Autoionizing States Research Articles

Formation of Free Electrons in Collisions of Xe Atoms with α Particles

We have measured the cross sections of elementary processes occurring during collisions of impinging 3He2+ particles with kinetic energies in the range 1–100 keV with Xe atoms, which are accompanied with the formation of free electrons and ions in different final charge states: Xen+ (n = 1–6) and He(2 – m)+ (m = 0–2). We distinguish between ionization processes, in which the impinging α particle does not change the charge state, and processes of capture of one or two electrons, which lead to the formation of a singly charged helium ion or atom with additional removal of electrons from the xenon atom to the continuum. We have determined the cross sections of formation of free electrons in each of these processes and calculated the contribution of multielectron processes to the formation of free electrons. It is shown that upon a change in the energy of collisions between 3He2+ ions from 1 to 100 keV (the velocity of an impinging ion changes from 0.12 to 1.2 a.u.), the mechanism of removal of electrons from the xenon atom changes basically. For low approach velocities of particles, the formation of an autoionization state of the quasi-molecule and its subsequent decay take place. At higher velocities (V > 0.7 a.u.), free electrons are mainly formed because of an abrupt change in the potential energy of electrons when a fast α particle approached the nucleus of the xenon atom.

Theoretical Calculations of Electron Excitation of the Lowest Autoionization States of the Rb Atom

Purpose. The value of the autoionization cross section and its energy dependence are determined to a great extent by excitation and subsequent electron decay of the lowest energy states. Recently, experimental data on the excitation cross sections of the rubidium atom lowest doublet (4p55s2)2P3/2,1/2 and quartet (4p54d5s)4P1/2,3/2,5/2 autoionizing states were obtained which promoted their theoretical investigations. Methods. Theoretical calculations were performed using the Flexible Atomic Code universal program package which takes account of the relativistic nature of the complex atoms excitation by the use of the Dirac-Coulomb Hamiltonian. The radial orbitals for basic relativistic wavefunctions were obtained by solving the Dirac-Fock-Slater equation. Results. Comparison of the experimental excitation cross sections for the doublet (4p55s2)2P3/2,1/2 and quartet (4p54d5s)4P1/2,3/2,5/2 autoionizing states with available theoretical data and our relativistic calculations is presented. The results of the study of the parameter R0 for the intensity ratio for the 2P3/2 and 2P1/2 states of the 4p55s2 configuration are presented. Our calculations in the distorted waves approximation is shown to well describe the experimental data in the energy region above 70 eV. At low collision energies our calculations qualitatively predict the presence of the threshold resonant excitation; however in the energy region 20-50 eV only the R-matrix method qualitatively describes the complex nature of excitation and decay of the lowest quartet autoionizing states of the rubidium atom. Conclusions. Electron excitation of the doublet (4p55s2)2P3/2,1/2 and quartet (4p54d5s)4P1/2,3/2,5/2 autoionizing states of the rubidium atom in the collision energy range from the excitation thresholds up to 700 eV was studied in the terms of the distorted waves relativistic approximation. A comprehensive analysis of the obtained results using the available experimental and theoretical data showed that the relativistic distorted waves approximation taking into account the configurational mixing is a suitable tool for describing the autoionizing states of the rubidium atom in a wide range of collision energies and allowed one to obtain the absolute values of the experimental excitation cross sections of the (4p55s2)2P3/2,1/2 and (4p54d5s)4P1/2,3/2,5/2 states.

The effect of exchange and absorption potential in the distorted wave calculation of electron impact excitation of the autoionizing state of caesium

We have calculated the differential and integral cross sections for electron impact excitation of the lowest autoionizing level of caesium atoms in the energy range 13 – 500 eV using the distorted wave method. In this study we have used a complex distortion potential as the distortion potential both at the initial and the final channels. Our results are compared amongst themselves to establish the effect of exchange and imaginary absorption potentials. We also compared with other available theoretical and experimental results to determine the suitability of our present method. From the results it is seen that when an absorption potential is included in the distortion potential, the integral cross section moves closer to the experimental result in the intermediate energy range 19 – 40 eV.

Perturbative theory of statistically averaged atomic dynamics in fluctuating laser fields

We have developed a perturbative method to model the resonant ionization of atomic systems in fluctuating laser fields. The perturbative method is based on an expansion in terms of the multitime cumulants, a suitable combination of moments (field's coherence functions), used to represent the field's statistical properties. The second-order truncated expansion is expressed in terms of the radiation's power spectrum and the intensity autocorrelation function. We investigate the range of validity of the model in terms of the field's coherence temporal length and peak intensity and have compared the results with conventional Monte-Carlo calculations. We apply the theory in the case of a near-resonant ionization of the Helium 2s2p autoionizing state with a SASE FEL pulse with square-exponentially dependent 1st-order coherence function. The ionization lineshape profile acquires a Voight profile; the degree of the Gaussian or Lorentzian character of which to depend crucially on the field's coherence time.

Pulse length effects on autoionizing states under the influence of intense SASE XUV fields

The Fano absorption line shape of an autoionizing state encodes information on its internal atomic structure and dynamics. When driven near-resonantly with intense extreme ultraviolet (XUV) electric fields, the absorption profile can be deliberately modified, including observable changes of both the line-shape asymmetry and strength of the resonance, revealing information on the underlying dynamics of the system in response to such external driving. We report on the influence of the XUV pulse parameters at high intensity that can be achieved with a free-electron laser (FEL) with statistically broadened spectra based on self-amplified spontaneous emission (SASE). More specifically, the impact of the FEL pulse duration is studied for the example of the doubly excited 2s2p resonance in helium, where line-shape modifications have been measured with XUV transient absorption spectroscopy in Fraunhofer-type transmission geometry. A computational few-level-model provides insight into the impact of different average pulse durations of the stochastic FEL pulses. These findings are supported by measurements performed at the Free-Electron Laser in Hamburg (FLASH) and provide further insight into XUV strong-coupling dynamics of resonant transitions driven by intense high-frequency FEL sources.

Dielectric Recombination of the Cd+ Ion via 4d95s25p Autoionizing States

Dielectronic recombination of the Cd+ (4d105s2S1/2) ion via formation of the 4d95s25p3 $$ {D}_1^0 $$ Cd atom autoionization state and its radiative decay to the 4d105p2 3PJ autoionization states were detected and studied in intersecting electron and ion beams using UV spectroscopy. The 4d95s25p3 $$ {D}_1^0 $$ → 4d105p2 3P0 radiative transition is found to be the strongest. The effective cross-section of the dielectronic recombination at the maximum equals (5.0 ± 2.0) × 10–17 cm2, which is comparable with those for electron excitation of the spectral lines (including resonance ones) of the investigated ion. Radiative decay of the 4d95s25p3 $$ {D}_1^0 $$ autoionization state has a high probability due to interconfigurational interaction (4d105s5p + 4d105p5d + 4d95s25p + 4d95p3).

Contribution of Autoionization Processes to the Electronic Stopping Power

It is shown that the excitation of autoionization states during the collisions of keV-energy ions with a solid plays a determining role in the formation of inelastic energy losses and, accordingly, the electronic-stopping cross section Se. It is proposed to estimate the Se values using the relationship between the cross section for autoionization-state excitation and the ionization cross section. For cases where the ionization cross sections are unknown, scaling is used to calculate the ionization cross sections when the L and M shells are excited. The threshold dependence of Se versus the energy of bombarding ions is predicted.

Polarization-dependent laser resonance ionization of beryllium

Using TRIUMF's off-line laser ion source test stand with a system of tunable titanium sapphire lasers, the polarization dependence of laser resonance ionization has been investigated using beryllium. A significant polarization dependence was observed for the excitation path 1S0 → 1P1∘ → 1S0, which are typical transitions for alkaline-earth and alkaline-earth like elements. This polarization dependence was further verified on Be radioactive isotopes at TRIUMF's isotope separator and accelerator facility (ISAC). Laser polarization was proven to be an important parameter in operating resonance ionization laser ion sources (RILIS).The polarization spectroscopy was performed off-line both on the 2p2 1S0 autoionizing (AI) state and high-n Rydberg states of the 2sns1S0 and 2snd1D2 series. The energy of the 2p2 1S0 AI state and ionization potential (IP) of beryllium were extracted as 76,167(6) cm−1 and 75,192.59(3) cm−1. Polarization spectroscopy can be used to determine the J values of newly found states in in-source spectroscopy of the complex/radioactive alkaline-earth like elements such as Ra, Sm, Yb, Pu and No.

Intensity of Resonant Harmonic Generated in the Multiphoton Ionization Regime

The relative intensities of the resonant and nonresonant high-order harmonics, generated by singly charged gallium and indium ions in the field of a laser pulse with a duration of 20 fs and intensity ranging from 5 × 1013 to 2 × 1014 W/cm2, are calculated. The harmonic intensity is found via the matrix element of multiphoton transition within the perturbation theory including the resonant contribution. The latter increases the harmonic intensity at the transition from the ground state to the autoionizing state. We consider laser wavelength when this transition corresponds to 7 and 15 photons.

Bound-State Electron Dynamics Driven by Near-Resonantly Detuned Intense and Ultrashort Pulsed XUV Fields

We report on numerical results revealing line-shape asymmetry changes of electronic transitions in atoms near-resonantly driven by intense extreme-ultraviolet (XUV) electric fields by monitoring their transient absorption spectrum after transmission through a moderately dense atomic medium. Our numerical model utilizes ultrashort broadband XUV laser pulses varied in their intensity (1014–1015 W/cm2) and detuning nearly out of resonance for a quantitative evaluation of the absorption line-shape asymmetry. It will be shown how transient energy shifts of the bound electronic states can be linked to these asymmetry changes in the case of an ultrashort XUV driving pulse temporally shorter than the lifetime of the resonant excitation, and how the asymmetry can be controlled by the near-resonant detuning of the XUV pulse. In the case of a two-level system, the numerical model is compared to an analytical calculation, which helps to uncover the underlying mechanism for the detuning- and intensity-induced line-shape modification and links it to the generalized Rabi frequency. To further apply the numerical model to recent experimental results of the near-resonant dressing of the 2s2p doubly excited state in helium by an ultrashort XUV free-electron laser pulse we extend the two-level model with an ionization continuum, thereby enabling the description of transmission-type (Fraunhofer-like) transient absorption of a strongly laser-coupled autoionizing state.

Desorption processes in the measurement of weak currents

It was previously shown that the experimental measurement of the lifetime of gas-phase negative ions formed in autoionization states during resonant electron capture by molecules is inherently fraught with inaccuracy due to adsorption of ions on the walls of the ionization chamber, with some of them surviving there and staying stabilized as ‘eternal’ till they eventually desorb back into the vacuum and merge with the total ion flow; this leads to an upward distortion of lifetime measurements. This paper describes a technique for removing desorbed ions from the total flow using an ion filter that lets only gas-phase ions pass into the detection system but detains the desorbed ions due to the differences among their kinetic energies.

Influence of nuclear motion on resonant two-center photoionization

Photoionization of an atom in the presence of a neighboring atom of different species is studied. The latter first undergoes resonant photoexcitation, leading to an autoionizing state of the diatomic system. Afterwards, the excitation energy is transferred radiationlessly via a two-center Auger process to the other atom, causing its ionization. Assuming a fixed internuclear distance, it has been predicted theoretically that this indirect ionization pathway can strongly dominate over the direct photoionization. Here, we extend the theory of resonant two-center photoionization by including the nuclear motion in van-der-Waals molecules. An analytical formula is derived reflecting the influence of molecular vibrational dynamics on the relative strength of the two-center channel. For the specific example of Li-He dimers we show that the two-center autoionizing resonances are split by the nuclear motion into multiplets, with the resonance lines reaching a comparable level of enhancement over direct photoionization as is obtained in a model based on spatially fixed nuclei.

Resonances in Systems Involving Positrons

When an incident particle on a target gets attached to the target, the cross-section at that energy could be much larger compared to those at other energies. This is a short-lived state and decays by emitting an electron. Such states can also be formed by the absorption of a photon. Such states are below the higher thresholds and are called autoionization states, doubly excited states, or Feshbach resonances. There is also a possibility of such states to form above the thresholds. Then they are called shape resonances. Resonances are important in the diagnostic of solar and astrophysical plasmas. Some methods of calculating the resonance parameters are described and resonance parameters occurring in various systems are given.

Features of the Excitation of Principal Series Lines of Atoms of the Zinc Subgroup by an Electron Impact: I. Cadmium

Using the technique of intersecting beams of slow electrons and atoms of zinc, excitation functions of four spectral lines of Zn I principal series corresponding to the 4snp $${}^{1}P_{1}^{^\circ }$$ → 4s2 1S0 transitions for n = 4, 5, 6, and 7 are measured. Their wavelengths are 213.9, 158.9, 145.8, and 140.4 nm, respectively. Within the energy range of 11–18 eV, the effect of postcollision interaction of electrons (slow scattered and fast ejected ones) has been revealed on these functions with the decay of the autoionizing state. At incident electron energies of ∼11–13 and ∼15–18 eV, such interaction leads to an additional population of the initial levels for these lines and, correspondingly, to the formation of maximum on their excitation functions due to the capture of a scattered electron into these excited levels. The terms of atomic autoionizing states responsible for the observed maximums are determined. The combined influence of the decay of these autoionizing states and postcollision interaction is expressed in the energy shift of the maximums on the excitation functions relative to positions of the corresponding autoionizing states. Using a straightforward calculation in the classical approximation, effective widths of the electron decay of the autoionizing states are estimated. The estimates involve an exact formula and approximation formulas. The latter are valid for different relations between the value of the postcollision shift of maximums on the excitation functions and electron bonding energy at the atomic level. They are used for the determination of effective widths when approximating experimental shifts by the least squares method.

Momentum transfer dependence behavior of the 3 s-1nl autoionization states of argon studied by high-resolution fast electron impact

SynopsisMomentum transfer dependence behavior of the autoionization states plays an unique role in understanding the electronic correlation effect. In the present work, the 3s-1nl autoionization states and their peak profiles of argon have been measured at different scattering angles by fast electron impact. The momentum transfer dependence behaviors of the dipole-allowed and dipole-forbidden autoionization states of argon have been elaborated.

Resonance Parameters, Autoionisation of <sup>2S+1</sup>L<sup>o</sup> Doubly Excited N<sup>5+</sup> Ion States Associated with n=3 and 4, N<sup>6+</sup> Threshold

The motivation of the diagonalization method is to take into consideration the coupling between closed and opened channels in term of perturbation theory and to neglect the indirect coupling as well but also the autoionisation states through the opened channels. This procedure leads to a relatively simple mathematical problem consisting of solving a system of linear algebraic equations instead of a system of coupled differential equations or integro-differential equations. Diagonalization method under LS coupling scheme for the states ^1,3P^o; ^1,3D^o; ^1,3F^o; ^1,3G^o; ^1,3H^o was performed. The partial widths for multi-channel autoionizing levels to sublevels of N⁶⁺ were carried out by neglecting the direct coupling between opened channels. The calculations of total and partial widths of the (nln'l') ^1,3L^o (L=1, 2, 3, 4 and 5) multiple-decay-channel system N⁵⁺ were performed. From L=1 to 2, the (4l4l') ^1,3Lo states located under n=3 and the (3lnl') ^1,3Lo</sup> states follow the same rules. All the (4l4l') ^1,3Go and (4l4l') ^1,3H^o states located under n=3 observe the rule 1.

Spin-orbit dynamics in atomic krypton observed through decay of autoionising states

SynopsisWe study the spin-orbit interaction within excited states of krypton by extending R-Matrix with time-dependence theory to include spin-orbit effects. We study an attosecond pump-probe scheme where we produce an autoionising state with the probe pulse. We observe dynamics through the subsequent decay of the autoionising state, from which we find a time-dependent signal when spin-orbit interactions are included. We examine the effect of the relative polarisation of the pulses, enabling an interpretation of the dynamics in terms of the quantum number mℓ of the core of the excited krypton atom.

Особенности возбуждения линий главной серии атомов подгруппы цинка электронным ударом. II. Цинк

Using the technique of crossed beams of slow electrons and zinc atoms, the excitation functions of four spectral lines of Zn I principal series corresponding to the 4snp 1Po1  4s2 1S0 transitions for n = 4, 5, 6, 7 are measured. Their wavelengths are 213.9, 158.9, 145.8, 140.4 nm, respectively. In the energy range 11–18 eV, these functions revealed the manifestation of the effect of collision interaction of electrons (slow scattered and fast emitted) during the decay of the autoionization state. At incident electron energies of 11–13 and 15–18 eV, this interaction leads to an additional population of the initial levels for these lines and, correspondingly, to the formation of maxima on their excitation functions due to the capture of the scattered electron at the indicated excited levels. The terms of autoionizing states of the atom responsible for the observed maxima are established. The combined effect of the decay of these autoionizing states and the post-collision interaction is expressed in the energy shift of the maxima on the excitation functions relative to the positions of the corresponding autoionizing states. In the classical approximation, the effective widths of the electronic decay of autoionizing states are estimated by direct calculations. For the estimations, exact and approximate formulas are used. The latter are valid for various relations between the magnitude of the post-collision shift of the maxima on the excitation functions and the electron binding energy at the atomic level. They are used to determine the effective widths when the experimental shifts are approximated by the least squares method.

Disentangling spectral phases of interfering autoionizing states from attosecond interferometric measurements

SynopsisThe spectral phase of the autoionising neon continuum has been probed via interferometric pump-probe measurements as well as ab initio, full-electron, time-dependent, theoretical calculations in energy intervals containing several auto-ionising. Theory and experiment are in excellent agreement. Despite the complex energy-dependence of the phase, a very simple model has been used to disentangle the contribution by different resonances. This work extends the applicability of reconstruction methods to multi-resonance regions of autoionising continua - a common scenario in multi-electron systems.

Contribution of molecular orbital promotion to inelastic energy losses in ion-solid collisions

A model for estimating the contribution of autoionization state formation to inelastic energy losses in collisions of keV-energy ions with solids is proposed. Using the suggested scaling for L and M shell excitation it is possible to estimate the ionization cross sections and the contribution of the ionization processes to the electronic stopping power dE/dx. The model predicts a threshold behavior of dE/dx. In the cases studied the formation of autoionization states gives important contribution to the value of dE/dx.