The most efficient harmonic generation occurs at (pi/2)alpha(0) due to less electron wavefunction expansion.