Frontiers in Nuclear and Particle Physics

Total and differential cross sections for positron and electron impact on argon atoms are compared in order to show their similarities and differences. These comparisons provide information as to how antimatter-matter atomic interactions are like, or different from, matter-matter interactions that are normally encountered. Plus such comparisons provide information about how simply changing the direction of the coulomb field in atomic interactions influences the interaction probabilities and the kinematics. Data taken from the literature are used for these comparisons. The selected data are considered to be the most reliable available and representative of the many studies performed to date.

The kinetic energy release (KER) of molecular fragments is of major interest in molecular reaction dynamics. When dissociation reactions of polyatomic ions occur, some of the excess internal energy of the ion is released as kinetic energy of the two fragments. The KER provides important information about the structures of the molecular species involved and on the energetics and dynamics of the reaction. To meet this end, the translational kinetic energy release distribution spectra of selected ionic fragments, produced through dissociative single and double photoionization of dichloromethane (DCM) molecule at photon energies around the chlorine L edge, were measured from the shape and width of the experimentally obtained time-of-flight (TOF) distributions. The kinetic energy release distributions (KERD) spectra exhibit either smooth profiles or structures, depending on the ionic fragment and photon wavelength. In general, the heavier the ionic fragments, the lower are their average KERDs. In contrast, the light H+ fragments are observed with kinetic energies centered around 4.5-5.5 eV, depending on the photon wavelength. It was noticed that the change in the photon wavelength involves a change in the KERDs, pointing out different processes or transitions taking place in the break-up process. In the particular case of double ionization with the ejection of two charged fragments, the kinetic energy distri-butions present own characteristics compatible with the Coulombic fragmentation model. Intending to interpret the experimental data singlet and triplet states at the chlorine L edge of the dichloromethane molecule, associated to the Cl (2p → 10a1*) and Cl (2p → 4b1*) transitions, were determined at multiconfigurational self-consistent field (MCSCF) level and multi reference configuration interaction (MRCI). These states were selected to form the spin-orbit coupling matrix elements, which after diagonalization results in a spin-orbit manifold. Minimum energy pathways for dissociation of the molecule were additionally calculated aiming to give support to the presence of the ultra-fast dissociation mechanism in the molecular break-up.

Here we revisit electron transfer processes yielding negative ion formation in gas-phase collisions of fast neutral potassium atoms (electron donor) and biomolecular target molecules (electron acceptor) in a crossed molecular-beam arrangement. The negative ions formed in the interaction region are time-of-flight (TOF) mass analysed as a function of the collision energy. Selective site and bond excision in the unimolecular decomposition of the transient negative show clear dependence on the collision energy.

Electron correlation (EC) in atoms is at the same time very important and challenging investigation topic due to its many-body interactions, nevertheless it may be the main process to describe the absolute electron impact cross section for the ionization of atoms and molecules. The outer-shell double photoionization of a multielectron target is usually a less important process in comparison to ionization of a single electron by a photon and it is determined completely by electron-electron interaction. The objective in this chapter has been upon a review of the recently found relationship between SO amplitudes and the number of target electrons as well as their density (electrons per volume). Through the comparison between the asymptotic values of the experimental ratios of the double-to-single photoionization cross-sections from the literature, a scaling law was achieved. This scaling allows us to predict the SO amplitudes for several atomic elements up to xenon within a factor two. The electron SO amplitudes following outer-shell photoionization have been plotted as a function of the target atomic number, Z, and static polarizability, α . Our results are in qualitative agreement with the experimental data.

In this chapter we analyze the multiple ionization by impact of |Z|=1 projectiles: electrons, positrons, protons and antiprotons. Differences and similarities among the cross sections by those four projectiles allows us to have an insight on the physics involved. Mass and charge effects, energy thresholds, and relative importance of collisional and post-collisional processes are discussed. For this purpose, we performed a detailed theoretical-experimental comparison for single up to quintuple ionization of Ne, Ar, Kr and Xe by particles and antiparticles. We include an extensive compilation of the available data for the sixteen collisional systems, and the theoretical cross sections by means of the continuum distorted wave eikonal initial state approximation. We underline here that post-collisional ionization is decisive to describe multiple ionization by light projectiles, covering almost the whole energy range, from threshold to high energies. The normalization of positron and antiproton measurements to electron impact ones, the lack of data in certain cases, and the future prospects are presented and discussed.

The fragmentation of the tetrachloromethane molecule, following core-shell photoexcitation and photoionization in the neighborhood of the chlorine K-edge has been studied by using time-of-flight mass spectroscopy and monochromatic synchrotron radiation. Branching ratios for ionic dissociation were derived for all detected ions, which are informative of the decay dynamics and photofragmentation patterns of the core-excited species. In addition, the absorption yield has been measured, with a new assignment of the spectral features. The structure that appears above the Cl 1s ionization potential in the photoionization spectrum, has been ascribed in terms of the existing connection with electron-CCl4 scattering through experimental data and calculations for low-energy electron-molecule cross sections. In addition, the production of the doubly ionized Cl2+ as a function of the photon energy has been analysed in terms of a simple and appealing physical picture, the half-collision model. PACS numbers: 33.80.-b, 33.80.Eh

We present a theoretical description of the spectra of electrons elastically scattered from various samples. The analysis is based on very large scale Monte Carlo simulations of the recoil and Doppler effects in reflection and transmission geometries. Besides the experimentally measurable energy distributions the simulations give many partial distributions separately, depending on the number of elastic scatterings (single, and multiple scatterings of different types). Furthermore, we present detailed analytical calculations for the main parameters of the single scattering, taking into account both the ideal scattering geometry, i.e. infinitesimally small angular range, and the effects of the real, finite angular range used in the measurements. The effect of the multiple scattering on intensity ratios, peak shifts and broadening, are shown. We show results for multicomponent and double layer samples. Our Monte Carlo simulations are compared with experimental data. We found that our results are in good agreement with the experimental observations.

Most of the knowledge of the atomic nucleus was obtained from experimental data involving stable nuclei or nuclei in the vicinity of the stability line. Since the 1980s, several intermediate energy laboratories in the world started to produce nuclei out of the stability line, Rare Ion Beams (RIB). Many new interesting phenomena related to these nuclei have been discovered so far. Light nuclei far away from the stability line such as 6,8He, 11Be, 11Li, 22C, 24O and others have been produced in laboratory. Some of these nuclei present a pronounced cluster structure formed by a core plus one or more loosely bound neutrons forming a kind of low density nuclear matter around the core (nuclear halo). Most of the research involving RIB was developed at intermediate energies, from 30 up to hundreds of MeV/nucleon, and more recently, some facilities are producing secondary beams to perform scattering experiments at energies around the Coulomb barrier. Heavy ion elastic scattering angular distributions at incident energies close to the Coulomb barrier, when plotted as a ratio to the Rutherford cross section, frequently exhibit a typical Fresnel type diffraction pattern, with oscillations around σ/σRuth ≅ 1 in the forward angle region, followed by a strong peak and a subsequent fall of the ratio σ/σRuth at backward angles. This behaviour is a consequence of the interference between the Coulomb and nuclear scattering amplitudes. Due to the low binding energies of exotic projectiles, the coupling between the elastic channel and the breakup states of the projectile is very important and strongly affects the elastic angular distributions, with a damping of the Fresnel oscillations and the complete disappearance of the Fresnel peak in some cases. To describe the effect of the breakup of the projectile in the elastic scattering, new theoretical approaches have been developed. We present 6He + 58Ni elastic scattering angular distributions measured at three energies a little above the Coulomb barrier. The angular distributions have been analyzed by Continuum-Discretized Coupled-Channels calculations to take into account the effect of the 6He breakup on the elastic scattering.Two different approaches were used to describe the structure of the projectile. One considering the 6He as a three-body system consisting of an alpha particle and 2 neutrons which, in addition to the target, form a four-body problem. To compare, in a second approach, the projectile is described as a two-body cluster formed by an alpha particle plus a di-neutron. A new kind of effect due to the projectile breakup in the elastic scattering angular distributions has been reported.

Twin photons, pairs of photons with entangled properties, are now easily produced from nonlinear optical crystals and experiments with these pairs are routinely done at different laboratories [1]. Is it possible to study the same kind of properties with twin atoms, i.e., pairs of massive particles also with entangled properties obtained simply by breaking diatomic homonuclear molecules? In this article we discuss experiments, calculations and proposal that point to this possibility.