Table of contents

The autoionization of high n, n∼280-430, strontium Rydberg states through excitation of the 5s ²S_1/2→5p ²P_1/2 transition in the core ion is investigated. The autoionization rates decrease rapidly as L is increased paving the way for production of long-lived two-electron-excited planetary atoms.

The B-spline R-matrix method is used to investigate the photoionization of neutral iron from the ground and metastable states in the energy region from the ionization thresholds to 2 Ry. The sensitivity of the results is checked by comparing the predictions obtained in different approximations. Inclusion of all terms from the 3d⁶4p and 3d⁵4s4p configurations considerably changes the low-energy resonance structure.

The photodetachment of the negative ion of Al is investigated by employing the B-spline R-matrix method for photon energies ranging from threshold to 12 eV. Several prominent resonance features are predicted, thereby providing new challenges in the study of this highly correlated process.

Absolute cross sections for double and triple photodetachment of O⁻ ions have been measured at photon energies around the threshold for K-shell ionization using the photon-ion merged-beams technique at a synchrotron light-source. In addition, corresponding large-scale ab-initio calculations were carried out at a level of complexity that has never been invoked before. From the comparison between experiment and theory it becomes apparent that the inclusion of multi-electron processes such as double shake-up in the theoretical calculations is crucial for the explanation of the experimental findings.

We use R-matrix method to study the near threshold (17.5eV-20eV) photoionization of neutral fluorine atom and obtain total photoionization cross sections in ³P to ¹D range. The theoretical calculation results fit the experimental results better after we consider core-valence electron correlation.

Dynamic changes due to relativity are very prevalent effects in the photoionization of heavy atoms. Presented is an exploration of the evolution of these dynamic changes as the nuclear charge, Z, increases and how they affect the cross sections.

Electron shakeoff probabilities accompanying inner-shell vacancy production are calculated in the screened hydrogenic model using pseudostates. The results are compared with the values calculated with continuum wave functions.

The Opacity Project is a highly successful venture in which energy levels, oscillator strengths, and photoionization cross sections were calculated for astrophysically important elements up to iron. However, metals heavier than iron are being discovered in stellar atmospheres. The photoionization cross sections for elements and ions not covered by the Opacity Project are usually approximated with hydrogenic formulae. We present a series of valence and inner-shell distorted wave photoioniza-tion cross section calculations for use in model atmosphere calculations covering Ni - Ni¹⁰⁺. We discuss current progress, and future work.

A detailed theoretical study on photoionization of polarized positive neon ion including its autoionizing states is presented, with reference to recent experiments at the free-electron laser FERMI. The feasibility for a complete photoionization experiment on positive ions is demonstrated.

In this work, the influence of the ambient gas (N₂, O₂, Ar and He) on the plasma emission signal, plasma lifetime, electron number density and temperature, as well as the crater morphology of laser-ablated Al has been studied experimentally.

We resolve the discrepancy between the existing theoretical oscillator strengths and the recent X-ray free electron laser (XFEL) experiment for the intensity ratio between two of the strongest emission lines from Ne-like Fe XVII (Fe¹⁶⁺) ion. Our study shows that, by considering the dynamic resonance induced population transfer between Fe XVII and Fe XVI (Fe¹⁵⁺) ions, which coexist in the XFEL experiment, we are able to successfully resolve this difference in theory and ex-periment. Further experimental works are suggested for a more detailed understanding of the dynamic resonance processes for ions under the plasma environment.

We study single- and two-photon double ionization of helium by short XUV pulses by numerically solving the time-dependent Schrödinger equation in full dimensionality within a finite element discrete variable representation scheme. We discuss the joint energy and angular distributions, identify sequential and non-sequential contributions in the double ionization by ultrashort pulse, and track the dynamics of the ionization process in distinct double ionization regimes.

The initial insight into the impact of fullerene polarizability on photodetachment time delay from fullerene anions ${{\rm{C}}}_{N}^{-}$ is exposed. The impact is demonstrated to be significant.

We perform time-dependent local density functional calculations of the quantum phase and Wigner-Smith time delays in the valence photoionization of Xe versus I atoms at higher energies where coupling with the core emission becomes important. Results for the 5s emission are presented.

We demonstrate controlling over the localization of high-lying Rydberg wave packets in argon atoms with orthogonally-polarized two-color laser fields. Experiments and accompanying semiclassical simulations show clear evidences on asymmetric localization of high-lying Rydberg electrons in argon atoms after the interaction with two-color laser fields.

The photoionization by short twisted XUV pulses (X-waves) is investigated in the presence of an intense infrared beam within the S-matrix formalism. It is demonstrated how sidebands are affected by the twist of the ionizing pulse as the pulse duration increases.

We present a theoretical study of ionization of the hydrogen atom due to an XUV pulse in the presence of an IR laser with both electric fields linearly polarized in the same direction. By means of a very simple semiclassical model we explain the photoelectron (PE) spectrum as the interplay of intra- and intercycle interferences.

We present a non-perturbative semiclassical model for strong-field ionization that accounts for path interferences of tunnel-ionized electrons in the ionic potential within the framework of a classical trajectory Monte-Carlo representation of the phase-space dynamics.

We present an intensity dependent study of transverse electron momentum distribution obtained from strong field ionization of argon using linearly polarized few-cycle (6 fs) laser pulses in the intensity range 10¹⁵W/cm²-10¹⁶W/cm².

Wigner-Eisenbud-Smith (WES) photoionization time delay is calculated for atomic barium 5s subshell using Relativistic Random Phase Approximation (RRPA), RRPA with relaxation (RRPA-R) and the Relativistic Multi-Configuration Tamm-Dancoff (RMCTD) techniques. Electron correlations are treated differently in different many body methods; the WES time delay is sensitive to these differences. The present study is aimed at determining the suitability of various many-body methods for the study of the WES time-delay.

The effect of confinement on the angle dependent Wigner-Eisenbud-Smith time delay in photoionization of atomic xenon is studied. A finite spherical annular well potential is used to model the nearly spherical confinement due to fullerene C₆₀ molecular cage. The confinement is seen to have a significant effect on the time delay.

Using a semiclassical approach, we find an enhancement of low-energy structure in above-threshold ionization spectra of Magnesium by intense, mid-infrared laser pulses. By performing electron trajectory and recollision-time distribution analysis, we demonstrate this enhancement results from a focusing of electrons by laser induced dynamical polarization.

We present an ab initio precision investigation of the time-frequency characteristics of high-order harmonic generation (HHG) calculated by solving accurately and efficiently the time-dependent Schrödinger equation by means of the time-dependent generalized pseudospectral method. We extend a new synchrosqueezing transform (SST) to obtain the time-frequency spectra of the HHG and the dynamical phase associated with the dipole-emission time profile. The SST time-frequency analysis allows us to explore the in depth dynamics and contributions of the multi-rescattering trajectories and resonant trajectories in HHG. It also exhibits the subtle details of the spectral and temporal fine structures of the HHG, providing novel insights regarding the dynamical origin of the HHG in below- and above-threshold regimes.

The angular dependence of Wigner-Eisenbud-Smith photoionization time delay has been investigated for the nd → ε f dipole channels in atomic Zn, Cd and Hg using the relativistic random phase approximation (RRPA). The studies have been carried out also in the nonrelativistic limit. This technique enables identification of the relativistic effects on the angular dependence of photoionization time delay.

We study the effects of length and pressure of gas jet on the conversion efficiency of macroscopic harmonics. Our results show that an optimal flux for certain range of harmonics can be obtained by optimizing macroscopic properties of the gas jet.

The intra-pulse interference effects appearing during the ionization of atoms by few-cycle laser pulses is studied. These are the result of coherent superposition of electronic wave packets, which follow different spatio-temporal paths. With the help of a wave function analysis tool, the spatial and temporal interference patterns are disentangled and studied separately.

We simulate the macroscopic Xe high-order harmonic spectra generated by strong mid-IR 1500-nm laser pulse. Our simulations confirm the experimental findings that the intensity of high-energy harmonics can be enhanced effectively by putting gas jet correctly to a position before laser focus.

Intense, circularly polarized extreme-ultraviolet (XUV) and near-infrared (NIR) laser pulses are combined to double-ionize atomic helium via the oriented intermediate He⁺(3p) resonance state. A strong and NIR-intensity dependent circular dichroism is accounted to a helicity dependent AC-Stark shift.

The strong-field ionization of atomic hydrogen induced by elliptically polarized light is studied in detail. In particular, we investigate the ionization spectra for various pulse characteristics, consider the circular dichroism from an excited oriented electronic state, and finally analyze the tunneling regime for few-cycle pulses.

The relativistic intense-field ionization of heavy hydrogenlike ions is investigated. The investigation is performed by numerical solving of the time-dependent Dirac equation. The interaction of intense laser pulses with ions is considered beyond the dipole approximation. As an example, the ionization probabilities of the hydrogenlike tin ion exposed to a strong laser field are calculated.

We follow a new approach for calculating high-harmonic spectra for multi-electron atoms and molecules by propagating the two-particle reduced density matrix. Calculated results are in very good agreement with state-of-the-art many-body wavefunction-based benchmark calculations.

We present a theoretical and experimental study of the subcycle interference in laser assisted XUV ionization of Ar atoms. Averaging over the focal volume happens to blur the intracycle interference, which thus cannot be measured directly. We show that even at these conditions, the intracycle interference can be obtained through the subtraction of two different angle and energy-resolved distributions at slightly different laser intensities.

We investigate dynamic interference in photoionization processes by intense spatially inhomogeneous fields. Simulations from numerical solutions of the three-dimensional (3D) time-dependent Schrödinger equation show that the phenomenon manifests by a multiple peak structure in the momentum distribution of the ejected electron. Its sensitivity to the inhomogeneity strength of the laser field offers the possibility to manipulate the mechanism.

We investigate trichromatic photoionization of neon induced by linearly polarized XUV femtosecond pulses. The fundamental frequency is tuned near an optically allowed transition to create an interference between one-photon and two-photon ionization pathways. The asymmetry in the photoelectron angular distribution is analyzed for a variety of pulse characteristics.

The effective control on the intensity, polarization and the beam width of the THz radiation based on laser-plasma interaction is realized in a magnetized rippled density plasma channel.

Laser-electron interaction in a plasma channel is presented, taking into the action of superluminosity of the laser phase velocity induced by the dispersion of the plasma and the channel.

We present the results of attosecond angular streaking of atomic Hydrogen using elliptically polarized, 5.5 fs pulses that are centered around 770 nm, within the intensity range of 1.65 to 4 × 10¹⁴ W/cm². We measure angular offsets in the photo-electron momentum distribution relative to the peak of the electric field of light in the polarization plane. We find a strong agreement in these results with the solutions of complete 3D-TDSE simulations. Further, we compute the contribution of coulomb effects (between the ionized electron-parent ion) to the measured angular offsets using Yukawa potential, subtracting which yields the real 'tunneling delays'.

In contrast to one-photon transitions and non-resonant multiphoton transitions, time delay in resonant multi-photon electron emission can exhibit large positive and negative values that have no scattering equivalent, due to the interference of multiple ionization paths.

Non-resonant inelastic x-ray scattering (NRIXS) is a powerful tool to reveal the electronic structures of matter while it has a very low cross section. Recently we have extended the NRIXS to atomic and molecular physics with a low-density target. The electronic structures of the ground/excited states and the photoabsorption cross sections of some atoms and molecules have been determined by NRIXS and the newly-developed dipole (γ,γ) method.

We demonstrated photodissociation of sympathetically crystallized CaH⁺ toward rovibrational spectroscopy by UV-UV or IR-UV double resonance. The photodissociation of CaH⁺ was successfully confirmed at λ = 283-287 nm.

A new scheme is presented for determining the hyperfine splitting of highly charged ions via angle-resolved spectroscopic analysis. In particular, we propose two measurements on the angular distribution and linear polarization of the emission line 1s2p³P₁, F = I − 1, I, I + 1 → 1s²¹S₀, F_f = I of He-like isotopic ions, following the stimulated decay of the 1s2s¹S₀, F_i = I level. We find that the angular and polarization behavior of the emitted photons strongly depends on the splitting of the overlapping hyperfine 1s2p³P₁, F resonances, which could be a promising "signature" for the hyperfine splitting determination.

Absolute cross sections for photoprocesses near the K edge of Ne⁺ ions and neutral Ne atoms have been determined at resolving powers up to 27000 facilitating the precise determination of natural widths of the most prominent resonances as well as the associated transition probabilities and branching ratios.

Experimental spectra of the Eu 4f⁷6p_3/2 nl autoionizing states are investigated with three-step isolated-core excitation technique, while the impact of laser polarization on the spectra is studied for the first time.

A new method is described for the detection and study of cold Rydberg atoms. Cold atoms held in an AC-driven magneto optical trap (AC-MOT) are selectively laser excited to a Rydberg state in a stepwise process. Ions or electrons from interactions between Rydberg atoms are then electrostatically extracted and detected via a channel electron multiplier. The apparatus is capable of studying Rydberg atoms from n = 15 to n ∼ 220.

We present a simplified radiation hydrodynamic model based on the fluid dynamic equations and the radiative transfer equation to rapidly investigate the radiation properties and dynamics in laser-produced tin plasmas. The self-absorption features of EUV spectra measured at an angle of 45^° to the direction of plasma expansion have been successfully simulated and explained. The evolution of the plasma temperature has also been evaluated.

We have measured the radiative lifetime of the ²P_3/2 metastable level in Xe³⁺ using an electrostatic ion beam trap. The lifetime was obtained by measuring the number of photons emitted during the ²P_3/2→²D_3/2 transition of Xe³⁺ as a function of the storage time. The lifetime is compared with previously reported theoretical and experimental values.

The hyperfine stark shifts of ground states of ^{87, 85}Rb are calculated by using relativistic configuration interaction plus core polarization (RCICP) approach, which agree with other theoretical and experimental results very well. The hyperfine Stark shift of ⁸⁷Rb is larger than that of ⁸⁵Rb.

The electric-dipole dynamic polarizabilities of the 6s²¹S₀ and 6s6p³P₁ states of Yb atom are calculated by using the CI+MBPT method. Magic wavelengths for the 6s²¹S₀ - 6s6p³P₁ transition are identified.

Measurements of the radiative lifetimes of the two excited states of the platinum anion Pt⁻ are presented. Pt⁻ ions stored in the cryogenic ion storage ring DESIREE were photodetached at different photon wavelengths and the resulting yield of neutral Pt measured as a function of time was recorded. Analysis of the neutral decay curves show a 2.54 ± 0.10 s lifetime for the higher-lying 5d¹⁰6s²S_1/2 excited state and a lifetime in the range of 50–200 ms for the lower-lying 5d⁹6s²²D_3/2 excited state. This is the first study to report the lifetime of a bound anion excited state with an electron configuration different from that of the anion ground state.

We report a spectral measurement of laser-produced Pr plasma and corresponding calculation results for 4d excitations of Pr³⁺ to Pr⁷⁺ ions with the Hartree-Fock method, in which the importance of configuration interaction effects has been evaluated. The plasma parameters have been obtained by comparison of experimental and simulated spectra based on a steady-state collisional-radiative (CR) model.

We present a simplified radiation hydrodynamic model based on the fluid dynamic equations and the radiative transfer equation, which can be used to investigate the radiation properties and dynamics evolution of highly-charged ions in a laser-produced plasma in vacuum.

Measurements of Lamb shift in antihydrogen atom have been planned to extract the antiproton charge radius which has never been experimentally investigated. A hyperfine spin filter and a microwave cavity can be employed.