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Journal of Physics B Atomic Molecular and Optical Physics

Under-threshold RABBITT in argon

Anatoli S. Kheifets

Published Mar 22, 2023• 12 citations
Publisher

Abstract

Abstract The process of reconstruction of attosecond bursts by beating of two-photon transitions (RABBITT) can involve a transition from under the ionization threshold. Such an under-threshold RABBITT (or uRABBITT) was demonstrated experimentally and analyzed theoretically in He and Ne. In the present work, we explore an analogous process in the argon atom. The uRABBITT in Ar reveals the familiar physical effects: a phase transition across the threshold and the symmetry modification of the photoelectron momentum distribution. It can also be used for mapping the electronic structure of the target atom bound states.

DOI
10.1088/1361-6455/acc6aa

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Moscow University Physics Bulletin•Jun 1, 2023•2 citations
Abstract The process of reconstruction of attosecond bursts by beating of two-photon transitions (RABBITT) can involve a transition from under the ionization threshold. Such an under-threshold RABBITT (or uRABBITT) was demonstrated experimentally and analyzed theoretically in He and Ne. In the present work, we explore an analogous process in the argon atom. The uRABBITT in Ar reveals the familiar physical effects: a phase transition across the threshold and the symmetry modification of the photoelectron momentum distribution. It can also be used for mapping the electronic structure of the target atom bound states.
Abstract The mutual angle formed by the non-collinear polarization axes of two laser pulses is used to control two-photon extreme ultraviolet and infrared ionization of noble gas atoms in the process of reconstruction of attosecond bursts by beating of two-photon transitions (RABBITT). The magnitude and the phase of this beating can be controlled very efficiently by the mutual polarization angle. The mechanism of this control can be understood within the lowest order perturbation theory and the soft photon approximation. We offer a very sensitive test on the polarization control of the angular dependent RABBITT process which validates our numerical simulations. We apply this test to the recent theoretical and experimental results of polarization controlled RABBITT on hydrogen and helium by Boll et al (2023 Phys. Rev. A 107 043113) and heavier noble gases by Jiang et al (2022 Nat. Commun. 13 5072).
The reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) method is a leading technique for time-resolved investigation of photoionization processes. Interpretation of RABBITT spectra is limited to a lowest-order description, while new variants of the technique necessitate higher intensity infrared fields where this description breaks down. Here, we present an extension of RABBITT theory which can account for higher-order processes (RABBITT-HOP), and demonstrate RABBITT-HOP's ability to predict and describe higher-order processes through simulation of a RABBITT experiment targeting the high-lying 1 s n p Rydberg series of helium. We demonstrate that the signature line shape imparted by the resonances in the 2 ω phase of the lowest sideband can also be observed in the 2 ω phase of the next mainband, as well as in the 4 ω phases of the next sideband and mainband—each with additional phase shifts predicted under RABBITT-HOP. Repeating this analysis for a range of infrared intensities, we determine the lower limits of intensities required for these higher-order processes to become observable and upper limits beyond which nonperturbative effects begin to dominate. This new theoretical framework forms a much-needed basis for accurate interpretation of RABBITT spectra in experiments involving more intense fields, and paves the way for the development of new RABBITT-like techniques which take advantage of this new understanding of how atomic resonance effects may be propagated beyond their nearest sideband.
We introduce circular under-threshold RABBITT (cuRABBITT) as a new interferometric method to probe discrete electronic excitations in atoms with attosecond resolution. By combining circularly polarized attosecond pulses with broadband ("rainbow") spectral analysis, we directly access two-photon ionization amplitudes and their relative phases. Time-dependent Schrödinger simulations, supported by Green's function theory, reveal strong resonances in helium and argon and a Cooper-like minimum in xenon. These results demonstrate that cuRABBITT provides continuous spectral mapping of bound-state resonances and extends Fano's propensity rule into the under-threshold regime. Our Letter establishes cuRABBITT as a powerful attosecond metrology technique, opening the way to polarization-resolved studies of resonant dynamics in atoms and molecules.
Reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) is a technique that can be used to determine the phases of atomic transition elements in photoionization processes. In the traditional RABBITT scheme, the so-called asymptotic approximation considers the measured phase as a sum of the Wigner phase linked to a single-photon ionization process and the continuum-continuum phase associated with further single-photon transitions in the continuum. In this paper, we extend the asymptotic approximation to multi-sideband RABBITT schemes. The predictions from this approximation are then compared with results obtained by an ab initio calculation based on solving the time-dependent Schr\"odinger equation for atomic hydrogen.
We present numerical simulations of the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) on lithium by solving the three-dimensional time-dependent Schr\"odinger equation. In our scheme, the infrared (IR) field couples the 2s2s2s and 2p2p2p states of lithium and leads to the Rabi oscillations of populations between these two states. We analyzed the RABBITT phases of the two peaks in the Rabi-oscillation-induced Autler-Townes splittings. Our results show that the relative phase between these two peaks changes with the photoelectron energy and depends on the intensity of the IR field. Moreover, in the angle-resolved RABBITT measurement, the phases of the two peaks depend differently on the emission angle of photoelectrons. These behaviors are traced back to the different initial phases of the electron wave packets emitted from the dressed 2p2p2p states and the competition among different ionization channels.
Abstract In the paper we present the angular distributions of photoelectrons in ionization of neon atom by a field of several multiple frequencies. The considered setup is referred to the RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) spectroscopy under condition that the field frequencies are selected in such a way that resonant transitions through discrete states play an important role. The role of the phase of the seeding infrared field on the angular distributions of photoemission is analyzed. A significant difference in the anisotropy parameters at the near-threshold sideband caused by transitions through discrete states is shown. Two methods are compared: numerical solution of the rate equations with continuum discretization and third-order perturbation theory.