HIGH-ORDER HARMONIC-GENERATION AT LONG-RANGE IN INTENSE LASER-PULSES

The physics of high-order harmonic generation in single-electron molec ular ions at long range (large internuclear distances) is examined by exact numerical solutions of the three-dimensional time-dependent Schr odinger 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 en ergy. It is shown that preferential high-order harmonic generation occ urs 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 electr ons with neighbouring ions as opposed to recollision with the parent i on. The most efficient harmonic generation occurs at (pi/2)alpha(0) du e to less electron wavefunction expansion.