Autoionization Resonances Research Articles

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.

Photoionization features of the ground and excited levels of Cl II and benchmarking with experiment

This report presents i) various characteristic features in photoionization cross sections (σPI) of Cl II + hν → Cl III + e for many fine structure levels of Cl II, 392 in total with n ≤ 10 and l ≤ 9, ii) comparison of features with those observed in an experiment carried out at the Advanced Light Source at Lawrence Berkeley National Lab, and iii) partial photoionization cross sections of the ground level for ionization leaving the core ion in to various excited levels for applications in plasma modeling. The features correspond to resonant structures, the shape of the background, and their interference effect in σPI of this near neutral ion Cl II with 16 electrons. σPI for the 5 levels of the ground configuration 3s23p4(3P0,1,2, 1D2, 1S0) of Cl II show regions of narrow Rydberg resonances at and near threshold energies, and resonant structures at higher energies in contrast to typical smooth decrease in the background. Various other features in σPI of levels of excited equivalent electron states and broad Seaton resonances in single valence electron excited levels are illustrated with examples. Comparison of calculated σPI of the 15 lowest levels with the combined features of the measured photoionization spectrum shows excellent agreement by reproducing and thus identifying them to the levels that they belong to. The calculations were carried out in relativistic Breit-Pauli R-matrix (BPRM) method using a close coupling wave function expansion of 45 levels up to 4s of the core ion Cl III. These levels were optimized using a set of 12 configurations going up to orbital 5s, 3s23p3, 3s3p4, 3p5, 3s23p23d, 3s23p24s, 3s23p24p, 3s23p24d, 3s23p24f, 3s23p25s, 3s3p33d, 3s23p3d2, 3p43d producing 283 levels of Cl III. The autoionizing resonances are delineated with a fine energy mesh to observe the fine structure effects. The present results will provide high precision parameters for various applications involving this less studied ion.

Ab Initio Complex Transition Dipoles between Autoionizing Resonance States from Real Stabilization Graphs.

Electronic transition dipoles are crucial for investigating light-matter interactions. Transition dipoles between metastable (autoionizing resonance) states become complex within non-Hermitian formalism, analogous to the resonance energies. Herein, we put forward a robust method for evaluating complex transition dipoles based on real ab initio stabilization calculations. The complex transition dipoles are obtained by analytical continuation via the Padé approximant and are identified as stationary solutions in the complex plane. The capability of the new approach is demonstrated for several transition dipoles of the doubly excited helium resonance states, for which exact values are available for comparison. Nevertheless, the method presented here has no inherent limitation and is suitable for polyatomic systems.

Dissociative Electron Attachment to MgCN

SynopsisDissociative electron attachment (DEA) to the molecule MgCN and its isomer MgNC have been proposed as possible sources of CN− in the interstellar media. We have carried out electron scattering calculations using the Complex Kohn Variational Method as a function of the internal degrees of freedom to obtain the resonance energy surfaces and autoionization widths. We use this data as input to form the Hamiltonian relevant to the nuclear dynamics. The multidimensional wave equation is solved using the MultiConfiguration Time-Dependent Hartree (MCTDH) approach. We compute the DEA cross sections.

Dissociative electron attachment to MgCN

Dissociative electron attachment (DEA) to the molecule MgCN and its isomer MgNC has been proposed as a possible source of CN− in the interstellar media. We have carried out electron scattering calculations using the complex Kohn Variational Method as a function of the internal degrees of freedom of the molecule to obtain the resonance energy surfaces and autoionization widths. We use these data as input to form the Hamiltonian relevant to the nuclear dynamics. The multidimensional time-dependent Schrodinger equation is solved using the MultiConfiguration Time-Dependent Hartree (MCTDH) approach. We compute the DEA cross sections and discuss the implications for CN− formation in circumstellar envelopes.

Photoionization of C60: Effects of Correlation on Cross Sections and Angular Distributions of Valence Subshells.

Calculations of the photoionization cross section and asymmetry parameter, β, are performed at the density functional theory (DFT) and time-dependent density functional theory (TDDFT) levels for all 32 valence levels of C60. Accurate numerical results are obtained for the isolated molecule in icosahedral symmetry. A detailed analysis based on the comparison between the DFT and TDDFT results allows the identification of four types of resonances: the well-known confinement resonances of mainly geometrical origin, shape resonances native to the ionization channel, induced shape resonances, and autoionization resonances brought about by interchannel coupling, as well as their different prominence in cross section or asymmetry parameter. Generally, cross sections are enhanced at the TDDFT level, which includes contribution from the bound-state excitations from closed channels, neglected at the DFT level, and the effect persists even well above the highest ionization threshold. This effect is best seen in the total cross section, although not as dramatic as found from simpler models, probably due to the stiffer electronic structure inherent in the full molecular description. The effects of interchannel coupling on individual native resonances are rather less predictable, leading to both enhancement and decreases and often altering the details of the structure significantly. A comparison with the previous accurate total cross-sectional calculations, as well as with the available experimental data, is very good for cross sections but slightly inferior for β's. The results reported can serve as a reference to compare the effects of different environments on C60, as well as chemical substitution, notably endohedral fullerenes.

Enhancement of photoionization by applying polarization-based common level excitation scheme for the selective photoionization of odd isotopes of samarium

Due to the importance of 149Sm isotope in the nuclear industry, isotope-selective photoionization of the odd isotopes of samarium has been studied using multi-color photoionization technique. The present work has been carried out in an atomic beam coupled to a linear time-of-flight mass spectrometer using multimode dye lasers. A new photoionization scheme: 292.58 cm−1 →17,769.71 cm−1→34,929.9 cm−1→ continuum with the J-value sequence 1-1-0-continuum has been identified for the enrichment of only odd isotopes using linearly polarized lasers in the same direction. This photoionization scheme in combination with the first-step transition: 0 cm−1 →17,769.71 cm−1 (J = 0→J = 1) has been utilized to realize a common level excitation scheme which has significantly increased the photoionization yield of the odd isotopes and suppressed the ionization of the even isotopes of samarium using suitable choice of laser polarization. To enhance the photoionization yield further, a search for the third-step transition terminating into an autoionization level has been carried out. A strong autoionization resonance at the third-step laser wavelength 644.14 nm, corresponding to the transition 34,929.9 cm−1→ 50,450.3 cm−1 has been identified. The J-value of the newly discovered autoionization level at 50,450.3 (±0.5) cm−1 has been uniquely assigned as J = 1. Finally, the photoexcitation/photoionization cross sections of all the transitions involved in the common level excitation scheme including that of the third-step transition terminating into the autoionization level have been measured by saturation method.

Ab-initio theory of photoionization via resonances.

We present an ab initio approach for computing the photoionization spectrum near autoionization resonances in multi-electron systems. While traditional (Hermitian) theories typically require computing the continuum states, which are difficult to obtain with high accuracy, our non-Hermitian approach requires only discrete bound and metastable states, which can be accurately computed with available quantum chemistry tools. We derive a simple formula for the absorption line shape near Fano resonances, which relates the asymmetry of the spectral peaks to the phase of the complex transition dipole moment. Additionally, we present a formula for the ionization spectrum of laser-driven targets and relate the "Autler-Townes" splitting of spectral lines to the existence of exceptional points in the Hamiltonian. We apply our formulas to compute the autoionization spectrum of helium, but our theory is also applicable for nontrivial multi-electron atoms and molecules.

Forced autoionisation in the Yb 6snd series

We have observed the conversion of a level perturbing the Yb series into a forced autoionisation resonance by the application of a static field. The perturbing level is barely visible in the zero field excitation spectrum, leading us to expect the forced autoionisation resonance to be manifested as a dip in the spectrum as a Fano 0 feature. Instead, we observe a pronounced peak with a 3. As shown by a quantum defect theory analysis, the zero field spectrum is consisitent with 3. The apparent discrepancy arises from the combination of the rapid radiative decay of the perturbing level, the weakness of the configuration interaction and the discreteness of the zero field spectrum.

Single photoionization of the Kr-like Rb ii ion in the photon energy range 22–46.0 eV

Abstract Single photoionization cross-sections for Kr-like Rb+ ions are reported in the energy (wavelength) range from 22 (564 Å) to 46 eV (270 Å). Theoretical cross-section calculations for this trans-Fe element are compared with measurements from the ASTRID radiation facility in Aarhus, Denmark and from the dual laser plasma technique, at respectively 40 and 35 meV FWHM energy resolution. In the photon energy region 22–32 eV the spectrum is dominated by excitation auto-ionizing resonance states. Above 32 eV the cross-section exhibits classic Fano window resonances features, which are analysed and discussed. Large-scale theoretical photoionization cross-section calculations, performed using a Dirac Coulomb R-matrix approximation are benchmarked against these high-resolution experimental results. Comparison of the theoretical work with the experimental studies allowed the identification of resonance features and their parameters in the spectra in addition to contributions from excited metastable states of the Rb+ ions.

Advanced computational approach to studying Rydberg and autoionization resonances in spectra of lanthanides: Ytterbium

We present the results of computing the Rydberg autoionization resonances in spectra of the ytterbium atom, in particular, data on the energies and widths of the autoionization resonances 4f13 [2F7/2]6s2np[5/2]2, 4f13 [2F7/2]6s2nf[5/2]2, which are arisen in result of excitation of the 4f-shell electrons. For some values of the principal quantum number the presented data are compared with the corresponding available experimental data by Jong-Hoon Yi et al, which are obtained by method of three-photon laser polarization spectroscopy. As the method of computing we apply the method of a relativistic many-body perturbation theory with the Dirac-Kohn-Sham zeroth approximation combined with the generalized relativistic energy approach. This approach takes effectively into account the important exchange-correlation corrections as the corrections of the perturbation theory second and higher orders (for example, the polarization interaction, quasiparticles screening corrections) and effect of the non-Coulomb grouping levels in the Rydberg spectra etc.

Effects of field fluctuations on driven autoionizing resonances

The excitation of an autoionizing resonance by intense radiation requires a theoretical description beyond the transition probability per unit time. This implies a time-dependent formulation incorporating all features of the radiation source, such as pulse temporal shape and duration, as well as stochastic properties, for pulses other than Fourier limited. The radiation from short wavelength free electron lasers is a case in point, as it is the only source that can provide the necessary intensity. In view of ongoing experiments with such sources, we present a systematic study for an isolated autoionizing resonance. We find that intensity, pulse duration and field fluctuations conspire in producing unexpected excitation profiles, not amenable to a description in terms of the usual Fano profile. In particular, the role of intensity fluctuations turns out to pose challenging theoretical problems part of which have been addressed herein.

The effect of large autoionization decay rates (resonance widths) on cold molecular cross-sections and the reflection phenomenon

Autoionization resonance states are characterized by a complex potential energy surfaces, where the imaginary part is proportional to the decay rate. Sharp changes in the autoionization decay rates, with respect to the reaction coordinates, introduce reflections. That is, when the autoionization decay rate is very small outside the interaction region and large within it, the reactants will be reflected away from the interaction region. Consequently, the reaction is expected to be suppressed. However, no reflections was observed in the collision of H2 with He in the 3P state [Nature Chemistry 7 (11) (2015) 921–926], for which the autoionization decay rate is very large in the interaction region and it is small elsewhere. In this paper, we theoretically investigate the reflection phenomenon within cold-chemistry collisions. Moreover, we present the conditions for which such reactions will not be suppressed due to reflections.

Role of intermediate continuum states in exterior complex scaling calculations of two-photon ionization cross sections

The calculation of partial two-photon ionization cross sections in the above-threshold energy region is discussed in the framework of exterior complex scaling. It is shown that with a minor modification of the usual procedure, which is based on the calculation of the outgoing partial waves of the second-order scattering wave function, reliable partial ionization amplitudes can be obtained. The modified procedure relies on a few-term least-squares fit of radial functions pertaining to different partial waves. To test the procedure, partial and total two-photon ionization cross sections of the helium atom have been calculated for a broad range of incident photon energies. The calculated cross sections may be seen to agree well with the results found in the literature. Furthermore, it is shown that, using a similar approach, partial photoionization cross sections of an atom in an autoionizing (resonance) state may be calculated in a relatively straightforward way. Such photoionization cross sections may find their use in enhanced few-parameter models describing the atom-light interaction in cases where a direct solution of the time-dependent Schroedinger equation becomes too resource-intensive.

Harmonic Generation from Neutral Manganese Atoms in the Vicinity of the Giant Autoionization Resonance.

High harmonics from laser-ablated plumes are mostly generated from ionic species. We demonstrate that with ultrashort infrared (∼1.82 μm) driving lasers, high harmonics from laser-ablated manganese are predominantly generated from neutral atoms, a transition metal atom with an ionization potential of 7.4eV. Our results open the possibility to advance laser-ablation technique to study the dynamics of neutral atoms of low ionization potential. Moreover, as manganese contains giant autoionizing resonance, intense and broadband high harmonics have been demonstrated from this resonance at energies from 49 to 53eV. This opens the possibility to generate intense attosecond pulses directly from the giant resonances, as well as to study these resonances using high-harmonic spectroscopy.

The study of Eu 4f76p6d autoionizing states with RIS and VMI techniques

The spectra and dynamic properties of Eu [Formula: see text] autoionizing states are studied systematically with the resonance-ionization spectroscopy (RIS) and the velocity-map imaging (VMI) techniques. The RIS technique is utilized to obtain the spectra of Eu [Formula: see text] autoionizing states with the three-step excitation, while the VMI technique is used to detect the dynamic process of autoionization, from which the branching ratio (BR) of ions and the angular distribution (AD) of ejected electrons from Eu [Formula: see text] autoionizing states are achieved. Not only the energy levels, line width and line shape are obtained, but also the q-reversal phenomenon is observed in the autoionization spectra. The BR represents energy distribution (ED) of ions by observing the radical information of the VMI image, while the AD, characterized by anisotropy parameters, reveals the differential cross-section of autoionization extracted from the angular information of the VMI image. Furthermore, the variation of BR and AD are analyzed within the entire region of autoionization resonance. Based on the results of BR between [Formula: see text] and [Formula: see text] ionic states, the possibility of the population inversion is discussed.

Collision strengths for FIR and UV transitions in P iii and the phosphorus abundance

ABSTRACT Phosphorus abundance is crucial for DNA-based extraterrestrial life in exoplanets. Atomic data for observed spectral lines of P-ions are needed for its accurate determination. We present the first calculations for collision strengths for the forbidden [P iii] fine structure transition $\mathrm{ 3s^23p (^2P^o_{1/2{\text{--}}3/2})}$ within the ground state at 17.9 $\mu$m , as well as allowed UV transitions in the $\mathrm{ 3s^23p (^2P^o_{1/2,3/2}) \rightarrow 3s3p^2 (^2D_{3/2,5/2}, ^2S_{1/2}, ^2P_{1/2,3/2})}$ multiplets between 915 and 1345 Å. Collision strengths are computed using the Breit–Pauli R-Matrix method including the first 18 levels, and they exhibit extensive auto-ionizing resonance structures. In particular, the Maxwellian averaged effective collision strength for the FIR 17.9 $\mu$m transition shows a factor 3 temperature variation broadly peaking at typical nebular temperatures. Its theoretical emissivity with solar phosphorus abundance is computed relative to H β and found to be similar to observed intensities from planetary nebulae; the abundances derived in earlier works are 3–5 times sub-solar. The results pertain to the reported paucity of phosphorus from preferred production sites in supernovae, and abundances in planetary nebulae and supernova remnants.

Wigner–Eisenbud–Smith photoionization time delay due to autoioinization resonances

An empirical ansatz for the complex photoionization amplitude and Wigner–Eisenbud–Smith time delay in the vicinity of a Fano autoionization resonance are proposed to evaluate and interpret the time delay in the resonant region. The utility of this expression is evaluated in comparison with accurate numerical calculations employing the ab initio relativistic random phase approximation and relativistic multichannel quantum defect theory. The indisputably good qualitative agreement (and semiquantitative agreement) between corresponding results of the proposed model and results produced by the ab initio theories proves the usability of the model. In addition, the phenomenology of the time delay in the vicinity of multichannel autoionizing resonances is detailed.

AN ADVANCED LASER PHOTOIONIZATION SEPARATION SCHEME FOR RADIOACTIVE ISOTOPES AND PRODUCTS OF ATOMIC ENERGETICS: URANIUM AND THE TRANSURANIUM ELEMENTSV.

We present an advanced, optimal laser-photoionization scheme of separating heavy radioactive isotopes and nuclear isomers, in particular, the uranium isotopes and the transuranium elements. The advanced scheme is based on the selective laser excitation of the isotope atom into the highly-excited Rydberg states and further electric field ionization mechanism. Some preliminary data for the autoionization resonance characteristics for U and transuranium isotopes are obtained. An appreciable dependence of the reorientation decay velocity of the autoionization resonances at an electric field is analyzed. Two autoionization resonances decay channels are predicted as well as the effect of the giant autoionization resonance width broadening in the relatively weak electric field for uranium and transuranium elements. Using the optimal laser action model and density matrices formalism it has been carried out modelling the optimal scheme of the U isotopes separation. It is presented The optimal scheme g- laser on quickly decayed nuclear isomers with using laser photoionization sorting excited nuclei with autoionization and electric field ionization mechanisms.

Autoionizing states driven by stochastic electromagnetic fields

We have examined the profile of an isolated autoionizing resonance driven by a pulse of short duration and moderately strong field. The analysis has been based on stochastic differential equations governing the time evolution of the density matrix under a stochastic field. Having focused our quantitative analysis on the resonance of helium, we have investigated the role of field fluctuations and of the duration of the pulse. We report surprisingly strong distortion of the profile, even for peak intensity below the strong field limit. Our results demonstrate the intricate connection between intensity and pulse duration, with the latter appearing to be the determining influence, even for a seemingly short pulse of 50 fs. Further effects that would arise under much shorter pulses are discussed.