Breakdown of Stabilization of Atoms Interacting with Intense, High-Frequency Laser Pulses

Abstract

An analysis of the influence of the magnetic field of an intense, high-frequency laser pulse on the stabilization of an atomic system is presented. We demonstrate that at relatively modest intensities the magnetic field can significantly alter the dynamics of the system. In particular, a breakdown of stabilization occurs, thereby restricting the intensity regime in which the atom is relatively stable against ionization. Counterpropagating pulses do not negate the detrimental effects of the magnetic field. We compare our quantum mechanical results with classical Monte Carlo simulations. PACS numbers: 32.80.Fb, 32.80.Rm, 42.50.Hz Theoretical studies of atoms interacting with highfrequency intense laser pulses have predicted a significant decrease in the ionization probability with increasing laser intensity. This phenomenon is referred to as atomic stabilization, and has been extensively studied over the past decade [1]. Many aspects of this phenomenon can be understood by performing a Kramers-Henneberger (KH) transformation to the rest frame of a classical electron in the laser field. In particular, by developing a highfrequency Floquet theory in the KH frame [2], stabilization can be seen to have its origin in the rapid quiver motion of the atomic electron in the laser field. This allows the electron dynamics to be described by an effective potential that, on average, localizes the electron away from the vicinity of the nucleus. Subsequent ab initio Floquet calculations confirmed that ionization rates decrease with increasing intensity in a high-frequency field [3]. By directly integrating the time-dependent Schrödinger equation numerically, simulations in one [4] and three dimensions [5] demonstrated reductions in the ionization probability with increasing laser intensity when an atom interacts with realistic laser pulses having a finite duration. Further work has been carried out in order to elucidate the effects of the pulse shape and duration [6,7]. We also note that evidence of atomic stabilization of Rydberg states has been observed experimentally [8].In the above-mentioned theoretical studies, the magnetic component of the laser pulse was neglected. However, as the laser intensity increases, relativistic effects that alter the stabilization dynamics become important. Classical Monte Carlo simulations have indicated that the magnetic field pushes the electron in the laser pulse propagation direction, reducing the degree of stabilization [9]. Relativistic wave equations have also been considered within the context of reduced dimensional models [10,11]. However, it is also of interest to study the effects that are neglected in the dipole approximation by using the fully space-and time-dependent vector potential in the nonrelativistic Schrödinger equation [12]. For atomic hydrogen, this results in the cylindrical symmetry of the system being broken, thereby requiring a fully three-dimensional calculation to be carried out for extremely high laser intensities. This is a computationally demanding task. Howev...

Two-color stabilization of atomic hydrogen in circularly polarized laser fields

“…Since all vector potential amplitudes considered in this work are ≤ 10 a.u. the dipole approximation is applicable, as has been shown in [20]. The survival probability increases with increasing peak field strengths, but not beyond a certain maximum.…”

“…In the limit of laser intensity I → ∞ but otherwise fixed pulse parameters the survival probability of an atom will thus approach zero. Moreover, it has been found that the magnetic field of the laser-usually neglected because of the dipole approximation-leads to increased ionization [20]. This is due to the ponderomotive force which pushes the electrons in propagation direction.…”

Laser-driven recollisions generalized to relativistic energies

“…In this case, the electrons are driven by a standing wave. Both linearly [29][30][31] and circularly polarized fields [28] can be superimposed to eliminate the drift. However, the electron dynamics differ for the two configurations [38].…”

“…Positronium is an appropriate bound system for relativistic recollisions since both particles drift at the same pace [26,27]. Moreover, standing waves can be applied where the Lorentz drift is circumvented [28][29][30][31] or two consecutive laser pulses, where the drift of the first pulse is compensated by the second one [32,33].…”

Relativistic ionization rescattering with tailored laser pulses

“…When the laser intensity approaches the relativistic regime, the laser magnetic field effect starts to play a role by inducing a drift of the ionized electron in the laser propagation direction which severely suppresses the probability of the electron revisiting the ionic core. There are several attempts to circumvent this effect in order to increase the efficiency of rescattering, particularly, using relativistic ions which propagate in laser propagation direction [6,8], or using two counter-propagating laser beams with linear [10] or equally handed circular polarization [7], or generating harmonics via exotic positronium atoms in strong laser fields [11]. On a different front, new techniques for generating attosecond pulses have recently emerged based on HHG in gases [12,13] and in plasmas interaction [14,15].…”