Ultrahigh Harmonics from Laser-Assisted Ion-Atom Collisions

We present a theoretical analysis of high-order harmonic generation from ion-atom collisions in the presence of linearly polarized intense laser pulses. Photons with frequencies significantly higher than in standard atomic high-harmonic generation are emitted. These harmonics are due to two different mechanisms: (i) collisional electron capture and subsequent laser-driven transfer of an electron between projectile and target atom; (ii) reflection of a laser-driven electron from the projectile leading to recombination at the parent atom.

Figure 1

Emission spectra for impact parameter $b=4\mathrm{a}\mathrm{.}\mathrm{u}\mathrm{.}$ and laser phase $\delta =0$. Upper curve (solid): polarization axis parallel to impact velocity. Lower curve (dashed): polarization axis perpendicular to impact velocity. The vertical lines indicate the cutoff energies for various mechanisms (see text). The inset shows the time dependence of the laser field and the capture probability for the two polarization directions.

Figure 2

Temporal analysis of harmonic emission for the same parameters as in Fig. 1. The polarization axis is parallel to the impact velocity. Each spectrum describes the emission during one laser cycle, and the value $Z=vt$ refers to the middle of the respective time interval. The dotted horizontal lines indicate the respective levels of $\log I=-10$.

Figure 3

(a) Solid curve: capture probability versus impact parameter $b$ for a laser with phase $\delta =0$; dashed curve: field-free capture probability. (b) Yield of ultrahigh harmonics (solid curve) and coherence parameter (dot-dashed curve) versus $b$. The polarization axis is parallel to the impact velocity. The insets show the same quantities as a function of the laser phase $\delta$ for fixed impact parameter $b=4\mathrm{a}\mathrm{.}\mathrm{u}\mathrm{.}$

Figure 4

Maximum classical kinetic energy versus internuclear distance for the recombination of a laser-driven electron in mechanisms (i) and (ii); see text (fixed-nuclei calculation).