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Physical Review A

Role of spin-orbit interaction in the production of spin-polarized photoelectrons using a dressing laser

Takashi Nakajima, Lampros A. A. Nikolopoulos

Published Jul 28, 2003• 6 citations
Publisher

Abstract

We investigate the role of spin-orbit interaction in the production of spin-polarized photoelectrons under the influence of a dressing laser. Although the scheme considered here is essentially based on the laser-induced continuum structure, it turns out that special care has to be taken for the spin states in the continua. This is because the spin-orbit interaction in the continuum cannot be neglected for our specific case. After the derivation of the general formula, specific theoretical results are presented for Rb and Cs atoms. Enhancement about a factor of 2 in the degree of spin polarization has been predicted for both systems by the introduction of a dressing laser.

DOI
10.1103/physreva.68.013413

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We investigate the role of spin-orbit interaction in the production of spin-polarized photoelectrons under the influence of a dressing laser. Although the scheme considered here is essentially based on the laser-induced continuum structure, it turns out that special care has to be taken for the spin states in the continua. This is because the spin-orbit interaction in the continuum cannot be neglected for our specific case. After the derivation of the general formula, specific theoretical results are presented for Rb and Cs atoms. Enhancement about a factor of 2 in the degree of spin polarization has been predicted for both systems by the introduction of a dressing laser.
We theoretically investigate the effects of laser polarization on the photoelectron angular distribution through laser-induced continuum structure. We focus on a polarization geometry where the probe and dressing lasers are both linearly polarized and change the relative polarization angle between them. We find that the total ionization yield and the branching ratio into different ionization channels change as a function of the relative polarization angle, and accordingly the photoelectron angular distribution is altered. We present specific results for the $4{p}_{1∕2}\text{\ensuremath{-}}6{p}_{1∕2}$ and $4{p}_{3∕2}\text{\ensuremath{-}}6{p}_{3∕2}$ systems of the K atom and show that the change of the polarization angle leads to a significant modification of the photoelectron angular distribution.
We present a scheme to control the spin-polarization of photoelectrons/photoions using short laser pulses. It is based on the pump-probe method. By exciting fine structure manifolds of a system by the short pump pulse with a sufficient bandwidth, a superposition of fine structure states is created. After the pump pulse, the coherently superposed excited state evolves in time under the field-free condition with a period determined by the inverse of energy difference between them. In terms of the spin state, this suggests that the different spin state evolves differently in time, leading to the time-varying spin-polarization. Therefore, varying the time delay between the pump and probe pulses leads to the control of spin states upon photoionization. Specific theoretical results are presented for two-valence-electron atoms, in particular for Mg, which demonstrate that, under certain conditions, not only the degree of spin-polarization but also its polarity can be manipulated through time delay. Furthermore, we propose a new technique to accelerate the coupling time by the introduction of a dressing laser with a few ns duration. Since the underline physics is rather general and transparent, the presented scheme may be potentially applied to nanostructures such as quantum wells and quantum dots.
We report on the observation of the modification of the two spin-orbit ionization continua of Xe through laser-induced continuum structure (LICS). The radiative decay of the Xe ground state into the two electronic continua that correspond to the two fine-structure levels of the ground state of the Xe+{\mathrm{Xe}}^{+}Xe+ ion can be varied by electromagnetically embedding a bound state of the atom into the two continua. The dressed 2P1/2{}^{2}{P}_{1/2}2P1/2​ continuum, probed from the atomic ground state through three-photon absorption, exhibits a clear induced structure, while the 2P3/2{}^{2}{P}_{3/2}2P3/2​ continuum possesses no observable structure under the present experimental conditions, as manifested through high-resolution photoelectron spectra. The modified ionization ratio is controlled by the wavelength of the dressing field.
We theoretically investigate how the photoelectron angular distribution is altered by the introduction of a dressing laser. The physical mechanism underlying this alteration is the so-called laser-induced continuum structure; namely, a strong dressing laser induces quantum mechanical interference, the degree of which is different for different ionization channels. Therefore the branching ratio into different ionization channels changes as a function of laser detuning, and accordingly the photoelectron angular distribution is altered. After a general argument, we present specific theoretical results for the K atom, which indeed exhibit significant modification of the photoelectron angular distribution.
An experiment is reported on two-step photoionization of Cs via the 6P 3/2 intermediate state using circularly polarized radiation from a GaAs laser for the first step (8521 AA) and from an Ar-ion laser for the second step. Values for the ratio of ion production rates for right- and left-handed circular polarization in the second step and for the spin polarization of the photoelectrons are presented. The measurements are interpreted on the basis of a calculation which takes into account the hyperfine splitting of the levels involved in the first transition. A description in terms of fine-structure splitting of the 6P state is shown to be totally inadequate, even when allowing for possibly large spin-orbit effects in the continuum. The process is evaluated as a possible source of a spin-polarized electron beam.
States behaving like autoionizing states can be selectively induced by laser radiation into one of the continuum spin channels in the photoionization of polarized excited alkaki atoms. As a result of destructive or constructive interference between the direct ionization channel and those introduced by the dressing laser radiation, the cross section of this specific spin component is completely suppressed or enhanced, while leaving the other spin channel unaffected. The qqq parameter which determines the line shape of the Fano-type resonance can be resonantly tuned as a function of the dressing laser frequency.