Academic literature on the topic 'Orbital Feshbach resonance'

Photoionization or photodetachment is an important process. It has applications in solar- and astrophysics. In addition to accurate wave function of the target, accurate continuum functions are required. There are various approaches, like exchange approximation, method of polarized orbitals, close-coupling approximation, R-matrix formulation, exterior complex scaling, the recent hybrid theory, etc., to calculate scattering functions. We describe some of them used in calculations of photodetachment or photoabsorption cross sections of ions and atoms. Comparisons of cross sections obtained using different approaches for the ejected electron are given. Furthermore, recombination rate coefficients are also important in solar- and astrophysics and they have been calculated at various electron temperatures using the Maxwell velocity distribution function. Approaches based on the method of polarized orbitals do not provide any resonance structure of photoabsorption cross sections, in spite of the fact that accurate results have been obtained away from the resonance region and in the resonance region by calculating continuum functions to calculate resonance widths using phase shifts in the Breit–Wigner formula for calculating resonance parameters. Accurate resonance parameters in the elastic cross sections have been obtained using the hybrid theory and they compare well with those obtained using the Feshbach formulation. We conclude that accurate results for photoabsorption cross sections can be obtained using the hybrid theory.

AbstractUsing a hypocycloidal electron spectrometer, the total scattering cross section of slow (0–9 eV) electrons and the dissociative electron attachment cross section for thymine molecules in the gas phase were measured. The ionization cross section for a thymine molecule was studied in the energy range of 9–32 eV. Some features were found in the scattering cross section, caused by the formation and decay of short-lived states of the molecular negative ion. Three of them ( E = 0.32, 1.71, and 4.03 eV) relate to shape resonances; the others, which are observed for the first time, refer to the Feshbach resonances (or core-excited resonances). In the total dissociative attachment cross section in the energy range of E < 4 eV, a clear structure is observed due to the formation of a negative ion (T–H)^–, and a less intense structure associated with the total contribution of fragment thymine ions is found above 4 eV. The correlation of the features found in the total scattering cross section and in the dissociative attachment cross section is assessed. The absolute total scattering cross section was obtained by normalizing the measured curve to the theoretical calculation. In the total ionization cross section, features are observed that are associated both with the effect of the formation of fr-agment ions and with ionization due to the ejection of electrons from the orbitals of the outer shell of the molecule.

In this thesis I report on the experimental results obtained during the years of my PhD in the laboratory of the University of Florence devoted to the investigation of quantum degenerate gases of Ytterbium. I discuss the main results that we achieved, focusing the attention on the experiments concerning two main research lines, the first related to the quantum simulation of synthetic gauge fields with ultracold Yb atoms and the second one to the investigation of two-orbital quantum physics exploiting the 1S0->3P0 clock transition. In particular we have been able to unify these fields of research simulating for the first time a synthetic gauge field for neutral atoms exploiting the orbital degree of freedom offered by two-electron atoms. The realization of artificial gauge fields for neutral atoms is a current trend in the context of quantum simulation and several techniques have been proposed and experimentally realized. Here we adopt a recently proposed quantum simulation scheme which relies on the concept of synthetic dimension. In this scheme an internal degree of freedom of the atom is interpreted as an extra dimension of the system and a hybrid 2D ladder is realized combining this synthetic dimension with a real one-dimensional optical lattice. An artificial magnetic field naturally arises in this hybrid 2D lattice as a consequence of the phase imprinted on the atoms by the laser coupling between the synthetic sites. We exploited this scheme in two different experiments with fermionic 173Yb, in which we map the synthetic dimension in the first case on the ground and clock states of the atom and in the second case on the nuclear spin states of the ground level. Couplings between synthetic sites are realized exploiting single-photon clock transitions and two-photon Raman transitions in the first and second experiment respectively. Despite their simplicity these systems feature some fundamental properties of larger quantum Hall bars, one of which is the presence of chiral currents counter-propagating on the synthetic edges. We have been able to induce and detect these chiral currents in ladders characterized by two and three (only in the Raman case) legs. In the case of the clock approach, for which the experimental realization is simpler, we have also been able to tune the artificial magnetic field and characterized for the first time the strength of the currents as a function of the synthetic flux, a result impossible to achieve in real solid-state systems where magnetic fields of the order of several thousand of Tesla would be required. In the three-leg Raman case we have also investigated the dynamics of the system observing the skipping-orbit-like trajectories performed by fermions in the hybrid space after a quenching of the synthetic tunnelling. In another experiment we used the orbital degree of freedom of 173Yb to demonstrate the possibility to implement Spin-Orbit Coupling (SOC) with single-photon clock transitions in a system of fermionic atoms trapped in a one-dimensional optical lattice, using as pseudospin states the fundamental level 1S0 and the clock state 3P0. This orbital approach to the synthesis of SOC in ultracold gases allows us to overcome some of the limitations imposed by Raman schemes in alkali atoms, where heating due to the presence of intermediate levels has detrimental effects in the observation of many-body processes. The emergence of SOC is detected by evaluating the broadening of the clock spectroscopic response which results from transitions connecting states with different lattice quasimomentum. Our ability to observe these narrow features relies on the high spectroscopic resolution of our clock laser system and is enabled by the long-term stabilization of the laser frequency on the metrological reference delivered by INRiM (the Italian metrological institute) from Turin to Florence through a 642-km-long fiber link. Remarkably, exploiting the long term accuracy provided by the fiber link, we have been able to improve the absolute value of the clock transition in 173Yb by two orders of magnitude with respect to the value previously reported in literature. We exploited the orbital degree of freeedom of 173Yb also to realize a new kind of Feshbach resonance which allows for the tuning of the scattering properties in a mixture of atoms in different orbital states. The possibility to tune interactions by means of standard Feshbach resonances lacked in two-electron atoms due to the absence of a hyperfine structure in the fundamental state. We instead experimentally demonstrated how a similar mechanism is possible also for this class of elements provided that atoms in two different electronic states are considered. In particular, we exploited the orbital Feshbach resonance mechanism to realize a strongly interacting two-orbital gas of 173Yb and characterized the resonance position evaluating the hydrodynamic expansion of the gas. The last part of the thesis reports, instead, some results in which the properties of clock excitation in bosonic 174Yb have been investigated. By means of high resolution spectroscopic measurements on particles confined in 3D optical lattices, the scattering lengths and loss rate coefficients for atoms in different collisional channels involving the ground level 1S0 and the metastable state 3P0 are derived. These quantities, that at our knowledge were still unreported in literature before our work, set important constraints for future experimental studies of two-electron atoms for quantum-technological applications.

There are a number of approaches to study interactions of positrons and electrons with hydrogenic targets. Among the most commonly used are the method of polarized orbital, the close-coupling approximation, and the R-matrix formulation. The last two approaches take into account the short-range and long-range correlations. The method of polarized orbital takes into account only long-range correlations but is not variationally correct. This method has recently been modified to take into account both types of correlations and is variationally correct. It has been applied to calculate phase shifts of scattering from hydrogenic systems like H, He+, and Li2+. The phase shifts obtained using this method have lower bounds to the exact phase shifts and agree with those obtained using other approaches. This approach has also been applied to calculate resonance parameters in two-electron systems obtaining results which agree with those obtained using the Feshbach projection-operator formalism. Furthermore this method has been employed to calculate photodetachment and photoionization of two-electron systems, obtaining very accurate cross sections which agree with the experimental results. Photodetachment cross sections are particularly useful in the study of the opacity of the sun. Recently, excitation of the atomic hydrogen by electron impact and also by positron impact has been studied by this method.