Autoionizing States Research Articles

Doubly excited states of H2 as studied by angle-resolved electron energy loss spectroscopy in coincidence with detecting Lyman-α photons

The differential cross sections of the dissociative double excitations resulting in H(2p) formation in 80 eV electron collisions with H2 have been measured at electron scattering angles of 3, 20, 30 and 40° by means of angle-resolved electron energy loss spectroscopy in coincidence with detecting Lyman-α photons. The doubly excited states of H2 involved in this study are the optically allowed Q1 1Πu(1) and Q2 1Πu(1) states and the forbidden state at 26 eV, which has not been assigned so far and is temporarily referred to as the F state in this study. At a 3° electron scattering angle the differential cross section of the dissociative excitation to the forbidden F state is the largest among those for the three doubly excited states and decreases with increasing the electron scattering angle more rapidly than those for the allowed Q1 1Πu(1) and Q2 1Πu(1) states. The differential cross sections for the forbidden and allowed states do not differ quite so much, which may show that the forbidden F state is a dissociative state rather than an autoionizing state. The effective generalized oscillator strengths for H(2p) formation have been compared with the dipole oscillator strengths according to the limit theorem.

Autoionization widths by Stieltjes imaging applied to Lanczos pseudospectra

Excited states of atoms and molecules lying above the ionization threshold can decay by electron emission in a process commonly known as autoionization. The autoionization widths can be calculated conveniently using Fano formalism and discretized atomic and molecular spectra by a standard procedure referred to as Stieltjes imaging. The Stieltjes imaging procedure requires the use of the full discretized spectrum of the final states of the autoionization, making its use for poly-atomic systems described by high-quality basis sets impractical. Following our previous work on photoionization cross-sections, here we show that also in the case of autoionization widths, the full diagonalization bottleneck can be overcome by the use of Lanczos pseudospectra. We test the proposed method by calculating the well-documented autoionization widths of inner-valence-excited neon and apply the new technique to autoionizing states of hydrofluoric acid and benzene.

Energy Resonances for $^1P^o,$ $^1D^e,$ $^1F^o,$ $^1G^e,$ and $^1H^o$ Autoionizing States of the Helium Isoelectronic Series Below the $n=2-4$ Hydrogenic Thresholds

Energy Resonances for $^1P^o,$ $^1D^e,$ $^1F^o,$ $^1G^e,$ and $^1H^o$ Autoionizing States of the Helium Isoelectronic Series Below the $n=2-4$ Hydrogenic Thresholds

Precision measurements of optical excitation functions for zinc atom

The excitation of zinc atoms by ultramonoenergetic electrons is experimentally studied. The optical excitation functions for 19 atomic spectral lines that originate from the n1S0, 41P1, n1D2, n3S1, 43P1, 63P2, and n3Dj levels are studied in detail. In the excitation functions measured from the excitation threshold to 19 eV for the spectral lines originating from the n1S0, n3S1, n1D2, and n3Dj levels, specific features caused by postcollision interactions of emitted and scattered electrons are observed for the first time in the energy region of 10.9–17.0 eV near the thresholds of autoionization states.

The decay of autoionizing states in electric field formed during collisions of multiply charged ions with metal surface

When a multicharged ion approaches the metal surface, a strong electric field occurs due to interaction between the ion and the charge induced in the metal. During ion-surface scattering, double excited autoionizing states can be formed, and their decay occurs in the presence of strong electric field. The Auger transition rates for such states are higher than those in the absence of the electric field; this is connected with the increase in the wave functions overlap and also with mixing of states with different orbital momentums. This effect changes our view on the population scheme of states with the lowest principal quantum numbers for the collisions considered.

Autoionization cross section of lithium atoms excited by electron impact

We measure the full autoionization cross section of lithium atoms excited by electrons in the energy range from the first autoionization threshold at 56.39 to 600 eV. Data are obtained by determining the total intensity of electron spectrum of autoionization states 1sn1l1n2l2 detected at the “magic” observation angle of 54.7°. The cross section behavior is characterized by a sharp increase to a maximum value of 1.7 × 10−18 cm2 in the energy interval of 56.4–60 eV and a subsequent monotonic drop to a value of 10−18 cm2 at 600 eV. We have discovered a “thin” cross section structure that reflects the presence of strong resonances of Li− ions in the near-threshold area of excitation of the lowest energy autoionization states (1s2s2)2S, (1s2s2p)4P, 1s(2s2p3P)2P), and 1s(2s2p1P)2P. We have established that the contribution of autoionization to the absolute cross section of single ionization of lithium atoms does not exceed 4%. We perform a comparative analysis of the data with analogous data for potassium and cesium atoms.

ChemInform Abstract: Autoionization Structure and Rotational Contours in the Multiphoton Ionization Spectrum of Carbon Dioxide

The structure of the multiphoton ionization spectra of carbon dioxide in the vicinity of 4f and 5f resonances is analyzed in detail. The rotational contour of the f complex is calculated using the frame transform method of Chang and Fano. It is found that only one electronic splitting constant k4f=7 cm−1 is needed to attain a reasonable approximation to the experimental spectrum. This would indicate that the Rydberg electron is relatively atomic‐like and excluded from the core. It is also found that the dominant contribution to the three‐photon excitation strength comes from the ΔΩ=3 path. The additional sharp structure to the blue of the 4f and 5f resonances is due to four‐photon resonances with autoionization states belonging to Rydberg series converging on the A and B states of the ion. The unusual strength of these lines arises from a coincidence between the photon energy and the energy of B←X core transitions and the predissociation of the nearby bound resonances, which limits their strength.

Laser-induced optical activity in range of Rydberg autoionizing states of xenon

Optical activity of xenon atoms in the vacuum UV range induced by circularly polarized laser light is studied theoretically. The optical activity arises in the vicinity of the autoionizing state 5p5(2P1/2)8s′\( \left[ {\frac{1} {2}} \right]_1 \) as a result of its coupling via the laser field with the discrete state 5p5(2P3/2)7p\( \left[ {\frac{1} {2}} \right]_1 \). Polarization variations of the vacuum UV radiation upon its propagation through the atomic medium are calculated, and the possibility of controlling this polarization is discussed. Manifestations of nonresonant coupling of the discrete state with the broad autoionizing state 5p5(2P1/2)6d′\( \left[ {\frac{1} {2}} \right]_1 \) induced by the overlap of the Rydberg autoionizing series in xenon are studied.

Role of Autoionizing State in Resonant High-Order Harmonic Generation and Attosecond Pulse Production

We suggest a high-order harmonic generation (HHG) model describing enhancement of the generation efficiency for the harmonic resonant with the transition between the ground and autoionizing state of the generating ion. The results of numerical and analytical calculations based on this model are in good quantitative agreement with the experiments showing HHG enhancement up to 2 orders of magnitude. Moreover, this model reproduces well the essential difference in HHG efficiency for different ions. We show that intense but relatively long attosecond pulses can be generated using the enhanced harmonics.

The post-collision energy shift of the (np5(n + 1)s2)2P3/2 lines in autoionizing spectra of sodium (n = 2) and caesium (n = 5) atoms excited by electron impact

The energy shift of lines arising from the decay of the (np5(n + 1)s2)2P3/2 autoionizing states in Na (n = 2) and Cs (n = 5) atoms has been studied for the first time in ejected-electron spectra measured over the excess energy range −0.1 to 5.4 eV. In Na spectra, the energy dependence of the shift possesses a clear oscillatory-like character. In particular, the shift becomes of negative or positive sign depending on the presence or absence of a negative-ion resonance structure in the excitation function of the (2p53s2)2P3/2 state. Earlier, we reported a similar behaviour of the line shift for the (3p54s2)2P3/2 autoionizing state in K atoms. Though in Cs atoms the strong negative-ion resonance is also present close to the excitation threshold of the (5p56s2)2P3/2 state, in contrast to Na and K data, the energy dependence of the line shift possesses a smooth asymptotic behaviour described well by the classical model of the post-collision effect. The comparative analysis of the spectroscopic classification of the resonance structure in the excitation of the (np5(n + 1)s2)2P3/2 autoionizing states in Na, K and Cs atoms has shown that the observed effect most probably reflects the complication of the compound character of resonances. In whole, the obtained data show that under the condition of the high polarizability of a resonantly excited target the relation between polarization attraction, centrifugal and Coulomb repulsion becomes a crucial factor which determines the final energy distribution of outgoing electrons.

Dissociative double photoionization of CO2 molecules in the 36–49 eV energy range: angular and energy distribution of ion products

Dissociative double photoionization of CO(2), producing CO(+) and O(+) ions, has been studied in the 36-49 eV energy range using synchrotron radiation and ion-ion coincidence imaging detection. At low energy, the reaction appears to occur by an indirect mechanism through the formation of CO(+) and an autoionizing state of the oxygen atom. In this energy range the reaction leads to an isotropic distribution of products with respect to the polarization vector of the light. When the photon energy increases, the distribution of products becomes anisotropic, with the two ions preferentially emitted along the direction of the light polarization vector. This implies that the molecule photoionizes when oriented parallel to that direction and also that the CO(2)(2+) dication just formed dissociates in a time shorter than its typical rotational period. At low photon energy, the CO(+) and O(+) product ions separate predominantly with a total kinetic energy between 3 and 4 eV. This mechanism becomes gradually less important when the photon energy increases and, at 49 eV, a process where the two products separate with a kinetic energy between 5 and 6 eV is dominant.

Modeling of population kinetics of plasmas that are not in local thermodynamic equilibrium, using a versatile collisional-radiative model based on analytical rates

We discuss the modeling of population kinetics of nonequilibrium steady-state plasmas using a collisional-radiative model and code based on analytical rates (ABAKO). ABAKO can be applied to low-to-high Z ions for a wide range of laboratory plasma conditions: coronal, local thermodynamic equilibrium or nonlocal thermodynamic equilibrium, and optically thin or thick plasmas. ABAKO combines a set of analytical approximations to atomic rates, which yield substantial savings in computer running time, still comparing well with more elaborate codes and experimental data. A simple approximation to calculate the electron capture cross section in terms of the collisional excitation cross section has been adapted to work in a detailed-configuration-accounting approach, thus allowing autoionizing states to be explicitly included in the kinetics in a fast and efficient way. Radiation transport effects in the atomic kinetics due to line trapping in the plasma are taken into account via geometry-dependent escape factors. Since the kinetics problem often involves very large sparse matrices, an iterative method is used to perform the matrix inversion. In order to illustrate the capabilities of the model, we present a number of results which show that the ABAKO compares well with customized models and simulations of ion population distribution. The utility of ABAKO for plasma spectroscopic applications is also outlined.

Probing H2 autoionizing states with femto- and attosecond laser pulses

We show the relevance that molecular autoionizing states display in some recent experiments related to the symmetry-breaking in molecular-frame photoelectron angular distributions in H2 when exposed to intense xuv femtosecond laser pulses, and others related to the electron (proton) localization when subject to attosecond pump-probe laser schemes. Our theoretical method solves the time-dependent Schrödinger equation with an spectral method that expands the wave function in terms of H2 correlated stationary vibronic states including all electronic and vibrational degrees of motion. Time-resolved asymmetric electron angular distributions are obtained at specific proton kinetic energies due to the delayed autoionization from H2 doubly excited states, which induces interferences between gerade (1sσg) and ungerade (2pσu) ionization channels. We also study photoionization of H2 exposed to a xuv attosecond pump pulse plus a time-delayed IR femtosecond probe pulse. Fast alternating asymmetries in the proton ejection (electron localization) are obtained as a function of the time delay between the pump and the probe pulses. Finally, we deal with the process of (xuv) two-photon double ionization of H2 under the assumption of having both sequential and non-sequential absorption processes.

Post-collision interaction in the near-threshold excitation of the (5p56s2)2P3/2 state in cesium atoms

The post-collision interaction (PCI) energy shift of the line resulting from the decay of the (5p56s2)2P3/2 lowest autoionizing state in cesium atoms was studied by precise measurement of the ejected-electron spectra in an incident electron excess energy range −0.1 – 1.1 eV. The energy dependence of the line shift possesses a smooth asymptotic behavior predicted by the known classical models of the PCI. The data are compared with the near-threshold excitation cross section for the (5p56s2)2P3/2 state.

Energy shift of the (2p53s2)2P3/2 line in ejected-electron spectra of sodium atoms excited by low-energy electron impact

The energy shift of the line resulting from the decay of the (2p53s2)2P3/2 lowest autoionizing state in Na atoms has been measured in the ejected-electron spectra obtained at near-threshold electron impact energies. Contrary to the smooth asymptotic behavior predicted by the known models of the PCI effect, the data reveal pronounced oscillatory character of the energy dependence of the shift in the energy regions where the strong negative-ion resonances are present in the excitation function of the 2P3/2 state.

Two-photon dissociative ionization of H2 by ultrashort xuv laser pulses

Ab initio calculations of H2 photoionization induced by low-intensity ultrashort xuv laser pulses are reported in the region of two-photon absorption. Our computational approach is based on the solution of the time-dependent Schrödinger equation including all electronic and vibrational degrees of freedom. We find that, as one approaches the one-photon ionization threshold, dissociative ionization largely dominates over non dissociative ionization. We show that this behavior is mostly induced by the presence of molecular autoionizing states in this photon energy region.

Efficient selection of single high-order harmonic caused by atomic autoionizing state influence

It is shown that the influence of atomic autoionizing states on the phase matching of high harmonic generation results in efficient selection of the single harmonic generated in the plateau region. The change of relative concentration of the atomic medium components and variation of the fundamental laser frequency can be used for tuning of the selected harmonic frequency. The possibility of achievement of contrast for the selected single harmonic of more than 10(4) at intensity conversion efficiency of approximately 10(-3) has been shown.

Selective generation of a higher harmonic in plasma

It is shown for the first time that the use of autoionisation states for phase matching leads to the efficient selection of a single harmonic generated in a plateau region in plasma. The selected harmonic frequency can be tuned by changing the relative concentration of plasma components and tuning the fundamental radiation frequency. It is shown that the contrast of the selected harmonic can exceed 104.

Theory and calculation of excited-state wave functions and properties

This paper discusses the following: (1) A newly developed first-order theory of oscillator strengths (FOTOS) which has yielded accurate results for f values involving single and multiple excitations to discrete and autoionizing states. Near-FOTOS results are reported for the N I 2p/sup 3/ /sup 4/S/sup 0/ ..-->.. 2s2p/sup 4/ /sup 4/P, 2p/sup 2/3s /sup 4/P isoelectronic sequences, the B I 2s/sup 2/2p /sup 2/P/sup 0/ ..-->.. ''2s2p2'' /sup 2/S transition, and the N II 2s2p/sup 3/ /sup 1/P/sup 0/ lifetime, are in good agreement with experiment, in contrast to earlier results. (2) The semiquanitative predictive capability of FOTOS in identifying the important correlations in photoelectron spectroscopy, photoionization of rare gases, and molecular transitions. (3) Improvements in the accuracy and efficiency of the correlation procedure which has now been applied to species as large as cesium. Positions of resonances in He/sup -/ and Li and binding energies of K/sup +/ and K are reported. (4) A new theorem which suggests nonexponential decays might be observed for autoionizing states sufficiently close to the ionization potential (perhaps O I 2p/sup 3/3s/sup 3/P/sup 0/). (5) The properties and means of theoretically and experimentally searching for bound states in the continuum, whose existence appears improbable.

CRDS study of auto-ionizing Rydberg states of argon in a microwave discharge This article is part of a Special Issue on Spectroscopy at the University of New Brunswick in honour of Colan Linton and Ron Lees.

Optical transitions from two microwave discharge excited states of argon have been observed using cavity ring-down spectroscopy. These transitions originate on the high-lying levels, 3d[1/2] 1° and 3d[3/2] 2° , and terminate on the nf ′[5/2] Rydberg (n = 8 to 22) levels, which, except for n = 8, lie between the 2P3/2 and 2P1/2 ionization thresholds. In total, 24 such spectral lines have been observed. The quantum defect for the f ′ series has been measured and is compared to previously measured values. We observe a nearly threefold jump in line width in going from n = 8 to n = 9, below and above the 2P3/2 threshold, respectively. The line widths are broad and increase monotonically with n (above 9), in contrast to the narrowing of line widths usually observed. We cannot attribute this to a single source but conclude that collisional, quasielastic l-mixing of the nf ′[5/2] Rydberg states plays a significant role.