Electronic Autoionization Research Articles

Population kinetics and M band emission spectra of gold plasmas in non-local thermodynamic equilibrium by using a detailed relativistic configuration approach

A collisional-radiative model based on the approach of detailed relativistic configurations is developed, where the complete set of atomic data including photo-excitation, photoionization, electron impact excitation, electron impact ionization and autoionization is calculated, and the data of the inverse processes are obtained by detailed balance. The population distribution is obtained by solving the rate equation under the steady-state condition. The present model is applied to calculate the charge state distribution and M band emission spectra of gold plasmas in non-local thermodynamic equilibrium under a variety of plasma conditions. Comparisons between the present work and experimental results were made and good agreement is found. For the strong transition lines, the intensities predicted by the present model agree with those of experimental spectra within 50%. The present work is useful in analyzing and interpreting experiments as well as in diagnosing the electron temperature in experiments.

Quenching H2 autoionization interferences with ultrashort xuv laser pulses

In contrast with atomic photoionization or molecular direct photoionization, in the autoionization region, electron and proton kinetic-energy differential probabilities resulting from a short pulse cannot be reconstructed by the incoherent superposition of those resulting from long pulses.

Electron diffraction self imaging of molecular fragmentation in two step double ionization of water

We doubly ionize H2O by single photon absorption at 43 eV leading to H+ + OH+. A direct double ionization and a sequential process in which single ionization is followed by rapid dissociation into a proton and an autoionizing OH* are identified. We obtain information about the internuclear distance at which the decay of the OH* occurs from the angular distribution of the autoionization electron.

STORAGE RING CROSS SECTION MEASUREMENTS FOR ELECTRON IMPACT IONIZATION OF Fe12 +FORMING Fe13 +AND Fe14 +

We report electron impact ionization cross section measurements for electron impact single ionization of Fe12 + forming Fe13 + and electron impact double ionization of Fe12 + forming Fe14 +. These are the first electron impact ionization data for any Si-like ion uncontaminated by an unknown metastable fraction. Recent distorted wave calculations agree with our single ionization results to within ~15%. Double ionization is dominated by inner shell ionization of a 2l electron resulting in autoionization of a second electron as the inner shell hole is filled.

Electron Diffraction Self-Imaging of Molecular Fragmentation in Two-Step Double Ionization of Water

We doubly ionize H(2)O by single photon absorption at 43 eV leading to H(+) + OH(+). A direct double ionization and a sequential process in which single ionization is followed by rapid dissociation into a proton and an autoionizing OH(*) are identified. The angular distribution of this delayed autoionization electron shows a preferred emission in the direction of the emitted proton. From this diffraction feature we obtain internuclear distances of 700 to 1100 a.u. at which the autoionization of the OH(*) occurs. The experimental findings are in line with calculations of the excited potential energy surfaces and their lifetimes.

Formation of inner-shell autoionizingCO+states below theCO2+threshold

We report a kinematically complete experiment on the production of ${\mathrm{CO}}^{+}$ autoionizing states following photoionization of carbon monoxide below its vertical double-ionization threshold. Momentum imaging spectroscopy is used to measure the energies and body-frame angular distributions of both photo- and autoionization electrons, as well as the kinetic energy release (KER) of the atomic ions. These data, in combination with ab initio theoretical calculations, provide insight into the nature of the cation states produced and their subsequent dissociation into autoionizing atomic (${\mathrm{O}}^{*}$) fragments.

3s Rydberg and Cationic States of Pyrazine Studied by Photoelectron Spectroscopy

We have studied 3s(n-1 and pi-1) Rydberg states and D0(n-1) and D1(pi-1) cationic states of pyrazine [1,4-diazabenzene] by picosecond (2 + 1) resonance-enhanced multiphoton ionization (REMPI), (2 + 1) REMPI photoelectron imaging, He(I) ultraviolet photoelectron spectroscopy (UPS), and vacuum ultraviolet pulsed field ionization photoelectron spectroscopy (VUV-PFI-PE). The new He(I) photoelectron spectrum of pyrazine in a supersonic jet revealed a considerably finer vibrational structure than a previous photoelectron spectrum of pyrazine vapor. We performed Franck-Condon analysis on the observed photoelectron and REMPI spectra in combination with ab initio density functional theory and molecular orbital calculations to determine the equilibrium geometries in the D0 and 3s(n-1) states. The equilibrium geometries were found to differ slightly between the D0 and 3s states, indicating the influence of a Rydberg electron on the molecular structure. The locations of the D1-D0 and 3s(pi-1)-3s(n-1) conical intersections were estimated. From the line width in the D1 <-- S0 spectrum, we estimated the lifetime of D1 to be 12 fs for pyrazine and 15 fs for fully deuterated pyrazine. A similar lifetime was estimated for the 3s(pi-1) state of pyrazine by REMPI spectroscopy. The vibrational feature of D1 observed in the VUV-PFI-PE measurement differed dramatically from that in the UPS spectrum, which suggests that the high-n Rydberg (ZEKE) states converging to the D1 vibronic state are short-lived due to electronic autoionization to the D0 continuum.

Theoretical exploration of the photosensitive properties of xanthurenic acid, a tryptophan metabolite in cataractous human lenses

Photochemical damage has been considered as an important reason for age-related cataracts. Recently, a photochemically active tryptophan metabolite, xanthurenic acid (Xan), has been isolated from aged cataractous human lenses. Previous studies demonstrated that Xan can promote the photo-oxidation of lens protein in model studies by the photoproduction of singlet oxygen and superoxide anion radicals and could therefore play an important role in the photoinduction of age-related cataracts. Thus, it is of significance to investigate further the photosensitive properties of Xan. The photo-physicochemical properties of Xan, including absorption spectrum, lowest triplet excited-state energy, vertical electron affinity and vertical ionization potential, were explored by means of time-dependent density functional theory (TD-DFT) calculations, as a means of gaining deeper insights into the photosensitive mechanisms of Xan. It was found that Xan can photo-generate various reactive oxygen species through different pathways. For example, Xan can produce singlet oxygen through direct energy transfer pathway, while it can generate the superoxide anion radical through both direct electron transfer and autoionization. Other photosensitive properties of Xan, e.g. its relatively lower singlet oxygen quantum yield, are also discussed.

Ab initio calculations of perturbed core-excited resonances in H2 associated with H, n =3: analogies and differences with doubly-excited helium

We have carried out ab initio R-matrix calculations of highly excited diatomic hydrogen (R=1.4 au, symmetry) in the region near the n=3 group of thresholds of H. Spheroidal coordinates were used and all channels with ℓ=0, 1 and 2 associated with the H n=2 and 3 thresholds were included in the calculations. Generalized multichannel quantum defect theory (GMQDT) has been employed to calculate the resulting resonance structures. Strong configuration interaction is predicted to occur between the various ionization and Rydberg channels, producing an irregular pattern of Rydberg resonances characterized by strongly varying electronic autoionization widths. Analogies with doubly excited resonances in He are uncovered despite the lowered symmetry of the molecule as compared to the atom.

YAC: A code using the detailed term accounting model for all- [formula omitted] elements

We are developing an atomic code, YAC, for elements from low to high Z that explicitly includes the detailed term structure. Detailed electron configurations are produced from a list of super-configurations. These detailed configurations are used as effective energy levels for population kinetics. Further, a list of all possible atomic processes such as photo-excitation (ionization), electron collision excitation (ionization) and autoionization is generated further from these configurations and, except for the radiative transitions, these are calculated by using the configuration-average model. The spontaneous radiative transitions are calculated using detailed term accounting model. The preliminary result for hard X-ray emission from laser-irradiated plasma of U is presented.

Valence double ionization of O2 at photon energies below and above the molecular double ionization threshold

A recently developed time-of-flight photoelectron-photoelectron coincidence spectroscopy technique, which gives complete two-dimensional e(-)-e(-) spectra in single photon double ionization, is applied to molecular oxygen at photon energies below and above the adiabatic double ionization threshold of O(2). Analysis of the two-dimensional coincidence maps reveals specific indirect pathways for the double ionization process. Dissociative ionization paths with subsequent autoionization of atomic oxygen are found to be the dominant processes for all chosen photon energies. Spectra of the photoelectrons coincident with the autoionization electrons show that intermediate O(2)(+) states are involved which do not autoionize to molecular O(2)(2+). In particular, the ground state of O(2)(2+) is vibrationally resolved and shows a regular progression which can be well described by direct Franck-Condon transitions at an internuclear distance R(e)(X (1)Sigma(g)(+))=1.054 A. Quantum yields of double ionization for O(2), of a form discussed in this paper, are determined.

Nature and energy structure of impurity and intrinsic defects in V-doped Cd1−xHgxTe

The nature and the energy structure of impurity and intrinsic defects, including the formation of anisotropic impurity centres, are determined in V-doped semi-insulating Cd1−xHgxTe (x ⩽ 0.037) crystals which were grown by the Bridgman technique for the first time. From detailed analysis of the spectral dependence of the photodiffusion current data obtained for the different directions of light propagation in crystals as well as the impurity absorption spectra, we established that for the investigated crystals anisotropic V2+ and V3+ centres are formed. Two mechanisms of the electron photogeneration from the ground impurity state to the conduction band (direct photoionization and auto-ionization from the excited state V2+ ions) are established. It is shown that the efficacy of the auto-ionization of electrons depends on the position of the excited state relative to the conduction band bottom. The photoinduced impurity centres are formed as a result of the illumination of the crystals with the light with energy about 1.50 eV. The nature and photoionization energy of these photoinduced centres are determined. The scheme of the impurity and intrinsic defects energy levels in the energy gap of the investigated crystals is presented.

Autoionization in I and I2 observed by multiphoton ionization and photoelectron spectroscopy: Two atomic iodine Rydberg series built on the …5s25p4 3P1 ion core and revised value for the I+(3P1) limit

Two well-developed Rydberg series of atomic iodine, built on the …5s25p4 3P1 ion core and assigned to nd[2]5/2 and ns[1]3/2 states with principal quantum numbers as high as n=47, have been observed by two photon (566⩽λ≲600 nm) ionization of I(6s 2P3/2), and time-of-flight photoelectron spectroscopy and mass spectrometry. The data reported here extend the nd[2]5/2 and ns[1]3/2 series by 27 and 23 terms, respectively. The limit for both series, 91 387.9±1.3 cm−1, revises upwards by 5.8 cm−1 the value for the I+(3P1) state energy suggested by Minnhagen in 1962 [Ark. Fys. 21, 415 (1962)]. The quantum defects for the nd[2]5/2 and ns[1]3/2 series were determined to be 2.44±0.03 and 4.03±0.02, respectively. Several terms of a third, weaker series were also detected and tentatively assigned to nd[2]3/2 states, also having the P13 ion core. Photoelectron peaks associated with direct, five photon ionization of I2 have been observed and are attributed to electronic autoionization of A 2Π(3/2)u ion core Rydberg states into the X 2Π(1/2)g,(3/2)g ionization continua.

Optical, photoelectric, and photorefractive properties of Ti-doped CdTe crystals

The photorefractive characteristics of Ti-doped CdTe semi-insulating crystals were measured. Additional optical and photoelectric measurement confirmed that titanium has advantages over other dopants and that this material has better characteristics for potential applications. The high optical holographic gain coefficient, Γ≈0.60 cm−1, low background absorption, kb≈0.2 cm−1, high optical quality and homogeneity, and almost monopolar (electronic) photoconductivity show that these materials can be effectively used for both optical and photoelectric applications in the near infrared region. Studies of the optical absorption and photodiffusion current made it possible to determine the nature and energy structure of impurity and intrinsic defects as well as to establish their role in the photorefractive effect. It is shown that the excited impurity T14(F) state is in resonance with the conduction band. As a result, autoionization of electrons to the conduction band under laser excitation takes place. An energy-level diagram, both of impurity and intrinsic defects, in the CdTe:Ti crystals was constructed.

Calculation ofn=3intrashell resonance states ofHe−and of isoelectronic atoms

We discuss the theory and computation of the lowest three, $n=3$ intrashell triply excited resonance states of ${\mathrm{He}}^{\ensuremath{-}}$ of Li, and of positive ions of the sequence, up to ${\mathrm{N}}^{4+}.$ These are the ${3s}^{2}3p{}^{2}{P}^{o},$ ${3s3p}^{2}{}^{4}P,$ and ${3s3p}^{2}{}^{2}D$ states, for which wave function characteristics, energies, and widths are reported. Contrary to recently published results for ${\mathrm{He}}^{\ensuremath{-}}$ and to earlier ones for ${\mathrm{N}}^{4+},$ we found that electron correlation and orthogonality to lower states are such that they make the ${}^{2}{P}^{o}$ state the lowest $n=3$ triply excited state (TES), as is the case with the $n=2$ shell. Our predictions for these states of ${\mathrm{He}}^{\ensuremath{-}}$ are in harmony with the measurements of Roy [Phys. Rev. Lett. 38, 1062 (1977)], which were interpreted only recently [C. A. Nicolaides and N. A. Piangos, J. Phys. B 34, 99 (2001)]. In addition, our value for the position of the Li ${3s}^{2}3p{}^{2}{P}^{o}$ TES, 175.15 eV, agrees with the measurement $(175.165\ifmmode\pm\else\textpm\fi{}0.050\mathrm{eV})$ of Diehl et al. [Phys. Rev. A 56, R1071 (1997)]. Apart from specifics, the paper discusses or points to certain basic aspects of computational quantum mechanics of such multiply excited states. For example, it refers to the utility of open-channel-like configurations toward proper convergence to a local energy minimum in the continuous spectrum, where quasibound and unbound states of the same symmetry lie below, and for which the normal eigenvalue properties of the discrete spectrum do not apply. Also, we discuss the possibility that is given by the state-specific theory for carrying out economic and physically transparent calculations and for deducing semiquantitative conclusions about the interplay between electronic structure, interference, and autoionization widths.

Ab initio calculations of the photoionization of diatomic molecules

A review is presented of the calculation of photoionization spectra, particularly in the spectral range where electron autoionization of diatomic molecules takes place. In addition to some interesting results obtained over years that compare favourably with experiment, the emphasis here is put on the relation between the methods developed for the calculation of observables associated with the continuum energy spectrum of the electrons and the Alchemy system of programs. This system of programs serves as a basis for initial and intermediate calculations. The examples presented show that diatomic molecules not only in gas phase but also oriented in space or physisorbed at surfaces may be studied readily.

Optical and photoelectric properties of vanadium-doped Cd1−xHgxTe crystals

Vanadium-doped semi-insulating Cd1−xHgxTe(x < 0.05)n-type crystals were grown by the Bridgman technique for the first time. Studies werecarried out of the low-temperature optical and photoelectric properties of Cd1−xHgxTe(x = 0.014),which provided information on the deep anisotropic impurity centres and intrinsicdefects. The nature and the position of their energy levels with respect to the crystalenergy band were determined. It was shown that for the investigated crystalsthere are two photogeneration mechanisms of electrons from deep impurity levels:the auto-ionization of electrons from the discrete state, which is in resonance withthe conduction band, and their direct photoionization. It was found that thephotosensitivity region for Cd0.986Hg0.014Te:V crystals is about 1.3 μ m.

Dissociative recombination of NO+: calculations andcomparison with experiment

Multichannel quantum defect calculations for NO+dissociative recombination (DR) for electron energies from thresholdto 8 eV are presented. The calculations use electronic energiesand autoionization widths of valence states obtained from ab initio R-matrix calculations with the correspondingpotential curves calibrated using available spectroscopic data.Six valence states open to dissociation are included in thefinal calculations. Excellent agreement with the measured crosssections is obtained for the low-energy DR, up to 3 eV and, forthe first time, the peak observed in the cross section at highenergy is accounted for. The importance of the variousdissociative states at different electron energies, as well asthe direct and indirect processes, is discussed. Compared toprevious theoretical studies, the inclusion of a thirddissociative state of 2Π symmetry and the largerautoionization width of the B' 2Δ state are found to beparticularly important for the agreement with experiment.

10.2172/10169172

We have investigated both photoionization and ion-pair formation in molecular hydrogen by using double-resonance excitation via the E,F ¹Σg⁺, v=6 level. The energetic threshold for ion-pair formation occurs just below the H₂⁺ X ²Σg⁺, v⁺=9 ionization. threshold. The spectrum in this region was studied by using both conventional and constant-ionic-state photoelectron spectroscopy, as well as by monitoring the H⁻ production. The decay dynamics in this region are extremely rich, because excited levels may decay by rotational and vibrational autoionization, by predissociation to neutral H + H* (n=2,3,4), by predissociation to the ion pair H⁺ + H⁻, and by fluorescence. In addition, the dissociative potential curve of the 2pσu3sσg ¹Σu⁺ doubly excited electronic state crosses the H₂⁺ X ²Σu⁺ potential curve in the same energy region, and the electronic autoionization of this state is found to significantly influence these decay processes.

A high-resolution angle-resolved electron spectroscopy study of the 5dnp Rydberg series in Ba

Angle-resolved measurements of the autoionization electrons from the decay of the 5dnp resonances in the wavelength region just above the first ionization limit of barium are presented. We have used a UV laser to provide high resolution and almost 100% plane-polarized light. Angular distribution parameters measured at the peak positions for some of the resonances are compared with the values expected for pure jK and jj couplings and with theoretical values obtained by R-matrix calculations. Experimental angle-resolved autoionization profiles are also compared with theoretical calculations.