Valence Vacancies Research Articles

High Thermoelectric Performance in Earth‐Abundant Cu3SbS4 by Promoting Doping Efficiency via Rational Vacancy Design

AbstractSulfides are well investigated as thermoelectric materials but their performance is typically limited by low electrical conductivity. High electrical performance in Cu3SbS4 is reported by creating high valence vacancies, which efficiently provides multiple carriers. It is revealed from the perspective of a chemical bond by calculations that Al can serve as vacancy stabilizer as its entry into the lattice forms intensified bonds with neighboring atoms and lowers the vacancy formation energy. As a result, the average power factor of Cu3SbS4 with 9 wt% CuAlS2 reaches 16.1 µW cm−1 K−2. Finally, by further addition of AgAlS2, a peak zT of 1.3 and an average zT of 0.77 are obtained due to the reduced thermal conductivity. The attained average power factor and average zT are superior to other low‐toxic thermoelectric sulfides. The findings shed light on the new strategy for creating favorable vacancies to realize high‐efficiency doping in thermoelectric materials.

High-order harmonic generation with resonant core excitation by ultraintense X-rays

High-order harmonic generation (HHG) is combined with resonant X-ray excitation of a core electron into the transient valence vacancy that is created in the course of the HHG process. To describe this setting, I develop a two-active-electron quantum theory for a single atom assuming no Coulomb interaction among the electrons; one electron performs a typical HHG three-step process whereas another electron is excited (or even Rabi flops) by intense X-rays from the core shell into the valence hole after the first electron has left the atom. Depending on the amplitude to find a vacancy in the valence and the core, the returning continuum electron recombines with the valence and the core, respectively, emitting high-order harmonic (HH) radiation that is characteristic of the combined process. After presenting the theory of X-ray boosted HHG for continuous-wave light fields, I develop a description for X-ray pulses with a time-varying amplitude and phase. My prediction offers novel prospects for nonlinear X-ray physics, attosecond X-rays, and HHG-based time-dependent chemical imaging involving core orbitals.

Strongly driven resonant Auger effect treated by an open-quantum-system approach

We present theoretical studies of a two-step resonant Auger process at high x-ray intensity. Tuning a short x-ray pulse to the initially closed resonant channel of the 1s-2p transition in singly ionized neon, the initially neutral neon target is valence ionized. Subsequently, the strong resonant x-ray field transfers an inner-shell electron to the created outer valence vacancy, thereby creating a core-excited state. The strong resonant coupling, giving rise to Rabi oscillations involving a core transition, results in a modification of the resonant Auger-electron spectral line profile. If the valence photoelectron remains unobserved, the system of the residual ion undergoing the resonant Auger decay can be treated by an open quantum system approach. The resonant Auger-electron spectral line shape is shown to be determined by an analog of the reduced density matrix that depends on two time arguments. The equations of motion of this reduced density matrix are derived and numerical results are presented, in support of the recent experimental verification [E. Kanter et al., Phys. Rev. Lett. 107, 233001 (2011)] of this nonlinear x-ray optical effect.

K-shell Auger lifetime variation in doubly ionized Ne and first row hydrides

We consider 1s Auger decay in doubly (core-core and core-valence) ionized Ne and in the isoelectronic first row element hydrides. We show theoretically that the presence of the spectator inner valence vacancy leads to Auger lifetime variation of up to about a factor of 2, relative to the Auger lifetimes in the singly ionized species. The origin of this effect is traced to spin selection rules. Implications on the modelling of the radiation damage in strong x-ray fields are discussed.

High-order harmonic generation enhanced by XUV light

The combination of high-order harmonic generation (HHG) with resonant XUV excitation of a core electron into the transient valence vacancy that is created in the course of the HHG process is investigated theoretically. In this setup, the first electron performs a HHG three-step process, whereas the second electron Rabi flops between the core and the valence vacancy. The modified HHG spectrum due to recombination with the valence and the core is determined and analyzed for krypton on the 3d→4p resonance in the ion. We assume an 800 nm laser with an intensity of about 10(14) W/cm2 and XUV radiation from the Free Electron Laser in Hamburg (FLASH) with an intensity in the range 10(13)-10(16)W cm2. Our prediction opens perspectives for nonlinear XUV physics, attosecond x rays, and HHG-based spectroscopy involving core orbitals.

Double Core Hole Creation and Subsequent Auger Decay inNH3andCH4Molecules

Energies of the hollow molecules CH(4)(2+) and NH(3)(2+) with double vacancies in the 1s shells have been measured using an efficient coincidence technique combined with synchrotron radiation. The energies of these states have been determined accurately by high level electronic structure calculations and can be well understood on the basis of a simple theoretical model. Their major decay pathway, successive Auger emissions, leads first to a new form of triply charged ion with a core hole and two valence vacancies; experimental evidence for such a state is presented with its theoretical interpretation. Preedge 2-hole-1-particle (2h-1p) states at energies below the double core-hole states are located in the same experiments and their decay pathways are also identified.

Soft-x-ray fragmentation studies of molecular ions

Imaging of photofragments from molecular ions after irradiation by soft x-ray photons has been realized at the ion beam infrastructure TIFF set up at the FLASH facility. Photodissociation of the two-electron system HeH+ at 38.7 eV revealed the electronic excitations and the charge-state ratios for the products of this process, reflecting the non-adiabatic dissociation dynamics through multiple avoided crossings among the HeH+ Rydberg potential curves. Dissociative ionization of the protonated water molecules H3O+ and H5O+2 at 90 eV revealed the main fragmentation pathways after the production of valence vacancies in these ionic species, which include a strong three-body channel with a neutral fragment (OH + H+ + H+) in H3O+ photolysis and a significant two-body fragmentation channel (H3O++ H2O+) in H5O+2 photolysis. The measurements yield absolute cross sections and fragment angular distributions. Increased precision and sensitivity of the technique were realized in recent developments, creating a tool for exploring x-ray excited molecular states under highly controlled target conditions challenging detailed theoretical understanding.

Biphenylnitrile-Based Self-Assembled Monolayers on Au(111): Spectroscopic Characterization and Resonant Excitation of the Nitrile Tail Group

A series of biphenylnitrile-based self-assembled monolayers (SAMs) on Au(111) was characterized by several complementary spectroscopic techniques, including resonant Auger electron spectroscopy (RAES). In spite of the disturbing role of nitrile, the SAMs exhibited high-quality and expected structural behavior, including the odd−even effects, even though with a lower extent than in the analogous nonsubstituted or methyl-substituted systems. A characteristic feature of the target films, and presumably also a general feature of oligophenyl-based SAMs, is a significant twist of the molecular backbone, similar to the situation in the analogous molecular crystals. No perceptible signal related to tail group-to-substrate transfer of the resonantly excited electron was recorded in the RAES spectra of the target films, even though such a transfer is energetically allowed in some of the cases. This was tentatively explained by the hypothesis that the charge transfer time is too long and lies beyond the range accessible by the core hole clock method via RAES in the present case. A characteristic feature of the RAES spectra of the target films at the excitation to the out-of plane orbital of the nitrile group is a strong suppression of the participator lines. This effect was qualitatively explained by the fact that the initial N1s core vacancy, the vacancy in the out-of-plane π1(C≡N*) orbital, and another valence vacancy participating in the decay are not localized on the same site in the case of this particular orbital and participator decay channels.

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.

Ab initiocalculation of interatomic decay rates by a combination of the Fano ansatz, Green’s-function methods, and the Stieltjes imaging technique

A new computational technique is introduced for the ab initio calculation of the rates of interatomic and intermolecular nonradiative decay processes occurring due to electronic correlation. These recently discovered phenomena are described theoretically using the configuration-interaction formalism first introduced by Fano [Phys. Rev. 124, 1866 (1961)] and later adapted to an Auger decay by Howat et al. [J. Phys. B 11, 1575 (1978)]. The boundlike and the continuumlike components of the wave function of the decaying state are constructed using a Green's-function method known as algebraic diagrammatic construction. A combination of atomic and distributed Gaussian basis sets is shown to provide an adequate description of both boundlike and continuumlike wave-function components. The problem of the normalization of the continuum (final state) wave function is addressed using the Stieltjes imaging technique. The new method is applied to the calculation of the rates of interatomic decay in alkaline-earth-rare-gas clusters. The obtained results help to verify our earlier conclusions [Phys. Rev. Lett. 93, 263002 (2004)] regarding the validity of the virtual-photon transfer model for the interatomic Coulombic decay. In addition, we demonstrate that the process of electron-transfer-mediated decay is responsible for the finite lifetimes of the outer valence vacancies in alkaline-earth-rare-gas clusters.

Mechanism of Interatomic Coulombic Decay in Clusters

Interatomic (or intermolecular) Coulombic decay is a general, very efficient mode of decay of inner valence vacancies in clusters. The physically appealing interpretation of such decays as a transfer of a virtual photon between two cluster units rests on the neglect of the orbital overlap between them. We show that even in loosely bound van der Waals clusters the orbital overlap is a crucial factor. At the equilibrium geometry of a cluster, the overlap effect can bring about an enhancement of the decay rate by 2-3 orders of magnitude, making the process dramatically more efficient than implied by the simple estimations.

Core-valence doubly ionized states: General aspects, examples, production mechanisms

Electronic double vacancies with one vacancy in the valence shell and one in the core play a role in several physical processes. Such core-valence double vacancies are theoretically analyzed and related to possible experiments. The corresponding wavefunctions and energies for CO, N2, and H2CO are computed using propagator and configuration interaction methods. The numerical results are analyzed in some detail and are compared to the corresponding single valence vacancies. The analysis is performed by breaking up the binding energy of the double vacancy into the most relevant components, such as hole–hole repulsion and relaxation contributions. It is shown that the double ionization potential is essentially given by single ionization quantities. In particular, we find a kind of ‘‘Koopmans theorem’’ for those dicationic states with an outer valence hole: the double ionization potential (shifted by the core ionization energy) is approximately given by the valence orbital energy of the core ionized state. As typical for double vacancies we encounter, in addition, an interesting singlet–triplet separation problem. Intensities for the production of the dicationic states by valence ionization out of a core ionized initial state are derived. The extent of valence hole localization in the dicationic states is analyzed by a two-hole population analysis. The analysis can be used to simulate the production of core-valence vacancies via Auger decay.