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Calculations of the photoionization cross section of the 2p and 3s subshells of free Ar and Ar@C60 as examples have been performed using the molecular structure of the confined system and time-dependent density functional theory for the dynamical quantities. The results for Ar2p in the combined system exhibit significant confinement resonances with the lower-energy ones being quite sharp, in contrast to the results of jellium-model calculations. In addition, calculations done with and without interchannel coupling between the photoionization channels of the 2p subshell of the Ar atom and the 1s subshell of the C60 shell show that, in this case, the coupling is of negligible importance, even though the C1s cross section is more than an order of magnitude larger than that of Ar2p in the 300 eV range. The Ar3s, which is not hybridized, also exhibits confinement resonances, but is very strongly affected by interchannel coupling with photoionization channels from the C60 shell. The phenomenology of both 2p and 3s subshells is explained in terms of the interchannel-coupling matrix elements. These results should be applicable to inner-shell ionization of essentially any endohedral fullerene system. [ABSTRACT FROM AUTHOR]

Experimental cross sections and cross-section ratios reported in the literature for direct double ionization of the outer shells of helium, neon, and argon atoms resulting from bare ions ranging from protons to uranium and for antiprotons are analyzed in terms of a first- and second-order interference model originally proposed by McGuire [J. H. McGuire, Phys. Rev. Lett. 49, 1153 (1982)]. Empirical formulas for the various contributions to double ionization plus information about the phase difference between the first- and second-order mechanisms are extracted from the data. Projectile and target scalings are also extracted. Total cross sections and their ratios determined using these formulas and scalings are shown to be in very good agreement with experimental data for lower-Z projectiles and impact velocities larger than 1 a.u. For very-high-Z projectiles, the amount of double ionization is overestimated, probably due to saturation of probabilities that is not accounted for in scaling formulas. [ABSTRACT FROM AUTHOR]

It is demonstrated that the effect of a static cage potential on the photoionization of endohedrally enclosed atoms can be incorporated into standard /¿-matrix calculations using one of two independent methods. For photoionization processes occurring entirely within the fullerene, the outer-region solutions can be modified by the additional cage potential to yield phase-shifted Coulomb functions that are matched to the inner-region R matrix. Alternatively, if the cage potential is contained within the /¿-matrix “box,” it can be directly incorporated into the formalism via simple one-electron integral contributions to the Hamiltonian, yielding a modified R matrix. Both methods are applied to the photoionization of Xe@C6o in the vicinity of the giant 4d → ∊f shape resonance and are found to be in excellent agreement with each other [ABSTRACT FROM AUTHOR]

Photoionization cross-section calculations are performed on the ground state ([Ne]3s23p64s2Se1/2) and the first two excited states {[Ne]3s23p63d2De3/2 and [Ne]3s23p63d2De5/2) of Ca+ ions for photon energies from threshold to 45.0 eV using the relativistic (Breit-Pauli) /f-matrix method. The discrete Ca2+ orbitals are generated using the computer program AUTOSTRUCTURE; 30 configurations are included in the configuration-interaction calculation for the states of Ca2+. The prominent 3p → 3d giant resonances are analyzed and identified, and our results are compared with experimental results, and rather good agreement is found. Using results of our previous photoionization calculations on Sc2+ and Ti3+ ions, the strongest and broadest resonances in the photoionization cross section of those three ions (Ca+, Sc2+, and Ti3+), in terms of width and oscillator strengths, are compared to show the evolution as a function of nuclear charge. [ABSTRACT FROM AUTHOR]

Photoionization cross sections and photoelectron angular distribution asymmetry parameters are calculated for the 4d10, 5s², 5p6, and 6s² subshells of atomic barium as a test of the relativistic multiconfiguration Tamm-Dancoff (RMCTD) method. The shape resonance present in the near-threshold region of the 4d subshell is studied in detail in the 4d photoionization along with the 5s, 5p, and 6s subshells in the region of the 4d thresholds, as the 4d shape resonance strongly influences these subshells in its vicinity. The results are compared with available experiment and other many-body theoretical results in an effort to assess the capabilities of the RMCTD methodology. The electron correlations addressed in the RMCTD method give relatively good agreement with the experimental data, indicating that the important many-body correlations are included correctly. [ABSTRACT FROM AUTHOR]

The effects of endohedral confinement on the correlation energy of Be, Mg, and Ca atoms have been investigated using modified Hartree-Fock and multiconfiguration Hartree-Fock methods where the endohedral system (A@C60) is approximated as an atom enclosed in an attractive spherically symmetric potential well of inner radius r∼5.8 a.u. and thickness of Δ∼1.89 a.u., and correlation energies are studied as a function of the depth of the confining potential (0≤U0≤1 a.u.) to give some idea as to how the correlation energy behaves in different endohedral environments. In general, we have found that as a function of well depth, starting from the free atom, valence electrons diffuse outward in the presence of the confining potential, which causes the electrons to be further apart, thereby decreasing the correlation energy; however, with further increase of well depth, the valence electrons become trapped in the confining well and, as a result of their being closer together, the correlation energy increases. [ABSTRACT FROM AUTHOR]

We conduct a systematic study of the dipole phase and the photoelectron group delay (Wigner time delay) in inner shell photoionization of noble gas atoms from Ne to Xe. Our study encompasses the tender x-ray spectral range and extends to 1 keV photoelectron energy. We employ both the relativistic and the nonrelativistic versions of the random-phase approximation with exchange. We identify the long-range Coulomb and short-range Hartree-Fock contributions to the dipole phase which governs the Wigner time delay variation from the threshold to the whole considered range of photoelectron energies. [ABSTRACT FROM AUTHOR]

Photoionization cross sections and dipole angular distribution asymmetry parameters, ß, of 5s and Ad shells of the closed-shell ions (La3+, La9+, and La11+) in the La isonuclear sequence have been studied using the relativistic random phase approximation. The positions of the 5s Cooper minima in La3+ and La9+ ions are found to be extremely sensitive to the details of electron correlation. The results show that the 5s cross sections of La3+ and La9+ do not lie along the same curve near the thresholds; the Ad cross sections, however, do match well in overlap regions so they lie along the same curve, over the isonuclear sequence, except for a shift in threshold towards higher energies with increasing degree of ionization. [ABSTRACT FROM AUTHOR]

Initial insight into time delay in Mn photoionization in the region of the 3p->,3d giant autoionization resonance is gained in the framework of the "spin-polarized" random-phase approximation with exchange. The dramatic effect of the giant autoionization resonance on the time delay of photoemission from the 3d and 4s valence subshells of the Mn atom is unraveled. Strong sensitivity of the time delay of the 4s photoemission to the final-state term of the ion remainder [Mn+(4s¹,5S) vs Mn+(4s¹', 7S)] is discovered. It is shown that photoionization time delay in the autoionizing resonance region is explicitly associated with the resonance lifetime, which can thus be directly measured in attosecond time-delay experiments. Similar features are expected to emerge in photoionization time delays of other transition-metal and rare-earth atoms with half-filled subshells that possess giant autoionization resonances as well. [ABSTRACT FROM AUTHOR]

Studies of 2s → np autoionization resonances in the neon isoelectronic sequence using relativistic multichannel quantum-defect theory are reported. The relativistic random-phase approximation is used to calculate the quantum-defect parameters. The autoionization resonances are characterized using Fano resonance parameters. The results are compared with available experimental and theoretical data, and the behavior of the resonances as a function of Z is discussed. [ABSTRACT FROM AUTHOR]

Time delay of photoemission from valence ns, np3/2, and np1/2 subshells of noble-gas atoms is theoretically scrutinized within the framework of the dipole relativistic random phase approximation. The focus is on the variation of time delay in the vicinity of the Cooper minima in photoionization of the outer subshells of neon, argon, krypton, and xenon, where the corresponding dipole matrix element changes its sign while passing through a node. It is revealed that the presence of the Cooper minimum in one photoionization channel has a strong effect on time delay in other channels. This is shown to be due to interchannel coupling. [ABSTRACT FROM AUTHOR]

Relative partial cross sections have been measured following photoexcitation of atomic chlorine near the Cl 2p and Cl 1s ionization thresholds. In addition, Breit-Pauli R -matrix calculations have been carried out in the region of the 2p thresholds, and the results are compared with experiment. Owing to angular-momentum considerations, it was found that the resonances associated with the higher 2p-1 thresholds should be significantly wider than the lower ones, and this is borne out in both the experimental and the theoretical results. It is shown that a large number of resonance series contribute to the cross section, which make it difficult to untangle, and suggestions for further work to better understand the spectra are presented. [ABSTRACT FROM AUTHOR]

Double photoionization accompanied by loss of n C atoms (n=0, 2, 4, 6) was investigated by merging beams of Xe@C+60 ions and synchrotron radiation and measuring the yields of product ions. The giant 4d dipole resonance of the caged Xe atom has a prominent signature in the cross section for these product channels, which together account for 6.2 ± 1.4 of the total Xe 4d oscillator strength of 10. Compared to that for a free Xe atom, the oscillator strength is redistributed in photon energy due to multipath interference of outgoing Xe 4d photoelectron waves that may be transmitted or reflected by the spherical C+60 molecular cage, yielding so-called confinement resonances. The data are compared with an earlier measurement and with theoretical predictions for this single-molecule photoelectron interferometer system. Relativistic R-matrix calculations for the Xe atom in a spherical potential shell representing the fullerene cage show the sensitivity of the interference pattern to the molecular geometry. [ABSTRACT FROM AUTHOR]

The photoionization cross section of the Ca atom, confined by a spherical annular potential, has been calculated using a modification to the Belfast i?-matrix code for a variety of depths of the confining potential. The results show that the external potential affects the cross section, which is dominated by doubly excited autoionizing states, quite significantly, causing much of the oscillator strength to move into the discrete region of the spectrum. In addition, increasing well depth causes a level crossing between the first two excited states of Ca+, thereby changing the ordering of resonances dramatically. The calculation for free Ca shows excellent agreement with experiment. [ABSTRACT FROM AUTHOR]

The multiconfiguration Tamm-Dancoff technique (MCTD) is applied to study photodetachment of negative ions of lithium and sodium. A cusplike structure is found in the photodetachment cross section just below the first detachment-plus-excitation threshold of Li- (Li 2p), and of Na- (Na 3p), in qualitative agreement with existing theoretical and experimental results. The current work emphasizes the importance of correlation in the form of configuration interaction in the photodetachment process and demonstrates the utility of MCTD in dealing with highly correlated systems. [ABSTRACT FROM AUTHOR]