Ultrahigh Harmonics from Laser-Assisted Ion-Atom Collisions

Abstract

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.PACS numbers: 42.65. Ky,34.50.Rk,32.80.Rm,34.70+e Over the last decades, a vast amount of work has been devoted to the study of ion-atom collisions [1] Kirchner [7] investigate fast collisions in the presence of a strong laser field. In Ref.[6], excitation mechanisms are discussed, while Ref. [7] focuses on ionization and electron capture. In both cases, the presence of the field leads to a significant modification of the collision process. On the other hand, there has been no study on the question how typical strong-field processes in atoms, such as high-order harmonic generation (HHG) [8] and above-threshold ionization [9], are modified due to the impact of an ion projectile. In HHG, a large number of incoming laser photons are converted into a single high-energy photon. HHG experiments are presently pursued with great effort [10,11] since the process serves as a source of coherent XUV radiation and attosecond pulses.In the present work, we investigate HHG in laserassisted ion-atom collision. We focus on impact velocities such that the time-scales of nuclear and electronic motion are comparable, i.e., we have significant probabilities for collisional electron transfer from the target to the projectile. For sufficiently long laser pulse durations, the laser-driven electron effectively sees a large range of internuclear distances during one laser pulse. When the laser polarization axis is parallel to the ion impact velocity, we show that this situation results in the generation of high harmonics with photon energies much higher than usually obtained in atomic HHG.The classical recollision model [12] describes atomic HHG as a sequence of strong-field ionization, acceleration of the electron in the laser field and recombination with the core. Within this model, the maximum return energy of the laser-driven electron is 3.17U p where2 ) is the ponderomotive potential for a laser with field amplitude E 0 and frequency ω. The maximum energy of the emitted photons is then equal to 3.17U p +I p where I p is the atomic ionization potential. For ion-atom collisions, we show below how the interplay between collisional electron capture and laser-driven electron transfer between target and projectile leads to new mechanisms of HHG with cutoffs at significantly higher energies.We consider collisions of protons on hydrogen atoms for proton energies of 2 keV (impact velocity v = 0.283 a.u.). Due to the large impact momentum, the projectile trajectory is assumed to be classical and along a s...

Circular dichroism in laser-assisted proton-hydrogen collisions

“…Reduced dimensionality numerical capture probabilities for field-free proton-hydrogen collisions have been published by Lein and Rost [16]. Their results are almost identical with our field-free capture probabilities (Fig.…”

“…Lattice calculations on a three-dimensional cartesian grid for laser-assisted proton collisions with lithium atoms in ground and excited states by Pindzola et al [15] show a significant modification of the charge-transfer process for moderate laser intensities of 10 12 W/cm 2 . Lein and Rost [16] applied a reduced dimensionality model, solved the Schrödinger equation on a two-dimensional cartesian grid, and predicted the generation of ultrahigh harmonics in laser-assisted collisions of 2 keV protons with hydrogen atoms in linearly polarized laser pulses with 16 optical cycles, a wavelength of 800 nm, and 10 14 W/cm 2 intensity. More work, both experimental and theoretical, has been done for laser-assisted electron scattering, but even a structureless projectile constitutes a serious challenge to present theories [17,18,19].…”

Laser-field-induced modifications of electron-transfer processes in ion-atom collisions

“…On the theoretical side refined methods for the description of laserassisted collisions have been worked out, implemented and tested on prototype scattering systems. These works include perturbative calculations of electron capture [8][9][10] and ionization cross sections [11][12][13] in fast collisions assisted by relatively weak fields of low frequency, classical simulations of laser-assisted ionization events [14], and quantummechanical calculations for excitation, ionization [14,15], electron capture [5,16], and high harmonic generation [17] within more intense fields. All these works predict significant modifications of collisional or laser-induced processes when both types of interactions are combined.…”

Generation of circularly polarized attosecond pulses by intense ultrashort laser pulses from extended asymmetric molecular ions

“…Another scheme for MHOHG is collision with neighboring ions, which allows for extension of the cutoff order as a function of ion separation [9][10][11]. In linearly polarized field laser-induced collisions with neighboring ions, maximum harmonic energies are given * andre.bandrauk@usherbrooke.ca from the initial zero velocity ionization model by I p + 8U p [11,12]. Most recently large-distance electron recombination has also been proposed for linearly polarized attosecond pulse generation [13].…”