Spin Asymmetry in an Intense-Field Ionization Process

Abstract

A relativistic analysis of the rate of spin flips in ionization of an ensemble of Dirac H atoms subjected to intense circularly polarized laser fields is made. A remarkable intensity-dependent asymmetry between the spin up and spin down electron currents is found. It is nonzero even when the retardation effect, hence the magnetic component of the field, as well as the spin-orbit interaction responsible for the well-known Fano effect, is negligible. Transformation properties of the amplitudes show that the sign of asymmetry can be controlled by changing the helicity of the laser photons from outside.

Intense-Field Dirac Theory of Ionization of Hydrogen and Hydrogenic Ions in Infrared and Free-Electron Laser Fields

“…The results are given for the ionization amplitudes in terms of simple algebraic formulas involving explicit functions. They permit us to readily calculate not only the energy and angle differential and integrated rates but also the individual spin-resolved ionization probabilities in intense fields [11]. The analytical formulas are given below for the general case of an elliptically polarized radiation field, which automatically reduces to the important special cases of linear and circular polarizations for the ellipticity parameter D 0 and D˙ =2 (right, left) helicity.…”

“…The evaluation of the four-dimensional integrations can be carried out analytically [11]. The results are given for the ionization amplitudes in terms of simple algebraic formulas involving explicit functions.…”

“…In fact, the distributions shown in the above figures correspond to the usual measurement of the unpolarized ionization signal that is spin-averaged over the initial spin states of the target atom and spin-summed over the final states of the free electron. The present four-component relativistic analysis provides explicit analytic expressions for the individual spin-dependent transition amplitudes (first obtained in [11]). This allows us to further examine the spin-resolved electron current, its dependence on polarized or un-polarized target atoms, the spin-flip ionization probabilities, and any asymmetry in the spin currents.…”

“…Finally, noting d 3 p D p 2 djpjd˝p, and performing the djpj integration OE0; 1, the rate of ionization for the final electron momentum p in the solid angle element d˝p takes the simple form [11]:…”

“…Thus for the ionization process, in strong fields the rates of spin-flips and the characters of up and down spin currents can be of much experimental interest. Intense-field 4-component Dirac analysis of such spin-dependence of the ionization rates has been reported in [11] that revealed among other things a remarkable spin asymmetry in the ionization currents even from spin unpolarized ground state H-atom. The asymmetry had been predicted to survive also for ionization by weak and long wavelength photons that possess a negligible photon momentum.…”