Accepted Manuscripts

We investigate the fundamental three-body Coulomb process of elastic electron-positronium (e^--Ps) scattering below the Ps(n = 2) threshold. Using the complex Kohn variational method and trial wave functions that contain highly correlated Hylleraas-type terms, we accurately compute ^1,3S-, ^1,3P-, and ^1,3D-wave phase shifts, which may be considered as benchmark results. We explicitly investigate the effect of the mixed symmetry term in the short-range part of the ^1,3D-wave trial wave function on the phase shifts and resonances. Using the complex Kohn phase
shifts we compute, for e⁻-Ps scattering, the elastic differential, elastic integrated, momentum-transfer, and ortho-para conversion cross sections and determine the importance of the complex Kohn D-wave phase shifts on these cross sections. In
addition, using the short-range part of the ¹S-wave trial wave function for the bound state of the purely leptonic ion of Ps⁻, and the complex Kohn ¹P trial wave function for the continuum state, we determine the Ps⁻ photodetachment cross section in the length, velocity, and acceleration forms.

Resonances leave prominent signatures in atomic and molecular ionization triggered by the absorption of single or multiple photons. These signatures reveal various aspects of the ionization process, characterizing both the initial and final states of the target. Resonant spectral features are typically associated with sharp variations in the photoionization phase, providing an opportunity for laser-assisted interferometric techniques to measure this phase and convert it into a photoemission time delay. This time delay offers a precise characterization of the timing of the photoemission process.
In this review, a unified approach to resonant photoionization is presented by examining the analytic properties of ionization amplitude in the complex photoelectron energy plane. This approach establishes a connection between the resonant photoemission time delay and the corresponding photoionization cross-section. Numerical illustrations of this method include: (i) giant or shape resonances, where the photoelectron is spatially confined within a potential barrier, (ii) Fano resonances, where bound states are embedded in the continuum, (iii) Cooper minima (anti-resonances) arising from kinematic nodes in the dipole transition matrix elements, and (iv) confinement resonances in atoms encapsulated within a fullerene cage.
The second part of this review focuses on two-photon resonant ionization processes, where the photon energies can be tuned to a resonance in either the intermediate or final state of the atomic target. Our examples include one- or two-electron discrete excitations both below and above the ionization threshold. These resonant states are probed using laser-assisted interferometric techniques. Additionally, we employ laser-assisted photoemission to measure the lifetimes of several atomic autoionizing states.

Faraday rotation spectroscopy and absorption spectroscopy are performed simultaneously in a dual comb spectroscopy arrangement with quantum cascade laser combs operating at ~8 μm. The system uses free-running laser combs that provide ~70 cm-1 spectral coverage and ~2 MHz spectral resolution. Detection of NO2 in an equilibrium mixture with N2O4 and N2O is used to demonstrate selective measurements of paramagnetic NO2 in the presence of spectrally interfering diamagnetic species.

We present absolute L-shell photoionisation cross sections for the S+, S2+, S3+ ions. The cross sections were obtained using the monochromatised photon beam delivered by the SOLEIL syn- chrotron source coupled with an ion beam extracted from an electron cyclotron resonance source (ECRIS) in the merged dual-beam configuration. The cross sections for single, double and triple ionisation were measured and combined to generate total photoionisation cross sections. For each of the S+, S2+ and S3+ ions, the photon energy regions corresponding to the excitation and ionisation of a 2p or a 2s electron (∼ 175-230 eV) were investigated. The experimental results are interpreted with the help of multiconfigurational Dirac-Fock (MCDF) and Breit-Pauli R-Matrix (BPRM) or Dirac R-Matrix (DARC) theoretical calculations. The former generates photoabsorption cross sec- tions from eigenenergies and eigenfunctions obtained by solving variationally the multiconfiguration Dirac Hamiltonian while the latter calculate cross sections for photon scattering by atoms. The cross sectional spectra feature rich resonance structures with narrow natural widths (typically ≤ 100 meV) due to 2p → nd excitations below and up to the 2p thresholds. This behaviour is consistent with the large number of inner-shell states based on correlation and spin-orbit mixed configurations hav- ing three open subshells. Strong and wide (typically ∼ 1 eV) Rydberg series of resonances due to 2s → np excitations dominate above the 2p threshold.

Gaining insight into the interaction of charged particles with nitrous oxide (N₂O) is crucial for advancing our understanding of atmospheric processes and the environmental impacts of N₂O. N₂O plays a pivotal role in climate change, specifically contributing to global warming in the troposphere and ozone depletion in the stratosphere. In addition, it is of great importance in the fields of plasma physics, atomic and molecular physics, laser physics, and medicine. The cross sectional data obtained from collision studies provide fundamental information about the dynamics involved in the few-body system under investigation. This paper presents electron impact double differential cross sections (DDCSs) for secondary electrons emitted from N₂O molecules. The measurements were conducted within fixed incident electron energy ranges of 50-350 eV, covering emission angles between 30° and 130°. Notably, a forward-backward angular asymmetry has been observed in the angular distribution of the DDCS for the detected electrons.

Molecular ions formed in cold hybrid ion-atom experiments may find interesting applications ranging from precision measurements to controlled chemical reactions. Here, we investigate electronic structure of the Sr₂⁺ molecular ion, which may be produced by photoassociation of laser-cooled Sr⁺ ions immersed into an ultracold gas of Sr atoms or by ionization of ultracold Sr₂ molecules. Using ab initio electronic structure methods, such as the coupled cluster and configuration interaction ones with small-core relativistic energy-consistent pseudopotentials and large Gaussian basis sets, we calculate potential energy curves for the ground and 41 excited electronic states, and electric dipole transition moments between them. We show that alkaline-earth molecular ions despite of their apparently simple structure with three valence electrons only are challenging for state-of-the-art quantum chemistry methods due to their multireference nature and high density of states. Finally, we calculate and analyze Franck-Condon factors governing the photoionization of ground-state Sr₂ molecules into ²Σ_u⁺ and ²Σ_g⁺ states of Sr₂⁺ molecular ions. The present results may be useful for studying and guiding formation and spectroscopy of cold Sr₂⁺ molecular ions.