S. T. Manson

Interference in the molecular photoionization and Young's double-slit experiment

The photoabsorption by an electron bound by a two-centre potential has been investigated within the framework of the zero-range potential model. Expressions for total photoabsorption cross sections and for the photoelectron angular distributions have been derived for fixed-in-space and randomly oriented targets. The analytical formulae for gerade and ungerade molecular states have been used to analyse separately the molecular effects due to the two-centre ground state of quasi-molecule and diffraction effects that are connected with the spherical waves in the molecular continuum. It is shown that the interference of these waves significantly influences the magnitude of the cross sections near threshold but does not significantly distort the shape of the photoelectron angular distribution and it depends rather weakly on the character of the forces acting between the electron and molecular residue: Coulomb forces for neutral molecular photoionization or the short-range forces in the case of photodetachment of molecular negative ions. It is shown that despite the fact that the photoionization of diatomic molecules is reminiscent of Youngs double-slit experiment, the similarity between these processes has been grossly exaggerated. This is confirmed by comparing the results of the classical interference of an electron scattered by two spatially separated centres with molecular photoelectron angular distributions.

Jellium model potentials for the C60 molecule and the photoionization of endohedral atoms, A@C60

Approximating the C60 shell as a collection of carbon atoms, the potential experienced by a confined atom has been calculated within the framework of the self-consistent spherical jellium model. It has been found that the potential well in this model has a cusp-shaped Lorentz-like profile. The parameters of the model Lorentz-bubble potential (depth and thickness) have been selected so that in the potential well there would be an electronic level corresponding to the experimental electron affinity of the C60 molecule. The spatial distribution of the positive charge of the C-atomic nuclei and the negative charge of the electron clouds forming the electrostatic potential of C60, as a whole, has been analyzed using the Poisson equation. It is demonstrated that the often used radial square-well potential to approximate the C60 corresponds to a non-physical charge density for the C60 molecule. This analysis demonstrates that the phenomenological potentials simulating the C60 shell potential should belong to a family of potentials with a non-flat bottom and non-parallel potential walls similar to the Lorentz-bubble potential. The photoionization cross-sections of a hydrogen atom localized at the center of the C60 shell have been calculated as well. It is found that confinement oscillations in the cross-sections are exhibited within the framework of the cusp-shaped potential model and these oscillations are essentially the same as those in the case of the potential wells with well-defined borders (parallel walls), thereby demonstrating that the inherent characteristic distances of the potential, e.g., radii of the potential walls, or the distances between potential walls, are not necessary to produce confinement resonances; this should be a general result for atoms or molecules confined in near-spherical fullerenes.

Effects of the fullerene (C 60 ) potential and position of the atom (A) on spectral characteristics of endohedral atoms A@C 60

Within the framework of a model representing the potential of a C60 cage as a spherical electro-neutral layer U(r) formed by smeared carbon atoms, the effect of the details of the potential on spectral characteristics of atoms localized inside the fullerene shell has been studied. Using examples of encapsulated H and He atoms, it is shown that for potential shell thickness not exceeding 1.3‐1.5 au, confinement resonance oscillations in the photoionization cross section weakly depend on the shape of the function U(r). With increasing width of the potential well, the confinement resonances in the energy dependence of the photoionization cross section disappear. In addition, it is demonstrated that displacing the doped atom from the centre of the cavity also diminishes the amplitude of the confinement resonance. (Some figures in this article are in colour only in the electronic version)

Strong LSJ dependence of fluorescence yields: Breakdown of the configuration-average approximation

Using the five-electron K-shell vacancy 1s2s22p2 configuration as an example, we show that the fluorescence yields of the eight LSJ states of the configuration exhibit a dramatic dependence on LSJ. These results demonstrate that, in general, configuration-average fluorescence data are inappropriate for astrophysical modeling.

Photoionization of atomic krypton confined in the fullerene C60

The combined effects of interchannel coupling, relativistic interactions and endohedral confinement on the photoionization of atomic Kr are studied. The confinement of the Kr atom placed at the centre of the C60 cage is modelled by placing the atom inside an annular spherical potential. Cross sections for the photoionization and angular distribution of photoelectrons from the 4p, 4s, 3d and 3p subshells are reported within the framework of the relativistic-random-phase approximation.

Theoretical and experimental demonstrations of the existence of quadrupole Cooper minima

Calculations and measurements of the Xe 5s and 5p nondipole photoelectron asymmetry parameters are obtained, which present clear evidence of the existence of quadrupole Cooper minima, i.e., minima in quadrupole matrix elements as a function of energy, in the photoionization process. This verifies earlier predictions of quadrupole Cooper minima.

Nondipole effects in the photoabsorption of electrons in two-centre zero-range potentials

The photoabsorption by an electron bound by a two-centre potential has been investigated within the framework of the zero-range potential model. Expressions for the photoelectron angular distribution have been derived for fixed-in-space and randomly oriented targets and are used to analyse the role played by both nondipole and molecular effects in near-threshold photoabsorption. It is found that, as in the case of hydrogen-like atoms, the nondipole effects in this model system vary as v/c (v and c are the speeds of the photoelectron and light, respectively) and near threshold they are small. The molecular effects due to the two-centre initial state structure, and the photoelectron multiple scattering by the centres in the final continuum state, rapidly decrease as 1/R2, with the increase of the distance R between the atoms of the molecule.

Interchannel Coupling in Ionic Photoionization Far above Threshold: The Neon Isoelectronic Sequence

A study of the evolution of interchannel coupling in the photoionization of the Ne isoelectronic sequence from neutral Ne to Fe16+ has been performed. As in neutral Ne, the 2s cross section dominates the 2p at a high enough energy over the entire sequence and affects the 2p photoionization cross section via interchannel coupling. The importance of this coupling decreases gradually with increasing nuclear charge along the sequence. This has implications for databases of photoionization rates of astrophysical importance.

On the dependence of inner-shell photoionization on the outer-shell structure of

Since inner-shell photoionization cross sections in the region of purely inner-shell-to-inner-shell resonances are generally insensitive to the state of excitation or ionization of outer electrons, the most sensitive comparison between theory and experiment for photoionization cross sections of excited or ionized atoms is in the region of inner-shell-to-outer-shell resonance transitions. The experimental 3p absorption cross section of is compared with theory as an example.

Photoionisation of K+ ions

The cross section for K+ photoionisation has been calculated at the relativistic random-phase approximation level and found to be in excellent agreement with a recent experimental investigation.