Generalized three-photon ionization cross sections of Kr atom in the region between the 4p 3/25 and 4p 1/25 ionization thresholds are calculated. The calculations are performed using the correlation function method, which performs an exact summation over full sets of intermediate states of the three-photon transition. The strong impact of the rearrangement of the atomic core and its polarization by an excited electron on the three-photon ionization cross sections is demonstrated. The necessity of going beyond the active-electron model, in which all three photons are subsequently absorbed by the active 4p-electron, is shown to be important to achieve an agreement between the cross sections, calculated in the length and velocity gauges of the dipole transition operator. The presently computed three-photon ionization cross sections are in good agreement with the previously measured ones, available in the literature.

Angular distributions of photoelectrons with a defined spin orientation, released by the absorption of a single circularly polarized photon from randomly oriented chiral molecules, are derived and analyzed within the electric-dipole approximation. The revealed symmetry properties of the distributions support previous findings that the total spin polarization of photoelectrons is independent of the enantiomer of a chiral molecule, whereas the angular distribution of the spin polarization is enantiosensitive. Numerical applications to the achiral HI and chiral 1-iodo-2-methylbutane molecules illustrate that these analytical results also hold for multiphoton ionization processes.

We optimize the internuclear geometry and electronic structure of a model chiral system to achieve a maximal photoelectron circular dichroism (PECD) in its one-photon ionization by circularly polarized light. The electronic structure calculations are performed by the single center method, while the optimization is done using quantum alchemy employing a Taylor series expansion. Thereby, the effect of bond lengths and uncompensated charge distributions on the chiral response of the model is investigated theoretically in some detail. It is demonstrated that manipulating a chiral asymmetry of the ionic potential may enhance the dichroic parameter (i.e., the PECD) of the randomly oriented model system well beyond β1 = 25%. Furthermore, we demonstrate that quantum alchemy is applicable to PECD despite the unusually strong coupling of spatial and electronic degrees of freedom and discuss the relative impact of the individual degrees of freedom in this model system. We define the necessary conditions for the computational design of PECD for real (non-model) chiral molecules using our approach.

Angular distributions of photoelectrons emitted upon double core-hole (DCH) generation in nitrogen and oxygen molecules are studied theoretically in the frame of a molecular reference. The respective electronic structure calculations are performed by the single center method for photoelectron kinetic energies up to 40 eV in the relaxed-core Hartree–Fock approximation. The molecular frame photoelectron angular distributions are computed for single-site and two-site DCH creation processes and further analyzed for different orientations of the molecular axis with respect to the electric field vector of linearly polarized incident light and for localized or delocalized emitting atomic site scenarios. The present theoretical results provide reliable predictions for future experiments with high-repetition free-electron lasers.

Spin polarization in the multiphoton above-threshold ionization of 5p3/2- and 5p1/2-electrons of Xe with intense 395nm, circularly polarized laser pulses, is investigated theoretically. For this purpose, we solve the time-dependent Schrödinger equation on the basis of spherical spinors. We, thus, simultaneously propagate the spin-up and spin-down single-active-electron wave packets, driven by the laser pulses in the ionic potential, which includes the spin-orbit interaction explicitly. The present theoretical results are in good agreement with the recent experimental results [D. Trabert et al., Phys. Rev. Lett. 120, 043202 (2018)].

A multichannel single center (MCSC) method for the theoretical description of the electron continuum spectrum in molecules is reported. The method includes coupling between different continuum channels via electron correlations and describes, thereby, photoelectron continuum in the Tamm–Dancoff (configuration interaction singles) approximation. Basic equations of the non-iterative one-channel single center (SC) method and their extension to the MCSC method are presented, and an efficient scheme for their numerical solution is outlined. The method is tested on known illustrative examples of the Ar 3s-, HCl 4 σ - and N 2 1 σ -photoionization processes, where inter-channel coupling plays a very important role. Unlike our previous SC studies, the present MCSC method can be reliably applied to photoionization of outer and valence molecular orbitals, where inter-channel correlations in the continuum might be relevant.

Sequential two-photon ionization is a process that is experimentally accessible due to the use of new free-electron laser sources for excitation. For the prototypical rare Ar gas atoms, a photoelectron spectrum (PES) corresponding to the second step of the sequential two-photon double ionization (2PDIII) at a photon excitation energy of 65.3 eV was studied theoretically with a focus on the consequences of electron correlations in the considered process. The calculation predicts many intense lines at low photoelectron energies, which cannot be explained on the basis of a one-electron approximation. The processes that lead to the appearance of these lines include many-electron correlations, either in the first or second step of photoionization. A significant fraction of the intensity of the low-energy part of PES is associated with the Auger decay of the excited states formed at the second step of 2PDI. The shape of the low-energy part of the 2PDIII PES is expected to be dependent on both the energy of photon excitations and the flux of the exciting beam.

Synopsis The two-photon ionization of the Ne–CH 4 and Ar–SH 4 isoelectronic systems was studied theoretically for the photon energies exceeding the ionization threshold of the outer shells. The calculations demonstrate that the two-photon ionization of the investigated objects is almost entirely a collective process if the one-photon energy corresponds to the energy of the n ℓ → ε ℓ R ′ giant resonance. The largest contribution to the transition (2) amplitude stems from the Auger-like correlation n ℓ 2 → ( 2 ) ε ℓ R ′ 2 ⇒ n ℓ ε ″ ℓ ″ .

A detailed theoretical analysis of the 1s photoionization of neon is presented. It is found that the most significant many-electron correlation in computing photoionization of inner shells is the rearrangement of the outer shells caused by the inner vacancy. Further noticeable effects are: (i) the polarization of the ion core by the outgoing photoelectron and (ii) the coherent effect of double excitation/ionization. The core polarization increases the photoionization cross section by about 10% at the 1s threshold, and the coherent excitation results in further increases by about 5%. Incoherent excitation of the satellite channel leads to an additional 10% increase in the photoabsorption cross section in the double-ionization threshold region.

The two-photon above-threshold 3p-ionization of argon for the photon energies exceeding the 3p-ionization threshold is studied using the correlation function technique. Generalized two-photon ionization cross sections were calculated taking into account many-electron correlations. The calculations demonstrate that the two-photon ionization of Ar at the 3p4 threshold is almost entirely a collective process. The decisive contribution in the above-threshold two-photon 3p-ionization at the exciting-photon energies corresponding to the 3p→εd giant resonance comes from the Auger-like many-electron correlations of ε′ dε″d − 3pεf type.

Rearrangement of the electron shells accompanying inner-shell photoionization of the CH4 molecule is studied theoretically. For this purpose, the K-shell photoionization cross section σ1s (ω) and the respective photoelectron angular distribution (PAD) parameter βe1s(ω)are computed in different approximations using the single center method. It is obtained that a strong mixing of the partial ionization channels in the vicinity of the 1s ionization threshold, caused by the the non-spherical part of the molecular potential, results in a qualitative difference between the electronic rearrangement effects in molecules, as compared to atoms.

The progress and the chronology in understanding the influence of electron correlations on the electronic structure of atoms and the dynamics of atomic processes is reviewed focusing on benchmark rare-gas atoms. The contributions and the chronological development of Photon-Induced Fluorescence Spectroscopy (PIFS), measuring dispersed-fluorescence emission cross sections upon excitation by single photons provided by monochromatized synchrotron radiation is described. Selected experimental results obtained by complementary techniques are also discussed for comparison. The basic suites of computer programs used for the investigation of the many-electron effects in atoms and the obtained results are analysed. Special attention is paid to the Configuration Interaction Pauli–Fock approximation with Core Polarization (CIPFCP) method used to interpret the PIFS data.

The absolute generalized cross sections and angular distribution parameters of photoelectrons for the two-photon above threshold $3p$-ionization of Ar were calculated in the exciting photon energy range from 15.76 to 36 eV. The correlation function technique developed earlier was extended for the case when an intermediate-state function is of a continuum-type. We show that two-photon ionization of Ar near the $3p^{4}$ threshold to a large extent is determined by the $(3p\dashrightarrow\varepsilon d)^{2}$ two-photon absorption via the giant resonance. This many-electron correlation causes (i) an increase of the photoionization cross sections by more than a factor of 3; (ii) the appearance of resonances in the exciting-photon energy range of the doubly-excited states. The predictions are supported by a good agreement between length and velocity results obtained after taking into account of the higher-order perturbation theory corrections.

Photoionization is a process where interaction of matter with photons of sufficiently high energy removes electron(s) from the target. The phenomenon is one of the main tools used in material sciences, because if the energy of the incident radiation Ep is known and the kinetic energy Ek of the emitted electron can be measured, the binding energy Eb of the emitted electron in the matter is Eb=Ek-Ep. Then the spectrum of photoelectrons (Eb versus intensity) provide insight to the chemical environment and the electronic structure of the studied target. Despite of its importance, calculation of photoelectron spectra of materials is a notoriously difficult task. The complete picture requires calculation of both, binding energies and photoionization matrix elements, which can be then used to obtain further properties, such as angular dependence or spin polarization of photoelectrons. One of the main challenges in the calculation of photoionization is the description of the continuum wavefunction of the emitted electron. Even after of decades of research, the problem remains to challenge theoretical physicists. The continuum problem is fairly well understood in spherically symmetric systems, such as atoms, but even for the smallest manycenter systems description of continuum states remain as one of the main bottlenecks in theoretical calculations of photoionization. Different approaches and levels for describing continuum wavefunctions of molecules have been developed over the years. Recently one of the less known methods, known as single-center (SC) method has been investigated and revived [1]. SC method was developed in the 1960s [2] and was therefore one the first ways for treating the continuum problem. However, due to its considerable computer memory demands, it remained nearly unused until the very recent years. The method is based on the fact that spatial molecular orbitals can be always expanded with respect to a chosen single center as a linear combination of spherical harmonic functions. The expansion allows writing the 3dimensional molecular Hartree-Fock (HF) equations to an infinite (but converging) set of atomic-like radial HF equations, which are straightforward to solve using matrix algebra. The method has been recently used to predict photoelectron spectra, photoionization energy dependence and angular distribution parameters for small molecules (see, references in [1]) and lithium clusters [3]. The presentation to be given at the physics days will describe the mathematical formalism behind the SC method, recent results obtained for small lithium clusters and future development plans at Oulu university.

A method for the calculation of transition amplitudes of two-photon ionization processes is developed. It is based on computing a correlation function, which enables the summation over intermediate states of two-photon transitions. Two-photon ionization transition amplitudes were calculated in the lowest order of perturbation theory with taking into account many-electron correlations. The noniterative numerical scheme provided a solution of the differential equation for the correlation function at exciting-photon energies close to the intermediate discrete resonance states. Cross section and angular distribution parameters for photoelectrons of the two-photon 3p photoionization of Ar were calculated. The exciting-photon energy ranged from 8 eV to 15 eV. For the first time an ab initio polarization potential considers the polarization of the atomic core by the excited photoelectron was included in the calculation. This effect increases the photoionization cross section at the photon energy from 8 eV to 10 eV by approximately 15% and shifts the computed energies of the intermediate discrete-state resonances, bringing them to excellent agreement with the experimental energies.

Cross sections and angular distribution parameters for the single-photon ionization of all electron orbitals of Li2-8 are systematically computed in a broad interval of the photoelectron kinetic energies for the energetically most stable geometry of each cluster. Calculations of the partial photoelectron continuum waves in clusters are carried out by the single center method within the Hartree-Fock approximation. We study photoionization cross sections per one electron and analyze in some details general trends in the photoionization of inner and outer shells with respect to the size and geometry of a cluster. The present differential cross sections computed for Li2 are in a good agreement with the available theoretical data, whereas those computed for Li3-8 clusters can be considered as theoretical predictions.

The radiationless decay of the Xe N-4,N-5 levels is investigated theoretically. The strong influence of the giant resonance in the 5s5p(5)(P-1)epsilon f channel on the partial and total Auger widths via the interchannel mixing is revealed. The computed widths of the N-4 and N-5 levels, equal to 105.9 and 109.6 meV, respectively, are in excellent agreement with the measured values 104(3) and 111(3) meV (Jurvansuu et al 2001 Phys. Rev. A 64 012502). The shape of the computed N4,5OO Auger spectrum is in good agreement with the measured one. Interchannel interaction reduces the probability of the 5p5p-4d epsilon g Auger decay by more than an order of magnitude. This conclusion is supported by the good agreement between computed and measured Auger electron angular distribution parameters.

The influence of the interchannel interaction on the radiationless decay of the Xe 4d(9)np resonances is found to be extremely large due to the potential barrier effect. Interaction between the 5s5p-4d epsilon f and 5p5p-4d epsilon g decay channels reduces the decay transition amplitude associated with the outgoing g wave by a factor of about 3. The effect is confirmed by the good agreement between computed and measured photoionization cross sections and polarization parameters of the Xe 5p(4)mp levels.

Cross sections and angular distribution parameters of electrons ejected via two-photon ionization of the hydrogen molecule by linearly and circularly polarized light are computed in the exciting-photon energy range of 8-14 eV, which covers the first four optical resonant states of the molecule. Photoelectron partial waves in the continuous spectrum are obtained within the single center method in precise numerical potential of the molecular ion field. The correlation function technique is used to calculate the two-photon transition matrix element. The presently computed angular-resolved spectra are in good agreement with the theoretical results available for the photon energy of 8.854 eV and can be considered as reliable predictions in the respective energy range.

Many-electron effects in the photoionization of Kr near the 3d(9)np resonances were investigated theoretically. Cross sections for the population of the 4p(4)((1)D)mp states, their alignment and orientation, and angular distribution of the Auger electrons were computed considering the resonance Auger effect as a coherent process. Probabilities of the radiative cascades following the resonant Auger effect were estimated quantitatively within the transition-array technique. The role of intra-and inter-shell correlations was investigated. Lineshapes computed, taking into account interchannel interaction, are in good agreement with the measured ones. Fourfold excitations of the Kr ground state were proposed to influence the resonance population, alignment and orientation of the 4p(4)np levels.