Improved molecular strong-field approximation theory is used to calculate the ionisation probability for the high-order above-threshold ionisation process induced by a few-cycle pulse with two carrier frequencies and one envelope. The asymmetry in the photoelectron momentum distribution is due to the ultrashort nature of the driving pulse and due to the relative orientation of the molecule with respect to the laser field. We introduce the generalised asymmetry parameter, which can be used to quantitatively measure the asymmetry between the photoelectron spectra along arbitrarily many selected directions. We investigate the difference between the asymmetry parameters calculated for atomic and molecular targets and show that the contributions to the asymmetry strongly depend on the type of the employed driving pulse. For the driving pulse with components that are linearly polarised with mutually orthogonal polarisations, we find that the main source of the asymmetry, especially in the high-energy part of the spectrum, is the ultrashort nature of the pulse. The relative orientation of the molecule with respect to the laser pulse only affects the low- and medium-energy parts of the spectrum. On the other hand, for the driving pulse with circularly polarised counterrotating components, the asymmetry introduced by molecular orientation is more pronounced. We also analyse the influence of the characteristics of molecular orbitals on the asymmetries using the examples of N2 and O2 molecules.

Using a strong-field-approximation theory, we investigate the high-order above-threshold ionization of diatomic molecules exposed to the monochromatic and bichromatic elliptically polarized fields. We devote particular attention to the difference between the photoelectron momentum distributions obtained with fields with opposite helicity. This difference is quantified using the elliptic-dichroism parameter, which represents the normalized difference between the differential ionization rates calculated with driving fields with opposite helicity. We find that this parameter strongly depends on the molecular orientation with respect to the laser field. In addition, this dependence is different for molecules with different types of highest-occupied molecular orbital. In other words, we show that the molecular structure is imprinted onto the elliptic-dichroism parameter for both monochromatic and bichromatic driving fields. This is explained by analyzing the interferences between various partial contributions to the differential ionization rate. In this way, elliptic dichroism also serves as a tool to analyze the electron dynamics. Finally, for heteronuclear diatomic molecules, we show that the elliptic dichroism is different from zero even for the direct electrons, i.e., the electrons that after liberation go directly to the detector. In this case, the dependence on the molecular orientation is far more pronounced for a bichromatic driving field.

Computed tomography (CT) is a diagnostic imaging process that uses ionising radiation to obtain information about the interior anatomic structure of the human body. Considering that the medical use of ionising radiation implies exposing patients to radiation that may lead to unwanted stochastic effects and that those effects are less probable at lower doses, optimising imaging protocols is of great importance. In this paper, we used an assembled 3D-printed infant head phantom and matched its image quality parameters with those obtained for a commercially available adult head phantom using the imaging protocol dedicated for adult patients. In accordance with the results, an optimised scanning protocol was designed which resulted in dose reductions for paediatric patients while keeping image quality at an adequate level.

For more than two years, coronavirus disease 19 (COVID-19) has represented a threat to global health and lifestyles. Computed tomography (CT) imaging provides useful information in patients with COVID-19 pneumonia. However, this diagnostic modality is based on exposure to ionizing radiation, which is associated with an increased risk of radiation-induced cancer. In this study, we evaluated the common dose descriptors, CTDIvol and DLP, for 1180 adult patients. This data was used to estimate the effective dose, and risk of exposure-induced death (REID). Awareness of the extensive use of CT as a diagnostic tool in the management of COVID-19 during the pandemic is vital for the evaluation of radiation exposure parameters, dose reduction methods development and radiation protection.

We address ionization of a diatomic molecule by a bichromatic elliptically polarized field with co-rotating components. Using the strong-field approximation we investigate symmetry properties of the photoelectron momentum distribution and explore the minima which appear in the photoelectron spectra. We distinguish two types of minima: (i) two-center interference minima which appear due to the destructive interference of the contributions of two electron wave packets emitted from the two centers of the diatomic molecule and (ii) the one-center minima which are caused by the interference of the parts of the wave packet emitted from the same atomic center at different times. The position of the two-center interference minima depends on the molecular orientation. When a molecular orbital is modelled using the atomic orbitals of a specific parity, the position of the two-center interference minima does not depend on the ellipticity of our driving field. However, when a molecular orbital consists of both odd and even atomic orbitals the interference of their contributions and the position of the minima depend on the ellipticity. The position of the interference minima in the photoelectron momentum plane is confirmed using the saddle-point method. The position and the number of the one-center minima do not depend on the molecular orientation, but they strongly depend on the ellipticity of the field components. Finally, comparing the photoelectron spectra of the CO molecule with the spectra of homonuclear molecules and the NO molecule we show that the electron probability density distribution plays a significant role for the high-energy rescattered electrons.

Using our theory which is based on the strong-field approximation we analyze high-order above-threshold ionization and high-order harmonic generation processes for the case of the homonuclear diatomic molecules exposed to an orthogonally polarized two-color (OTC) laser field. The OTC field represents a superposition of two linearly polarized fields with mutually orthogonal polarizations and different frequencies. We analyze the photoelectron energy spectra and the harmonic ellipticity as a function of the ratio of the intensities of the OTC laser-field components and the relative phase. Some combinations of the values of these parameters lead to the high-energy electrons, while the harmonic ellipticity depends strongly on the ratio of the intensities of the laser-field components. It is possible to find the value of this ratio for which the ellipticity of the emitted harmonics is large. The signes of ellipticity are opposite for the molecular orientations which are connected through the reflection with respect to the axis along the first OTC field component. This symmetry is explained using the expression which relates the T-matrix element and the harmonic ellipticity.

By analyzing angular and energy distributions of the photoelectrons emitted in strong-laser-field-induced ionization of molecules, one can obtain information about the molecular structure and the ground-state symmetry. High-energy part of the photoelectron spectra in the above-threshold ionization (ATI) is characterized by a plateau region in which the ionization probability is practically energy independent. The photoelectron yield drops off exponentially for electron energies higher than some critical energy, i.e. the mentioned plateau is followed by an abrupt cutoff. We investigate the influence of the molecular ground state symmetry on this plateau region and show that, analyzing the corresponding high-order ATI spectra, one can obtain information about the highest occupied molecular orbitals (HOMOs) of the considered molecules. We present results for different homonuclear diatomic molecules: N2, O2, Ar2 and C2 having, respectively, the σ g , π g , σ u and π u symmetries of the HOMO. Particular attention is devoted to the C2 molecule since high-order ATI spectra for this molecule have not been analyzed yet. We consider ATI by a linearly polarized laser field for which the mentioned plateau can be well-developed, depending on the orientation of the molecular axis with respect to the laser-field polarization axis. The HOMO-symmetry-dependent (dis)appearance of the plateau is particularly pronounced for the parallel and perpendicular orientations. Our findings are valid for a wide range of the laser-field intensities and wavelengths, which is important for realization of the suggested experiments. Using the improved molecular strong-field approximation, the theory which is particularly suitable for the analysis of high-energy ATI spectra, for the case of the C2 molecule and different molecular and laser parameters, we investigate various features of the plateau, such as its length and the interference minima and their positions.

S-matrix theory is used in order to analyze the energy spectra of electron-atom potential scattering assisted by a bicircular (two-component circularly polarized) laser field having corotating field components. The double scattering (rescattering) is also included in the analysis by applying the second Born approximation in the expansion of the S-matrix element. We have investigated how the energy spectrum of scattered electrons is affected by the scattering angle. We have also analyzed the sensitivity of the energy spectrum to the relative phase and intensity ratio of the laser-field components. The calculated energy spectra are characterized by the plateau-like oscillatory structures with abrupt cutoffs. Positions of these cutoffs in the energy spectra are confirmed by a classical analysis. Rescattering effects can be observed in the calculated energy spectra for certain values of the scattering angle. These effects are represented by the second plateau in the energy spectrum. This is different from the process of above-threshold ionization/detachment by a bicircular laser field, where the (re)scattering effects in the photoelectron energy spectra cannot be observed in the case of corotating laser-field components.

Molecular strong-field approximation is applied to above-threshold detachment of homonuclear diatomic molecular negative ions. Differences between the photodetachment amplitudes for neutral diatomic molecules and diatomic anions, for both direct and rescattered electrons, are examined. Numerical results for the photoelectron spectra of $ {\rm C}_2^ - $C2− molecular anions for different intensities and wavelengths of a linearly polarized laser field and different molecular anion orientations are shown and discussed. Two-center destructive interference minima (suppression regions) in the rescattering part of the photoelectron spectra are observed. For molecules with molecular orientation defined by the angle $ {\theta _L} $θL with respect to the laser-field polarization axis, these minima manifest as two parallel straight lines in the distribution of the photoelectron yield presented in the photoelectron momentum plane. These lines make the angle $ {90^ \circ } - {\theta _L} $90∘−θL, with the momentum component parallel to the laser-field polarization axis. Focal-averaged photoelectron spectra are also presented and analyzed.