Photoionization from H@C ₆₀ and Ar@C ₆₀ is studied by employing two different model potentials to simulate the endohedral environment. The contrast between the diffusive Gaussian annular square well (GASW) and the compact annular square well (ASW) model potential on the photoionization dynamics is explored by studying the cross-section, phase shift, and Wigner time delay. A realistic description of the fullerene environment requires the right amount of diffuseness and compactness, and the proposed GASW model has both of these attributes. The dynamical properties of an encapsulated atom are studied systematically as a function of the well depth, and it is found that the GASW model induces mild and realistic confinement oscillations on the photoionization parameters in comparison with the ASW case. In addition, the present work brings out a few other interesting features. For a confining well depth of 1.03 a.u., the phase of the dipole matrix elements takes sharp jumps akin to the behavior of dipole phases at the Cooper minimum. In the case of free Ar, the correlation effects contribute less to the time delay in comparison with screening effects near the Cooper minimum. In search of a realistic potential to mimic confinement, the GASW model potential is recommended.

Characteristic features of Wigner photoemission time delay from endohedral anions A@${\mathrm{C}}_{60}{}^q$ along with their dependence on the anion charge $q$ are unraveled. Specifically, significant enhancement of the time delay in the innermost dipole photoionization channels near threshold is found, owing to the presence of the Coulomb confined resonances (CRs). Moreover, it is shown that interchannel coupling of the inner-shell Coulomb CRs with outer-shell photoionization channels results in resonantly enhanced time delay in the release of the outer-shell photoelectron well above, several hundreds eV, the outer-shell thresholds. It is also demonstrated that, and explained why, photoionization cross sections of the innermost subshells as well as outer subshells (near the inner-subshell threshold) depends only very weakly on the anion charge $q$, but the dependence of the corresponding time delays on $q$ can be significant. Furthermore, Coulomb CRs are found to emerge in the innermost quadrupole photoionization channels as well, thereby causing considerable time delay in the quadrupole photoemission. These findings are illustrated in calculations of the photoionization of inner and outer subshells of the endohedral anions Ne@${\text{C}}_{60}{}^{-1}$ and Ne@${\mathrm{C}}_{60}{}^{-5}$ that were chosen as case studies.

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.

An earlier work of Dolmatov et al. [ Phys. Rev. A 73, 013201 (2006)] on Ne inside a charged fullerene ($\mathrm{Ne}@{\mathrm{C}}_{60}^{\mathrm{q}=-3,-5}$) has revealed the presence of two unusually large confinement resonances termed Coulomb confinement resonances (CCRs); the first CCR is narrow and the second one is broad and their origins are attributed to the presence of charge on the fullerene surface. The present work extends this study to other subshells and also to other systems such as $\mathrm{Ar}@{\mathrm{C}}_{60}^{\mathrm{q}}$ and $\mathrm{Xe}@{\mathrm{C}}_{60}^{\mathrm{q}}$ using relativistic random phase approximation, but with an aim to investigate the genesis of the CCRs. Further, a detailed analysis of these resonances within a single active electron approximation unearths an interesting difference associated with the origin of these two CCRs. The photoionized electron is temporally trapped in the ${\mathrm{C}}_{60}$ confinement well for the case of narrow resonance and in the atomic well region for the case of broad resonance. Also, the present work shows that the broad resonance can occur even for the case of $q=0$ and is demonstrated for $\mathrm{Xe}@{\mathrm{C}}_{60}^{\mathrm{q}}$ as a test case.

Plane-wave electrons undergo momentum transfer as they scatter off a target in overlapping spherical waves. The transferred momentum leads to target structural information to be encoded in angle and energy differential scattering. For symmetric, periodic, or structured targets this can engender diffraction in the electron intensity both in real and momentum space. With the example of elastic scattering from a C_{60} molecule we show these simultaneous diffraction signatures. Simulated angle-momentum diffractograms can be imaged in experiments with a two-dimensional detector and an energy-tunable electron gun. The result may inspire the invention of technology to extend scopes of electron diffraction studies, open a track of electron crystallography using the momentum-differential diffraction, and motivate research about controlling the time delay between the pump laser pulse and probe electron pulse by tuning the electron impact speed in ultrafast electron diffraction experiments.