Atomic Photoionization Processes Research Articles

Attosecond ionic photoionization spectroscopy.

Photoionization is one of the most fundamental processes in light-matter interaction. Advanced attosecond photoelectron spectroscopy provides the possibility to characterize the ultrafast photoemission process in an extremely short attosecond time scale. Following scattering symmetry rules, residual ions encode ultrafast photoionization prints at the instant of electron removal forming an alternative electron emission chronoscope. Here, we experimentally illustrate the attosecond ion reconstruction of attosecond beating by interference of two-photon transition (RABBIT)-like interferometry through the development of high-resolution ion momentum detection in atomic photoionization processes. Our ion interferometry presents identical momentum- and time-dependent scattering phase shift, as we observed in photoelectron spectroscopy, and thus demonstrates that ion interferometry can be a possible alternative attosecond approach to resolve the photoionization process, without the electron homogeneity limitation.

Photoionization of Atomic Systems Using the Random-Phase Approximation Including Relativistic Interactions

Approximation methods are unavoidable in solving a many-electron problem. One of the most successful approximations is the random-phase approximation (RPA). Miron Amusia showed that it can be used successfully to describe atomic photoionization processes of many-electron atomic systems. In this article, the historical reasons behind the term “random-phase approximation” are revisited. A brief introduction to the relativistic RPA (RRPA) developed by Walter Johnson and colleagues is provided and some of its illustrative applications are presented.

Complete reconstruction of bound and unbound electronic wavefunctions in two-photon double ionization

To be complete, the characterization of the photoionization process of atoms and molecules requires the extraction of all quantum-mechanical phases and amplitudes. So far, complete experiments have accessed only the ionization process of neutral atoms and molecules. Here we report the quantum-mechanically complete characterization of the single and double ionization of neon to yield doubly charged ions. The first ionization step by intense, polarized extreme ultraviolet light from a free-electron laser leaves the ion in a polarized state (that is, one in which the angular momentum of the ion is aligned in space). By controlling the polarization of the light, we determine the bound and continuum components of the system in the first and second ionization steps leading to the formation of doubly charged neon ions. We test the validity of our approach by characterizing the influence of autoionizing ionic states on the two-photon double-ionization mechanism. Our results are important for understanding the physics of the interaction of extreme ultraviolet radiation with ions. Photoionization is one of the most important photophysical events. This process can now be characterized in a quantum-mechanically complete manner by use of polarization-controlled extreme-ultraviolet light derived from a free-electron laser

Influences of Doppler broadening of absorption lines on a multi-step photoionization process of atoms

Photoexcitation and photoionization of atoms, the central part of atom vapor laser isotope separation (AVLIS), relate to the ionization yield and isotope selectivity directly. Doppler broadening of absorption lines is one of the parameters that influence the photoexcitation and photoionization process of atoms. When evaporation temperature is high or beam equipment is absent, Doppler broadening has obvious influence on the ionization yield because most of atoms are non-resonantly excited. In this paper, we study the influences of Doppler broadening of absorption lines on a multi-step photoexcitation and photoionization process of atoms according to the facts of AVLIS. A Doppler-broadened three-level atom system with two resonant lasers is investigated. The interaction between laser field and atoms is described by a density matrix, which is calculated by fourth-order Runge-Kutta numerical method with variable steps. The results show that the ionization yield of atoms decreases with the increase of Doppler broadening of absorption lines under the same laser parameters. At a constant laser power, the ionization yield reaches its maximum value at the best laser bandwidths, which is different from that obtained without Doppler broadening, as reported in published papers. Meanwhile, the best laser bandwidth increases with the increase of Rabi frequency and Doppler broadening when other parameters are constant. Moreover, the best bandwidth of the second laser is smaller than that of the first laser in a multi-step photoionization process of atoms. Therefore, lasers should work at the best bandwidths in AVLIS for highest ionization yield. It is advantageous to make laser bandwidths slightly bigger than the best bandwidths technically for smaller fluctuation of ionization yield, owing to incoercible stochastic volatility in laser bandwidths. The ionization yield increases with the decrease of Doppler broadening, especially at the best laser bandwidths. Therefore, it is necessary to reduce Doppler broadening of atom vapor in laser ionization zone.

Intense field induced excitation and ionization of an atom confined in a dense quantum plasma

Exponential cosine screened Coulomb potential (ECSCP) has been widely used in various branches of physics e.g., solid-state physics, nuclear physics and plasma physics. The atomic photoionization processes under plasma shielding can serve as an efficient tool for study of plasma properties in various environments ranging from nano-scale devices to astrophysical objects. In the present study, ECSCP has been used to characterize a dense quantum plasma and its effect on the spectrum of an atom encaged in a spherical box has been investigated. The work has further been extended to study the response of such a system to a periodic laser field. Photoexcitation and ionization probabilities of the system have been studied as a function of applied laser field parameters using the non-perturbative Floquet technique. As the Floquet method requires exact energy values and oscillator strengths, the spectrum of confined system has been calculated using Bernstein-polynomial method. The variation of energy spectrum and oscillator strengths with screening as well as confinement parameters has also been explored.

Multiple ionization under strong XUV to X-ray radiation

We review the main aspects of multiple photoionization processes in atoms exposed to intense, short wavelength radiation. The main focus is the theoretical framework for the description of such processes as well as the conditions under which direct multiphoton multiple ionization processes can dominate over the sequential ones. We discuss in detail the mechanisms available in different wavelength ranges from the infrared to the hard X-rays. The effect of field fluctuations, present at this stage in all SASE free-electron-laser (FEL) facilities, as well as the effect of the interaction volume integration, are also discussed.

Time-dependent restricted-active-space configuration-interaction method for the photoionization of many-electron atoms

We introduce the time-dependent restricted active space Configuration Interaction method to solve the time-dependent Schr\"odinger equation for many-electron atoms, and particularly apply it to the treatment of photoionization processes in atoms. The method is presented in a very general formulation and incorporates a wide range of commonly used approximation schemes, like the single-active electron approximation, time-dependent Configuration Interaction with single-excitations, or the time-dependent R-matrix method. We proof the applicability of the method by calculating the photoionization cross sections of Helium and Beryllium, as well as the X-ray--IR pump-probe ionization in Beryllium

Nonequilibrium Green function approach to photoionization processes in atoms

We present a quantum kinetic approach for the time-resolved description of many-effects in photoionization processes in atoms. The method is based on the non-equilibrium Green functions formalism and solves the Keldysh/Kadanoff–Baym equations in second Born approximation. An approximation scheme is introduced and discussed, which provides a complete single-particle description of the continuum, while the atom is treated fully correlated.

Theoretical Studies on Expressions of Condensed-PhasePhotoionization Cross Section

A set of general expressions for photoionization cross sections ofatoms or molecules embedded in a medium and a dielectric influencefunction are derived based on Maxwell's equations and theBeer–Lambert's law in this work. The applications are performed forthe photoionization process of solid gold both in theClausius–Mossotti (virtual cavity) model and theGlauber–Lewenstein (real cavity) model firstly. The results showthat the present theoretical expressions of photoionization crosssection can be used to describe the photoionization process ofatoms in condensed matter properly.

Atomic photoionization processes under magnification

Recently, classical simulations of threshold photoionization in the presence of an electric field have shown that a clear distinction between direct and indirect trajectories followed by the outgoing electron can be observed in the patterns of electron impacts on a two-dimensional detector. Subsequently, slow photoelectron imaging experiments have been reported where this distinction could be observed in atomic xenon. Furthermore, using a magnifying electrostatic lens to improve the velocity-map imaging technique, oscillatory patterns were observed modulating the classical envelope that was measured in the experiments of Nicole et al. [Phys. Rev. Lett. 88, 133001 (2002)]. This extension of slow photoelectron imaging, called photoionization microscopy, relies on the existence of interferences between various trajectories by which the electron moves from the atom to the plane of observation. In this article we present the main experimental results obtained both in slow photoelectron imaging and in photoionization microscopy. The formation of the interference pattern is discussed in the framework of a semiclassical model that is described in detail elsewhere. The qualitative information that can be drawn from the experiments is discussed, and the potential applications of photoionization microscopy are considered. Particular attention is paid to the role of continuum Stark resonances that appear between the saddle point in the Coulomb+dc field potential and the field-free ionization limit.

Some recently found interchannel coupling effects in atomic photoionization processes

The present article highlights some recently found interchannel coupling effects that must be taken into account to explain an atomic photoabsorption process. The importance of interchannel coupling effects is illustrated in a few select cases of recent interest: (a) sustenance of interchannel coupling effects at photon energies high enough where the independent particle approximation was believed to hold good; (b) demonstration of the generality of a new spin–orbit activated correlation effect as an essentially many-body collective event; (c) recognition of the scope and limitations of the indispensable approximation techniques to account for each ‘kind’ of correlation effect in configuration interactions between ‘discrete’ and ‘continuum’ channels; and (d) the recognition in some recent studies on atomic mercury, that contrary to the earliest expectations, dynamical interchannel coupling between dipole and quadrupole channels is important as found in the studies of photoionization of atomic xenon as well.

Generalization of the atomic random-phase-approximation method for diatomic molecules:N2photoionization cross-section calculations

Partial and total photoionization cross sections of ${\mathrm{N}}_{2}$ molecule are calculated using the generalization of the random-phase approximation (RPA) which earlier has been successfully applied to the description of the atomic photoionization processes. According to this method, at first the Hartree-Fock (HF) ground-state wave functions are calculated in prolate spheroidal coordinates using the fixed-nuclei approximation. With their help the zero order basis set of single particle Hartree-Fock wave functions containing both discrete excited states and continuous spectrum is calculated in the field of a frozen core of a singly charged ion. The calculations are performed for all four valence shells of ${\mathrm{N}}_{2}$ molecule, $3{\ensuremath{\sigma}}_{g},$ $1{\ensuremath{\pi}}_{u},$ $2{\ensuremath{\sigma}}_{u},$ and $2{\ensuremath{\sigma}}_{g},$ with the intershell correlations fully taken into account within the RPA method. It is demonstrated that different intershell correlations, especially between three outer shells, play an important role in photoionization process. Examples of the influence of intershell correlations on several transitions are presented. Partial and total photoionization cross sections of ${\mathrm{N}}_{2}$ molecule obtained by this method in the photon energy range from ionization threshold up to 70 eV are in a good agreement with the existing experimental data and with the recent RPA calculations [Cacelli et al., Phys. Rev. A 57, 1895 (1998)].

Dynamical screening inK-shell photoionization

Starting from the Hartree-Slater independent electron model, we perform configuration interaction (CI) calculations for both the initial state and the final state in atomic photoionization processes. Our CI K-shell photoionization cross section for Ne displays an enhancement near the threshold due to the dynamical screening effect of the K vacancy, which is in good agreement with experiments. Furthermore, the K-shell ratio function of the CI Ne K shell to the CI ${\mathrm{Ne}}^{+}$ K-shell photoionization cross sections shows no oscillatory behavior, in contrast to the ratio in the Hartree-Slater independent electron calculation.

Trends in ESSR Studies of Atoms

In the past 10 years, photoionization cross sections and photoelectron angular distribution parameters have been measured for the s, p, d, and f subshells of closed-shell atoms and a good understanding has been achieved, in interplay with theory, for the single-electron photoionization process in these atoms. Current research is directed mainly toward the elucidation of two-electron processes, autoionization resonances and toward photoionization processes in open-shell atoms, excited atoms, and ions. Most of the current studies are closely related to many-body effects in the dynamic behavior of the electronic structure.

Recent progress in the study of photoionization processes of atomic species by electron spectroscopy using synchrotron radiation

The use of the synchrotron radiation emitted by the ACO storage ring has recently produced new significant results in the study of single and multiple photoionization processes in atoms. With photon flux in the 10 10-10 11 photons/sec in 1 % band width between 30 and 110 eV, we have been able to investigate photoionization processes with cross sections as low as 0.01 megabarn. It has thus been possible to study the correlation satellites in helium, argon and xenon, to determine subshell photoionization cross sections, including the variation of branching ratios due to the spin-orbit splitting, to produce photoemission spectra of metallic vapors and to observe threshold effects, like the post collision interaction.

R-matrix theory of photoionization. Application to neon and argon

The R-matrix method, which has been used recently to calculate electron-atom and ion collision cross sections and atomic polarizabilities is extended to enable atomic photoionization processes to be studied. Both the initial atomic bound state and the final atomic continuum state are expanded in terms of R-matrix bases. The method is programmed for a general atomic system and then used to calculate the photoionization cross sections of ground state neon and argon atoms leaving the residual ions in their ground or first excited states. Good agreement is obtained with recent experiments using synchrotron radiation both in resonant and non-resonant regions, showing that the method has a wide range of applicability.