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

Abstract

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 Schrödinger 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 = α 0 , (π/2)α 0 and πα 0 , where α 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 (π/2)α 0 due to less electron wavefunction expansion.

Asymmetry of above-threshold ionization of metal clusters in two-color laser fields: A time-dependent density-functional study

“…Quasistatic tunneling ionization models of single-electron atoms predict E max =10U p [20,34]. In the case of extended molecules, we have shown previously that collisions with neighboring ions can raise this up to 12U p , thus extending highorder harmonic generation plateaus beyond the atomic maximum E max = I p + 3.17U p [36]. This has been confirmed in simulations of chains of atoms [29].…”

Attosecond localization of electrons in molecules

“…We have shown that collision of ionized electrons with neighboring ions in molecules can lead to even larger harmonic orders with photon energies of 6U p up to 12U p , thus extending the recollision energy, cut-off law (1) [24].…”

“…The source of asec radiation is HOHG by recollision of the ionized electron with its parent [23] or neighboring ions [24]. Unfortunately, the efficiency of such spectra is low, as illustrated in Figure 1.…”

Effect of nuclear motion on high-order-harmonic generation ofH2+in intense ultrashort laser pulses

“…The HHG pro- * m.vafaee@modares.ac.ir cess in molecules is more complex than that in atoms because of nuclear motion [12], two-center interference [13] and different orientations of molecule with respect to the laser field [13]. Effects of different initial vibrational states [14,15], initial nuclear velocities [16] and relation between electronic wavepacket expansion and internuclear distance on the HHG produced by H + 2 [17] have been reported. In addition, extraction of nuclear dynamics from HHG spectra [18], effect of nuclear motion on the HHG efficiency by varying pump-probe time delay [19], on the broadening of cutoff regime [20], on the length of generated attosecond laser pulses [21] and on the generation of isolated attosencond laser pulses for H + 2 have been investigated [22,23].…”

“…In most theoretical works mentioned above, electron and nuclei are considered quantum mechanically in a one-dimensional (1D) model. The fulldimensional electron wavepacket expansion during laser interaction and its consequent HHG cannot be described properly by a one dimensional model [17].…”