Autoionization Processes Research Articles

Resonant photoemission of LaNiO3-δthin films

Thin LaNiO3−δ films with pseudocubic (100) preferred orientation were prepared by reactive DC magnetron sputtering and annealed at ultra high vacuum above dehydration temperature. The resonant photoemission spectroscopy was used to study the surface composition and electronic structure under La 4d→4f and Ni 3p→3d photoexcitation. The resonance features observed in core level and valence band spectra under La 4d→4f transition were explained in terms of autoionization process and lanthanum–oxygen valence band states’ hybridization. No resonant features were observed in the valence band spectra under Ni 3p→3d excitation indicating that nickel species are not present at the LaNiO3−δ film surface after heat treatment. Keywords: rare earth alloys and compounds, oxide materials, resonant photoelectron spectroscopy

Electronic feshbach resonances created in soft x-ray-induced 02 dissociation

When an atom or molecule is ionized by an x-ray, super-excited states can be created that then decay, or autoionize, by ejecting a second electron from the ion. Here we find that autoionization following soft x-ray photoionization of molecular oxygen follows a complex, multi-step, process[1]. By interrupting the autoionization process using a short laser pulse, we show that autoionization cannot occur until the internuclear separation of the fragments is greater than 30 Ångstroms. As the ion and excited neutral atom separate, we directly observe the birth of a new state. We follow the transformation of electronically bound states of the molecular ion into Feshbach resonances of the neutral oxygen atom that are characterized by both positive and negative binding energies. States with negative binding energies have not been predicted or observed in neutral atoms previously.

Doubly excited 2s2p 1,2P1 resonances in photoionization of helium

The multi-configuration Dirac–Fock (MCDF) method is implemented to study doubly excited 2s2p 1,3P1 resonances of the helium atom and the interference between photoionization and photoexcitation autoionization processes. In order to reproduce the total photoionization sprectra, the excited energies from the ground 1s2 1S0 state to the doubly excited 2s2p 1,3P1 states and the relevant Auger decay rates and widths are calculated in detail. Furthermore, the interference profile determined by the so-called Fano parameters q and p2 is also reproduced. Good agreement is found between the present results and other available theoretical and experimental results. This indeed shows a promising way to investigate the Fano resonances in photoionization of atoms within the MCDF scheme, although there are some discrepancies in the present calculations of the 2s2p 3P1 state.

Dipole approximation in theL2,3electron excited spectra in3dtransition metals

A theoretical model based on the autoionization and characteristic decay processes following electron impact ionization of a core electron in solids that has previously been used in calculating electron-energy-loss spectra of transition metals near the $3p$-excitation edge has been extended to the $2p$-excitation edge for $^{21}\text{S}\text{c}$ through $^{27}\text{N}\text{i}$ as well. In the first set of calculations, magnetic effects were ignored and the relative scattering intensity was formulated in terms of the electrostatic interaction $U(p,d)$ between the $3p$ and $3d$ electrons of the intermediate resonant configuration state ${p}^{5}{d}^{n+1}$, using many-body perturbation theory that led to a generalized Fano-type formula for the intensity profiles. In the second set of calculations in which magnetic effects were included as well, an analysis based on the Bethe-Born formalism of inelastic scattering of electrons on atoms was used. The nature of the relative magnitudes of $U(p,d)$ and the spin-orbit parameters ${\ensuremath{\varsigma}}_{3p}$ and ${\ensuremath{\varsigma}}_{3d}$ and the localized nature of the $3p$ state necessitated the diagonalization of the intermediate configuration state ${p}^{5}{d}^{n+1}$ to determine the multiplet splitting and their corresponding intensities in the $LS$-coupling limit using fractional parentage scheme. The nonrelativistic multiconfiguration Hartree-Fock (MCHF) code was used in determining the ground and continuum state wave functions, and the itinerant $3d$ states in the solid were approximated with an atomic MCHF-wave function. The outline above is applied to the $2p$-excitation edge, except that because of the relative magnitudes of $U(p,d)$, ${\ensuremath{\varsigma}}_{2p}$, and ${\ensuremath{\varsigma}}_{3d}$, it is found that $LK$ coupling is suitable for Sc, Ti, and V, while $jK$ coupling is appropriate for Cr to Ni when it comes to the diagonalization of the configuration ${p}^{5}{d}^{n+1}$ to determine the multiplet splitting and their associated scattering intensities. In the dipole approximation, the scattering intensities separate into two distinct manifolds that arise from the ${p}_{3/2}$ and ${p}_{1/2}$ states. The branching ratios of the white lines are extracted from the spectra and compared with x-ray-absorption spectra.

Observing the Creation of Electronic Feshbach Resonances in Soft X-ray–Induced O 2 Dissociation

When an atom or molecule is ionized by an x-ray, highly excited states can be created that then decay, or autoionize, by ejecting a second electron from the ion. We found that autoionization after soft x-ray photoionization of molecular oxygen follows a complex multistep process. By interrupting the autoionization process with a short laser pulse, we showed that autoionization cannot occur until the internuclear separation of the fragments is greater than approximately 30 angstroms. As the ion and excited neutral atom separated, we directly observed the transformation of electronically bound states of the molecular ion into Feshbach resonances of the neutral oxygen atom that are characterized by both positive and negative binding energies. States with negative binding energies have not previously been predicted or observed in neutral atoms.

On the use of photoelectron spectroscopy to probe the dynamics of a dissociative intermediate state

An in-depth experimental investigation into the use of photoelectron spectroscopy to probe the dynamics of a dissociative intermediate state is reported here. More than 70 velocity-mapped photoelectron images were acquired from either one-colour or two-colour two-photon ionization processes, covering the energy range from the ionization limit of CH3I to about 3600 cm−1 above the IP. Numerous autoionizing states, both vibrationless and vibrationally excited, were observed and spectroscopically assigned. The finial ion state distributions, derived from the photoelectron kinetic energy analysis, are highly selective both in vibrational modes and in quantum numbers, as well as exhibit sensitive dependences on photon energy. These findings were interpreted as a manifestation of vibronic autoionization processes rather than a direct reflection of the dissociation dynamics of the resonant intermediate state in the (1 + 1) resonance-enhanced multiphoton ionization of CH3I as suggested in previous studies.

The nonlinear Fano effect

The Fano effect is ubiquitous in the spectroscopy of, for instance, atoms, bulk solids and semiconductor heterostructures. It arises when quantum interference takes place between two competing optical pathways, one connecting the energy ground state and an excited discrete state, the other connecting the ground state with a continuum of energy states. The nature of the interference changes rapidly as a function of energy, giving rise to characteristically asymmetric lineshapes. The Fano effect is particularly important in the interpretation of electronic transport and optical spectra in semiconductors. Whereas Fano's original theory applies to the linear regime at low power, at higher power a laser field strongly admixes the states and the physics becomes rich, leading, for example, to a remarkable interplay of coherent nonlinear transitions. Despite the general importance of Fano physics, this nonlinear regime has received very little attention experimentally, presumably because the classic autoionization processes, the original test-bed of Fano's ideas, occur in an inconvenient spectral region, the deep ultraviolet. Here we report experiments that access the nonlinear Fano regime by using semiconductor quantum dots, which allow both the continuum states to be engineered and the energies to be rescaled to the near infrared. We measure the absorption cross-section of a single quantum dot and discover clear Fano resonances that we can tune with the device design or even in situ with a voltage bias. In parallel, we develop a nonlinear theory applicable to solid-state systems with fast relaxation of carriers. In the nonlinear regime, the visibility of the Fano quantum interferences increases dramatically, affording a sensitive probe of continuum coupling. This could be a unique method to detect weak couplings of a two-level quantum system (qubits), which should ideally be decoupled from all other states.

Experimental and theoretical study of the microsolvation of sodium atoms in methanol clusters: differences and similarities to sodium–water and sodium–ammonia

Methanol clusters are generated in a continuous He-seeded supersonic expansion and doped with sodium atoms in a pick-up cell. By this method, clusters of the type Na(CH(3)OH)(n) are formed and subsequently photoionized by applying a tunable dye-laser system. The microsolvation process of the Na 3s electron is studied by determining the ionization potentials (IPs) of these clusters size-selectively for n = 2-40. A decrease is found from n = 2 to 6 and a constant value of 3.19 +/- 0.07 eV for n = 6-40. The experimentally-determined ionization potentials are compared with ionization potentials derived from quantum-chemical calculations, assuming limiting vertical and adiabatic processes. In the first case, energy differences are calculated between the neutral and the ionized cationic clusters of the same geometry. In the second case, the ionized clusters are used in their optimized relaxed geometry. These energy differences and relative stabilities of isomeric clusters vary significantly with the applied quantum-chemical method (B3LYP or MP2). The comparison with the experiment for n = 2-7 reveals strong variations of the ionization potential with the cluster structure indicating that structural diversity and non-vertical pathways give significant signal contributions at the threshold. Based on these findings, a possible explanation for the remarkable difference in IP evolutions of methanol or water and ammonia is presented: for methanol and water a rather localized surface or semi-internal Na 3s electron is excited to either high Rydberg or more localized states below the vertical ionization threshold. This excitation is followed by a local structural relaxation that couples to an autoionization process. For small clusters with n < 6 for methanol and n < 4 for water the addition of solvent molecules leads to larger solvent-metal-ion interaction energies, which consequently lead to lower ionization thresholds. For n = 6 (methanol) and n = 4 (water) this effect comes to a halt, which may be connected with the completion of the first cationic solvation shell limiting the release of local relaxation energy. For Na(NH(3))(n), a largely delocalized and internal electron is excited to autoionizing electronic states, a process that is no longer local and consequently may depend on cluster size up to very large n.

Emission Characteristics of the Dielectronic Recombination Line of Atomic Cu and Selectively Excited Ionic Cu Line by Resonance Charge-Transfer Collision in a Low-Pressure Laser-Induced Plasma

Emisson spectra and time-resolved two-dimensional (2D) emission images of the electron-ion dielectronic recombination (i.e. a reversal process of auto-ionization) line of neutral Cu atoms, the selectively excited Cu ionic line, and normal Cu atomic line were observed for understanding the excitation mechanisms of Cu neutral and ionic lines in a low-pressure laser-induced plasma (LP-LIP) of Ar. From the observations, the number of charged particles around the emitting species seems to increase with increasing Ar pressure. Different time-resolved 2D emission images were observed among the selectively excited Cu ionic line and other Cu emission lines resulting from the different excitation mechanisms of the respective emission lines. Collisions of the second kind and electron-ion recombinations were found to be one of the major excitation mechanisms of Cu in Ar LP-LIP.

Helium atom in a box: a fully quantal solution

A complete non–perturbative solution of the Helium atom in a box problem is presented by developing two numerical techniques. The first technique is the direct solution by diagonalization of the Hamiltonian, and the second is based on a constrained relaxation of the wave functions. Time–dependent propagation of doubly–excited wave–functions is analyzed, allowing the calculation and the visualization of the autoionization process.

Ionization of H2 Rydberg molecules at a metal surface

The ionization of a beam of H2 Rydberg molecules in collision with a metal surface (evaporated Au or Al) is studied. The Rydberg states are excited in an ultraviolet-vacuum ultraviolet double-resonant process and are state selected with a core rotational quantum number N+=0 or 2 and principal quantum numbers n=17-22 (N+=2) or n=41-45 (N+=0). It is found that the N+=0 states behave in a very similar manner to previous studies with atomic xenon Rydberg states, the distance of ionization from the surface scaling with n2. The N+=2 states, however, undergo a process of surface-induced rotational autoionization in which the core rotational energy transfers to the Rydberg electron. In this case the ionization distance scales approximately with nu0(2), the effective principal quantum number with respect to the adiabatic threshold. This process illustrates the close similarity between field ionization in the gas phase and the surface ionization process which is induced by the field due to image charges in the metal surface. The surface ionization rate is enhanced at certain specific values of the field, which is applied in the time interval between excitation and surface interaction. It is proposed here that these fields correspond to level crossings between the N+=0 and N+=2 Stark manifolds. The population of individual states of the N+=2, n=18 Stark manifold in the presence of a field shows that the surface-induced rotational autoionization is more facile for the blueshifted states, whose wave function is oriented away from the surface, than for the redshifted states. The observed processes appear to show little dependence on the chemical nature of the metallic surface, but a significant change occurs when the surface roughness becomes comparable to the Rydberg orbit dimensions.

Photoelectron Imaging of Atomic Iodine Following A-Band Photolysis of CH3I

Photoionization of the iodine atom following methyl iodide A-band photodissociation was studied over the wavelength range of 245.5-261.6 nm by photoelectron imaging technique. Final state-specific speed and angular distributions of the photoelectron were recorded. Two types of the photoelectron resulted from ionizing the I atom from the photodissociation of CH3I were identified: (a) (2+1) REMPI of the ground state I atom, and (b) two-photon excitation of spin-orbit excited I(2P1/2) to autoionizing resonances converging to the 3P1 state of I+. In addition, some weaker signals were attributed to one-photon ionization of I atoms produced in some higher excited states from multiphoton ionization of CH3I followed by dissociation. Analysis of relative branching ratios to different levels of I+ (in case a) revealed that the final ion level distributions are generally dominated by the preservation of the ion-core configuration of the intermediate resonant state. A qualitative interpretation of the electron angular distribution from an autoionization process is also given.

Threshold photoemission spectroscopy in solids

Threshold photoemission spectroscopy (TPES) is used to measure the Fe 2p spectrum of a stainless steel sample. The obtained spectrum is compared with analogous spectra measured by X-ray photoemission and absorption spectroscopies. The results of this comparison suggest that resonant two-electron autoionization processes, rather than direct photoemission from the core level, are the main mechanisms contributing to the signal. Limits and applicability of this experimental approach to investigate bulk electronic properties in solids are discussed.

The efficiency of Interatomic Coulombic Decay in Ne clusters

In this article, we demonstrate that Interatomic Coulombic Decay (ICD) is the dominant relaxation channel of Ne 2s inner valence vacancies in free Ne clusters, with an efficiency close to 100%. ICD designates a novel autoionization process of a vacancy in a weakly bonded atomic or molecular cluster. Its main characteristic is the release of an electron from a site different than the original vacancy, which is mediated by ultrafast energy transfer. Results are shown for cluster sizes between approx. 50–600 atoms. A trend towards apparently increased efficiency for larger clusters may result from inelastic scattering processes inside the cluster.

Density-Matrix Study of the Hydrogen-Antihydrogen Molecule

The exotic hydrogen-antihydrogen molecule and its properties are investigated. The system is metastable due to matter-antimatter annihilation, but also due to the possible double autoionization process describing the decay into protonium and positronium. The latter decay channel is energetically favored at small internuclear distances where the protonium dipole cannot permenently bind positronium. The leptonic density and reduced one-particle density matrices are calculated and discussed for the ground and the first excited state.

Cluster Model for the Ionic Product of Water: Accuracy and Limitations of Common Density Functional Methods

In the present study, the performance of six popular density functionals (B3LYP, PBE0, BLYP, BP86, PBE, and SVWN) for the description of the autoionization process in the water octamer was studied. As a benchmark, MP2 energies with complete basis sets limit extrapolation and CCSD(T) correction were used. At this level, the autoionized structure lies 28.5 kcal.mol(-1) above the neutral water octamer. Accounting for zero-point energy lowers this value by 3.0 kcal.mol(-1). The transition state of the proton transfer reaction, lying only 0.7 kcal.mol(-1) above the energy of the ionized system, was identified at the MP2/aug-cc-pVDZ level of theory. Different density functionals describe the reactant and product with varying accuracy, while they all fail to characterize the transition state. We find improved results with hybrid functionals compared to the gradient-corrected ones. In particular, B3LYP describes the reaction energetics within 2.5 kcal.mol(-1) of the benchmark value. Therefore, this functional is suggested to be preferably used both for Carr-Parinello molecular dynamics and for quantum mechanics/molecular mechanics (QM/MM) simulations of autoionization of water.

Time-dependent wave packet approach to the pulse delay effect upon RbI photoelectron spectrum

The time-resolved photoelectron spectrum (TRPES) of RbI molecule is simulated using the time-dependent wave-packet method. Both the normal three-photon ionization process and auto-ionization process are involved in the simulation. The calculated results show that the change of delay time will influence the shape of the photoelectron spectrum (PES), and the influence is sub- stantially due to the existence of the crossing between excited states and the strong laser field which will change the position of relevant curves.

Ab Initio Theoretical Study of Temperature and Density Dependence of Molecular and Thermodynamic Properties of Water in the Entire Fluid Region: Autoionization Processes

The temperature and density dependence of the molecular and thermodynamic properties of water is investigated theoretically by means of the ab initio electronic structure theory combined with the reference interaction site model method, so-called RISM-SCF. We consider the autoionization process (H2O + H2O right harpoon over left harpoon H3O+ + OH-) by regarding H2O, H3O+, and OH- as "solute" molecules in an aqueous solution and evaluate molecular geometry, electronic structure, solvation structure, and the ionic product of water (pKw) of these species as functions of thermodynamic conditions. In our previous paper, we calculated these properties by using essentially the same method in a wide range of density values (0.6-1.4 g/cm3). However, the calculation was limited at rather higher density (>0.6 g/cm3) due to the difficulty of convergence, which is inherent to the hypernetted-chain (HNC) closure. The problem is overcome in this study by employing the Kovalenko-Hirata (KH) closure which hybridizes the HNC and the mean-spherical approximation (MSA). Here, we present the results for the thermodynamic range of densities from 0.025 to 1.0 g/cm3 and for temperatures from 300 to 800 K including the supercritical point.

Effect of the quantum confinement on the luminescent properties of sesquioxydes

Spectroscopic behaviour of Y 2O 3:Ce 3+ nanocrystals is presented. A new fluorescence peaking around 430 nm appears in small particles. This emission is interpreted as the 5d–4f radiative recombination of Ce 3+, which does not occur in bulk materials due to autoionisation processes. This change in behaviour could be due to both quantum confinement inside the nanoparticles and changes in the crystal field for small sizes.

Theoretical treatment of intense laser fields influence on the femtosecond time-resolved photoelectron spectrum of RbI molecule

The effect of laser fields on the RbI interaction potentials is obtained by the calculation of time-resolved photoelectron spectrum (TRPES) using the time-dependent wave-packet method. Both the normal three-photon ionization process and auto-ionization process are involved in the simulation. This complicated system which involves two ionization continua is successfully treated with a five-state model. The calculated results show not only that the crossings between excited states play important roles during the multiphoton ionization pump–probe process, but also that the intense laser field will change the position of relevant curves, and then influence the shape of the TRPES and ion signal.