Earlier, a significant enhancement of the nondipole parameters γ2p, δ2p, and ζ2p=γ2p+3δ2p in the photoelectron angular distribution for Ne 2p photoionization was predicted, owing to resonance interference between dipole (E1) and quadrupole (E2) transitions. This enhancement manifests as narrow resonance spikes in the parameters due to the low-energy 2s→3p and 2s→4p dipole, as well as the 2s→3d quadrupole autoionizing resonances. Given the unique nature of this predicted enhancement, it requires further validation, specifically regarding whether these narrow spikes in γ2p, δ2p and ζ2p will or will not retain their values for experimental observation if one accounts for a typical finite frequency spread in the ionizing radiation. To address this, we revisit the previous study, now incorporating the effect of frequency spread in the ionizing radiation, assuming a spread as large as 5 meV at the half-maximum of the radiation’s intensity. In the present paper we demonstrate that while the frequency spread does affect the resonance enhancement of γ2p, δ2p and ζ2p, these parameters still retain quantitatively significant values to be observed experimentally. The corresponding calculations were performed using the random phase approximation with exchange, which accounts for interchannel coupling in both dipole and quadrupole photoionization amplitudes.

We study the relationship between the results of two qualitatively different semi-empirical models for photoionization cross sections, σnℓ, of neutral atoms (A) and their cations (A+) centrally encapsulated inside a fullerene anion, CNq, where q represents the negative excess charge on the shell. One of the semi-empirical models, broadly employed in previous studies, assumes a uniform excess negative charge distribution over the entire fullerene cage, by analogy with a charged metallic sphere. The other model, presented here, considers the quantum states of the excess electrons on the shell, determined by specific n and ℓ values of their quantum numbers. Remarkably, both models yield similar photoionization cross sections for the encapsulated species. Consequently, we find that the photoionization of the encapsulated atoms or cations inside the CNq anion is influenced only slightly by the quantum states of the excess electrons on the fullerene cage. Furthermore, we demonstrate that the influence decreases even further as the size of the fullerene cage increases. All this holds true at least under the assumption that the encapsulated atom or cation is compact, i.e., its electron density remains primarily within itself rather than being drawn into the fullerene shell. This remarkable finding results from Hartree–Fock calculations combined with a popular modeling of the fullerene shell which is simulated by an attractive spherical annular potential.

We focus on the study of the photodetachment of bare, i.e., single-cage (CN)− as well as nested (multi-cage) (CN@CM@…)− singly charged fullerene anions. We calculate the attached electron’s wavefunctions, energies, oscillator strengths and photodetachment cross sections of the C60−, C240−, C540−, (C60@C240)−, (C60@C540)−, (C240@C540)− and (C60@C240@C540)− fullerene anions, where the attached electron is captured into the ground s-state by the resultant external field provided by all fullerene cages in the anion. The goal is to gain insight into the changes in behavior ofphotodetachment of this valence electron as a function of the different geometries and potentials of the various underlying fullerenes or nested fullerenes (fullerene onions) both due to their increasing size and due to “stuffing” of a larger bare fullerene with smaller fullerenes. To meet this goal, we opt for a simple semi-empirical approximation to this problem: we approximate each individual fullerene cage by a rigid potential sphere of a certain inner radius, thickness and potential depth, as in numerous other model studies performed to date. The results reveal a number of rather significant differences in the wavefunctions, oscillator strengths and photodetachment cross sections among these fullerene anions, some of which are completely counter-intuitive. The results obtained can serve as a “zeroth-order-touchstone” for future studies of single-cage and nested fullerene anions by more rigorous theories and/or experiments to build upon this work to assess the importance of interactions omitted in the present study.

We theoretically study the positron elastic scattering by an atom with a multielectron semifilled subshell in its structure. We focus on gaining the initial insight into the specifics of such a process. The positron scattering by the Mn(…3d54s2, 6S) atom with a 3d5 semifilled subshell (e+ + Mn scattering) is chosen as a case study. We account for both the electron correlation and the formation of a e+ + e− virtual positronium (Ps) in the intermediate states of the e+ + Mn system. Electron correlation is taken into account in the framework of the self-energy part of the scattering positron Green function generalized for the application to semifilled-shell atoms. The influence of the virtual Ps formation on the positron scattering is taken into account by the reduction of the energy of the virtual positron plus atomic-excited-configuration states by the Ps-binding energy, to a reasonable approximation. We unravel the importance and specificity of the influence of both the virtual Ps and electron correlation on e+ + Mn elastic scattering. We demonstrate spectacular differences between the electron and positron scattering processes.

Synopsis Wigner time delay in photodetachment of Br− has been studied. The absence of Coulomb phase in photodetachment process makes the Wigner time delay more sensitive to the centrifugal barrier shape resonance and threshold effects. Important relativistic effects have been found near the threshold region.

We examine giant resonant enhancement of high-order harmonics generation (HHG) in the Mn atom and its singly charged Mn+ ion. Our theoretical model combines single active electron tunneling and propagation with correlation-enhanced recombination. Previously, this approach demonstrated its useability for the Xe atom [Phys. Rev. A 100, 013404 (2019)]. Spin-polarized Hartree-Fock and random-phase approximation with exchange calculations are carried out to evaluate the correlation enhancement factor in Mn and Mn+, which is then compared with other reported calculations and the experiment.

We study theoretically the experimental angle-differential elastic electron scattering off a C-60 fullerene with the aim to elucidate whether its features depend primarily on the details of the full molecular potential of C-60 or they can be understood without reference to the full nature of the potential. We demonstrate the success of the latter option.

Initial insights into spin-polarized photoelectron fluxes from fullerene anions are presented here. Both the angle-dependent and angle-integrated degrees of spin polarization of said photoelectron fluxes are discussed. Empty C60−(2p) and endohedral H@C60−(2p) and He@C60−(2p) anions, where the attached electron resides in a 2p state, are chosen as case studies. We uncover the characteristics of the phenomenon in the framework of a semi-empirical methodology where the C60 cage is modeled by a spherical annular potential, rather than aiming at a rigorous study. It is found that the spin-polarization degree of photoelectron fluxes from fullerene anions can reach large values, including a nearly complete polarization, at/in specific values/domains of the photoelectron momentum. This is shown to correlate with an inherent feature of photoionization of fullerenes, the abundance of resonances, known as confinement resonances, in their photodetachment spectra owing to a large empty space inside fullerenes. Moreover, the results obtained can serve as a touchstone for future studies of the phenomenon by more rigorous theories and/or experiments to reveal the significance of interactions omitted in the present study.

We uncover dramatic variations of the Wigner photoemission time delay with energy and angle in the vicinity of a Fano resonance with the time delay taking opposite signs at different angles at the same energy as well as at the opposite sides of the resonance at the same angle. These variations are illustrated by choosing the Ne 2s -> 3p autoionizing state as a case study. Moreover, we demonstrate the existence of strikingly significant changes in time delay due to relativistic effects despite Ne being a low-Z atom. This finding shows the possibility for utilizing time delay chronoscopy as a route towards experimental probing of relativistic interactions and the phases of individual transition matrix elements upon atomic photoionization of low-Z atoms. Finally, we develop a practical parametrization to model and explain the angle and energy variation of the autoionizing resonance time delay in the nonrelativistic limit.

We show that, in contradiction to intuitive perception, the impact of polarization of a C-N cage by the outgoing photoelectron on photoionization of a A@C-N endohedral atom decreases as the size of C-N increases. This is counter-intuitive, owing to the fact that the polarizability of C-N drastically increases with the increase of N. Furthermore, we demonstrate that this is also in contrast to the polarization impact on elastic electron scattering off empty fullerenes; for the latter, the bigger the C-N cage (the greater its polarizability), the greater the polarization impact is on the scattering process. We provide researchers with a 'relative frame of reference' for understanding how the increase in the size of the cage affects the polarization impact on the corresponding photoionization and electron scattering processes. Our findings are supported by the direct calculations of the photoionization and electron scattering cross sections for selected fullerene cages from 'dwarf' C-20 to 'giant C-240. The calculations are performed in the framework of a semi-empirical modeling of C-N and A@C-N fullerene systems obtained with and without account for the dipole static polarizability of a fullerene cage.

We prove in the present paper that simple modeling of a complicated highly polarizable C-60 target by a rectangular (in a radial coordinate) square well potential in combination with the static polarization potential provides a viable approximation for a low-energy elastic-electron scattering off this target. The proof is based on the results of the comparison of the calculated angle-differential elastic-electron scattering cross section off C-60 versus corresponding experimental data.

We predict an observable Wigner time delay in outer atomic shell photoionization near inner shell thresholds. The near-threshold increase of time delay is caused by intershell correlation and serves as a sensitive probe of this effect. The time delay increase is present even when the inner and outer shell thresholds are hundreds of electron volts apart. We illustrate this observation by several prototypical examples in noble gas atoms from Ne to Kr. In our study, we employ the random phase approximation with exchange and its relativistic generalization. We also support our findings by a simplified, yet quite insightful, treatment within the lowest-order perturbation theory.

An empirical ansatz for the complex photoionization amplitude and Wigner–Eisenbud–Smith time delay in the vicinity of a Fano autoionization resonance are proposed to evaluate and interpret the time delay in the resonant region. The utility of this expression is evaluated in comparison with accurate numerical calculations employing the ab initio relativistic random phase approximation and relativistic multichannel quantum defect theory. The indisputably good qualitative agreement (and semiquantitative agreement) between corresponding results of the proposed model and results produced by the ab initio theories proves the usability of the model. In addition, the phenomenology of the time delay in the vicinity of multichannel autoionizing resonances is detailed.

We provide the initial insight into the angle-differential photodetachment spectra of fullerene anions beyond the dipole approximation by utilizing a broadly used modelling of C$_{n}$. In the model, the C$_{n}$ cage is approximated by a spherical attractive potential of a certain inner radius, thickness and depth which binds an external electron, thereby turning into a C$_{n}^{-}$ anion. It is demonstrated in the framework of the utilized model, which a single-electron model in its essence, that the dipole and dipole-quadrupole approximations might get broken down in the angle-differential photodetachment spectra of fullerene anions, C$_{n}^{-}$, at a great number of photon energies of only a few tens of eV. Moreover, the breakdown occurrences are shown to start developing at lower photon energies, and the frequency of the occurrences grows with increasing size of C$_{n}^{-}$. The findings are demonstrated by direct calculations of angle-differential photodetachment cross sections of the C$_{60}^{-}$, C$_{240}^{-}$, C$_{540}^{-}$ and C$_{1500}^{-}$ anions. It is not clear beforehand how electron correlation might affect the predicted anomalies in the C$_{n}^{-}$ angle-differential photodetachment spectra, so that the verification(s) of the predicted breakdowns by means of more sophisticated theories is urged.

Photoionization cross sections, photoelectron angular-asymmetry parameters and photoionization time delays for A@C_60 endohedral atoms are studied with account for both the individual and combined effects of dipole static polarization (DSP) of C_60 by the outgoing photoelectron, interior static polarization (ISP) of C_60 by the ion-remainder, A^+, and atomic-core relaxation of the encapsulated atom upon its ionization. It is unraveled that the DSP effect is weak; it changes the phase of confinement-resonant oscillations in σ_nℓ, β_nℓ and τ_nℓ without generally noticeable changes in their magnitudes, unless σ_nℓ concentrates a relatively large part of oscillator strength of the continuum spectrum near threshold. This is counter-intuitive in view of a large dipole static polarizability of C_60, α > 800 a.u.. Furthermore, it is demonstrated that the DSP effect results in the transmission of a part of oscillator strength of the continuum spectrum of A@C_60 into its discrete spectrum. It is shown that the DSP effect is counteracted by ISP. Possible reasons behind the made findings are provided. Photoionization of Xe@C_60, Ne@C_60, H@C_60 and some hypothetical C^-_60 fullerene anions is chosen as a case study. For Xe@C_60, the role of atomic-core relaxation of the ionized encapsuled Xe^+-ion-remainder on the 4d photoionization of Xe@C_60 is revealed to be of utter significance, as in free Xe. The random phase approximation with exchange (RPAE) and generalized RPAE were used in the study. The C_60 cage is modelled by a spherical attractive potential of a certain inner radius, thickness and depth. Its dipole static polarization potential is approximated by the Bates dipole static potential.

Sourav Banerjee, Pranawa C. Deshmukh, Valeriy K. Dolmatov, Steven T. Manson, and Anatoli S. Kheifets 1 Department of Physics, Indian Institute Technology Madras, Chennai 600036, India Department of Physics, Indian Institute of Technology Tirupati, Tirupati, 517 506, India Department of Physics and Earth Science, University of Northern Alabama, Florence, AL 35632, USA Department of Physics and Astronomy, Georgia State University, Atlanta, 30303, USA Research School of Physics and Engineering, The Australian National University, Canberra ACT 2601, Australia (Dated: August 14, 2018)