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arXiv:2204.09406 · Physical Review A

Polarization effects in the total rate of biharmonic ω+3ω ionization of atoms

J. Hofbrucker, S. Ramakrishna, A. V. Volotka, S. Fritzsche

Published Jul 29, 2022• 2 citations
arXivHTMLPDF

Abstract

The total ionization rate of biharmonic ($\ensuremath{\omega}+3\ensuremath{\omega}$) ionization is studied within the independent particle approximation and the third-order perturbation theory. Particular attention is paid to how the polarization of the biharmonic light field affects the total rate. The ratios of the biharmonic ionization rates for linearly and circularly polarized beams as well as for corotating and counter-rotating elliptically polarized beams are analyzed, and how they depend on the beam parameters, such as photon frequency or phase between $\ensuremath{\omega}$ and $3\ensuremath{\omega}$ light beams. We show that the interference of the biharmonic ionization amplitudes determines the dominance of a particular beam polarization over another and that it can be controlled by an appropriate choice of beam parameters. Furthermore, we demonstrate our findings for the ionization of neon $L$ shell electrons.

DOI
10.1103/physreva.106.013118

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The total ionization rate of biharmonic (\ensuremathω+3\ensuremathω\ensuremath{\omega}+3\ensuremath{\omega}\ensuremathω+3\ensuremathω) ionization is studied within the independent particle approximation and the third-order perturbation theory. Particular attention is paid to how the polarization of the biharmonic light field affects the total rate. The ratios of the biharmonic ionization rates for linearly and circularly polarized beams as well as for corotating and counter-rotating elliptically polarized beams are analyzed, and how they depend on the beam parameters, such as photon frequency or phase between \ensuremathω\ensuremath{\omega}\ensuremathω and 3\ensuremathω3\ensuremath{\omega}3\ensuremathω light beams. We show that the interference of the biharmonic ionization amplitudes determines the dominance of a particular beam polarization over another and that it can be controlled by an appropriate choice of beam parameters. Furthermore, we demonstrate our findings for the ionization of neon LLL shell electrons.
Abstract The biharmonic (ω, 2ω) photoionization of atomic inner-shell electrons opens up new perspectives for studying nonlinear light–atom interactions at intensities in the transition regime from weak to strong-field physics. In particular, the control of the frequency and polarization of biharmonic beams enables one to carve the photoelectron angular distribution and to enhance the resolution of ionization measurements by the (simultaneous) absorption of photons. Apart from its quite obvious polarization dependence, the photoelectron angular distributions are sensitive also to the (relative) intensity, the phase difference and the temporal structure of the incoming beam components, both at resonant and nonresonant frequencies. Here, we describe and analyze several characteristic features of biharmonic ionization in the framework of second-order perturbation theory and (so-called) ionization pathways, as they are readily derived from the interaction of inner-shell electrons with the electric-dipole field of the incident beam. We show how the photoelectron angular distribution and elliptical dichroism can be shaped in rather an unprecedented way by just tuning the properties of the biharmonic field. Since such fields are nowadays accessible from high-harmonic sources or free-electron lasers, these and further investigations might help extract photoionization amplitudes or the phase difference of incoming beams.
At scaled frequencies {Omega}{sub o} = n{sub o}{sup 3}{omega}{much_lt} 1, extreme sensitivity to elliptical polarization (EStoEP) occurs (9.904 GHz, half-sine pulse, 154 field osc.) for certain ranges of n{sub o}, the principal quantum number; n{sub o}=43,42,41 ({Omega}{sub o}=0.1197 to 1.1038), show EStoEP close to circular polarization (CP), while n{sub o} = 33,32,31 ({Omega}{sub o} = 0.0541 to 0.0449) show it farther from CP. Coherent control schemes usually exploit interference of quantal amplitudes, but here 3d classical Monte Carlo calculations (3dCL) are quantitatively accurate. Approximate classical theory reveals underlying resonance physics. In the corotating frame (CRF), a low-{Omega}{sub o} CP/g field of amplitude F induces (Stark) motion at {omega}{sub s} = 1.5n{sub o}F. For {Omega}{sub o} {much_lt} 1, classical ionization requires F{sub o} = n{sub o}{sup 4}F {ge} F{sub cl} = 0.13. In CRF, EP near CP gives driving terms at 2{omega}; EStoEP at n{sub o}=43-41 occurs when this resonates with the {omega}{sub s} motion and F{sub o} {ge}F{sub cl} EStoEP at n{sub o}=33-31 occurs when 4{omega} is close to {omega}{sub s}. At even larger F{sub o}, n{sub o}=31 data show yet more EStoEP reproduced by 3dCL. This is likely the 6{omega} resonance.
In multiphoton ionization of atoms, elliptical dichroism may arise in the photoelectron angular distributions due to the interference of the possible ionization pathways. We here consider the interaction of atoms with an elliptically polarized biharmonic ($\ensuremath{\omega} + 2\ensuremath{\omega}$) field which simultaneously allows one- and two-photon ionization of the atoms. The interference between these two ionization pathways introduces contributions to the elliptical dichroism in addition to the dichroism that arises from the two-photon ionization alone. We show that these additional dichroism contributions can lead to a stronger dichroism in comparison to the one arising from two-photon ionization only. We present a relativistic analysis of the corresponding photoelectron angular distributions and discuss individual contributions to the dichroic phenomena. Detailed computations have been performed for biharmonic ionization of neutral helium atoms.
Coherent control of the photoelectron angular distribution (PAD) in ionization by a bichromatic circularly polarized field is theoretically studied in neon in the photon energy range 16--20 eV. The breakdown of the PAD axial symmetry with respect to the light propagation direction is analyzed. The coherent control of the PAD manifests itself through: (1) a change from a one-lobe to a three-lobe shape, respectively, for co- and counter-rotating harmonics, (2) a variation of the polar asymmetry with the light frequency, and (3) a rotation of the PAD around the direction of the beam depending on the relative phase between the harmonics. Tuning the lowest harmonic at the 2p53s2{p}^{5}3s2p53s or 2p54s2{p}^{5}4s2p54s intermediate excited states strongly influences the PAD. In contrast to atomic hydrogen and the alkali atoms, the cases of equal and opposite helicity are not related in neon. Features of a conjugate polarization setup, when one harmonic is linearly polarized while the other one is circularly polarized, are also discussed.