Hole Decay Research Articles

Auger electron emission initiated by the creation of valence-band holes in graphene by positron annihilation

Auger processes involving the filling of holes in the valence band are thought to make important contributions to the low-energy photoelectron and secondary electron spectrum from many solids. However, measurements of the energy spectrum and the efficiency with which electrons are emitted in this process remain elusive due to a large unrelated background resulting from primary beam-induced secondary electrons. Here, we report the direct measurement of the energy spectra of electrons emitted from single layer graphene as a result of the decay of deep holes in the valence band. These measurements were made possible by eliminating competing backgrounds by employing low-energy positrons (<1.25 eV) to create valence-band holes by annihilation. Our experimental results, supported by theoretical calculations, indicate that between 80 and 100% of the deep valence-band holes in graphene are filled via an Auger transition.

Direct and simultaneous observation of ultrafast electron and hole dynamics in germanium

Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M4,5 edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 1020 cm−3. Separate electron and hole relaxation times are observed as a function of hot carrier energies. A first-order electron and hole decay of ∼1 ps suggests a Shockley–Read–Hall recombination mechanism. The simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.

Bounds on large extra dimensions from the Generalized Uncertainty Principle

The Generalized Uncertainty Principle (GUP) implies the existence of a physical minimum length scale [Formula: see text]. In this scenario, black holes must have a radius larger than [Formula: see text]. They are hotter and evaporate faster than in standard Hawking thermodynamics. We study the effects of the GUP on black hole production and decay at the LHC in models with large extra dimensions. Lower bounds on the fundamental Planck scale and the minimum black hole mass at formation are determined from black hole production cross-section limits by the CMS Collaboration. The existence of a minimum length generally decreases the lower bounds on the fundamental Planck scale obtained in the absence of a minimum length.

Multipartite entanglement and firewalls

Black holes offer an exciting area to explore the nature of quantum gravity. The classic work on Hawking radiation indicates that black holes should decay via quantum effects, but our ideas about how this might work at a technical level are incomplete. Recently Almheiri-Marolf-Polchinski-Sully (AMPS) have noted an apparent paradox in reconciling fundamental properties of quantum mechanics with standard beliefs about black holes. One way to resolve the paradox is to postulate the existence of a "firewall" inside the black hole horizon which prevents objects from falling smoothly toward the singularity. A fundamental limitation on the behavior of quantum entanglement known as "monogamy" plays a key role in the AMPS argument. Our goal is to study and apply many-body entanglement theory to consider the entanglement among different parts of Hawking radiation and black holes. Using the multipartite entanglement measure called negativity, we identify an example which could change the AMPS accounting of quantum entanglement and perhaps eliminate the need for a firewall. Specifically, we constructed a toy model for black hole decay which has different entanglement behavior than that assumed by AMPS. We discuss the additional steps that would be needed to bring lessons from our toy model to our understanding of realistic black holes.

Planck stars as observational probes of quantum gravity

A phenomenon recently studied in theoretical physics may hold considerable interest for astronomers: the explosive decay of primordial black holes through quantum tunnelling. Their detection would be of major theoretical importance.

Electron trapping energy states of TiO2–WO3 composites and their influence on photocatalytic degradation of bisphenol A

Impregnation method was used to synthesize TiO2–WO3 composites with two different TiO2 morphologies (nanorods (R-TiO2–WO3) and polyhedral nanoparticles (P-TiO2–WO3)). Their structural, morphological, surface properties and electron trapping states were analyzed and correlated to performance in photocatalytic bisphenol A oxidation. TiO2 nanorods were prepared with alkaline hydrothermal digestion of commercially available high surface TiO2 nanopowder (DT-51), which was used as a reference for TiO2 nanoparticles. TEM analysis showed that in R-TiO2–WO3 composites WO3 is dispersed over the surface of TiO2 nanorods in amorphous form and with increasing amount of WO3 the TiO2 crystal structure deteriorates. However, in the case of P-TiO2–WO3 composite, multiphase system with monoclinic WO3 intermixed with anatase TiO2 was observed. Moreover, P-TiO2–WO3 composite showed strong surface acidic sites, which were absent in R-TiO2–WO3 composites; this information is significant to understand the depth of the electron trapping states. UV light-induced electron–hole pair excitations and decay dynamics in both TiO2–WO3 composites were studied by infrared spectroscopy measurements and information about the conduction band (CB) electrons and surface trapping states of the composites were collected. In the case of composites with high density of shallow trapping states, enhanced photocatalytic activity was observed. On the contrary, lower photocatalytic activity of solids was observed in cases where deep trapping (TiO2 nanoparticles) or fast recombination (R-TiO2–WO3 composite) prevailed.

An experimental setup for studying the core-excited atoms and molecules by electron impact using energy analysed electron–ion coincidence technique

Operation and performance of an apparatus for studying the decay dynamics relevant to core–hole decay processes in atoms and molecules excited by energetic electrons using an energy analysed electron–ion coincidence technique are described in some detail. The setup consists of a time- and position sensitive double-field linear TOF mass spectrometer coupled with a dual MCP detector and a single-pass CMA to select the energy of detected electrons. Details of different components involved in the setup are presented and discussed. To demonstrate the performance and capability of the apparatus, we present some typical results extracted from the TOF argon ion-mass spectra observed in coincidence with 18-energy selected electrons emitted from interaction of a continuous beam of 3.5 keV electrons with a dilute gaseous target of argon atoms. Specifically, the variation of relative correlation probability for the final ion-charge states Ar1+ to Ar4+ produced in the considered collision reactions as a function of energy of emitted electrons is determined and discussed.

Effects of model approximations for electron, hole, and photon transport in swift heavy ion tracks

The event-by-event Monte Carlo code, TREKIS, was recently developed to describe excitation of the electron subsystems of solids in the nanometric vicinity of a trajectory of a nonrelativistic swift heavy ion (SHI) decelerated in the electronic stopping regime. The complex dielectric function (CDF) formalism was applied in the used cross sections to account for collective response of a matter to excitation. Using this model we investigate effects of the basic assumptions on the modeled kinetics of the electronic subsystem which ultimately determine parameters of an excited material in an SHI track.In particular, (a) effects of different momentum dependencies of the CDF on scattering of projectiles on the electron subsystem are investigated. The ‘effective one-band’ approximation for target electrons produces good coincidence of the calculated electron mean free paths with those obtained in experiments in metals. (b) Effects of collective response of a lattice appeared to dominate in randomization of electron motion. We study how sensitive these effects are to the target temperature. We also compare results of applications of different model forms of (quasi-) elastic cross sections in simulations of the ion track kinetics, e.g. those calculated taking into account optical phonons in the CDF form vs. Mott’s atomic cross sections. (c) It is demonstrated that the kinetics of valence holes significantly affects redistribution of the excess electronic energy in the vicinity of an SHI trajectory as well as its conversion into lattice excitation in dielectrics and semiconductors. (d) It is also shown that induced transport of photons originated from radiative decay of core holes brings the excess energy faster and farther away from the track core, however, the amount of this energy is relatively small.

Ion charge-resolved branching in decay of inner shell holes in Xe up to 1200 eV

Using a new multi-electron multi-ion coincidence apparatus and soft x-ray synchrotron radiation we have determined branching ratios to final Xen+ states with 2 < n < 9 from the 4d−1, 4p−1, 4s−1, 3d−1 and 3p−1 Xe+ hole states. The coincident electron spectra give information on the Auger cascade pathways. We show that by judicious choice of coincident electrons, almost pure single charge states of the final ions can be selected.

Subpicosecond to Second Time-Scale Charge Carrier Kinetics in Hematite-Titania Nanocomposite Photoanodes.

Water splitting with hematite is negatively affected by poor intrinsic charge transport properties. However, they can be modified by forming heterojunctions to improve charge separation. For this purpose, charge dynamics of TiO2:α-Fe2O3 nanocomposite photoanodes are studied using transient absorption spectroscopy to monitor the evolution of photogenerated charge carriers as a function of applied bias voltage. The bias affects the charge carrier dynamics, leading to trapped electrons in the submillisecond time scale and an accumulation of holes with a lifetime of 0.4 ± 0.1 s. By contrast, slower electron trapping and only few long-lived holes are observed in a bare hematite photoanode. The decay of the long-lived holes is 1 order of magnitude faster for the composite photoanodes than previously published for doped hematite, indicative of higher catalytic efficiency. These results illustrate the advantages of using composite materials to overcome poor charge carrier dynamics, leading to a 30-fold enhancement in photocurrent.

Search for low-scale gravity signatures in multi-jet final states with the ATLAS detector at s = 8 $$ \sqrt{s}=8 $$ TeV

We search for evidence of physics beyond the Standard Model in the production of final states with multiple high transverse momentum jets, using 20.3 fb$^{-1}$ of proton-proton collision data recorded by the ATLAS detector at $\sqrt{s} = 8$ TeV. No excess of events beyond Standard Model expectations is observed, and upper limits on the visible cross-section for non-Standard Model production of multi-jet final states are set. Using a wide variety of models for black hole and string ball production and decay, the limit on the cross-section times acceptance is as low as 0.16 fb at the 95% CL for a minimum scalar sum of jet transverse momentum in the event of about 4.3 TeV. Using models for black hole and string ball production and decay, exclusion contours are determined as a function of the production mass threshold and the gravity scale. These limits can be interpreted in terms of lower-mass limits on black hole and string ball production that range from 4.6 to 6.2 TeV.

Off the beaten path: a new approach to realistically model the orbital decay of supermassive black holes in galaxy formation simulations

We introduce a force correction term to better model the dynamical friction (DF) experienced by a supermassive black hole (SMBH) as it orbits within its host galaxy. This new approach accurately follows the orbital decay of a SMBH and drastically improves over commonly used advection methods. The force correction introduced here naturally scales with the force resolution of the simulation and converges as resolution is increased. In controlled experiments we show how the orbital decay of the SMBH closely follows analytical predictions when particle masses are significantly smaller than that of the SMBH. In a cosmological simulation of the assembly of a small galaxy, we show how our method allows for realistic black hole orbits. This approach overcomes the limitations of the advection scheme, where black holes are rapidly and artificially pushed toward the halo center and then forced to merge, regardless of their orbits. We find that SMBHs from merging dwarf galaxies can spend significant time away from the center of the remnant galaxy. Improving the modeling of SMBH orbital decay will help in making robust predictions of the growth, detectability, and merger rates of SMBHs, especially at low galaxy masses or at high redshift.

The Decay of a Black Hole in a GUT Model

I propose a phenomenological model for the decay of black holes near Planck mass. The decay takes place via a quantum state between general relativity and a grand unified field theory like SO(10). This group is favored also by a no-scale SUGRA GUT model for Starobinsky inflation by other authors.

Decay Processes of Si 2sCore Holes in Si(111)-7 × 7 Revealed by Si Auger Electron Si 2sPhotoelectron Coincidence Measurements

Decay processes of Si 2s core holes in a clean Si(111)-7 × 7 surface are investigated using coincidence measurements of Si Auger electrons and Si 2s photoelectrons at a photon energy of 180 eV. We show that Si 2s core holes exhibit two nonradiative decay processes: the first being a Si L1L23V Coster–Kronig transition followed by delocalization of the valence hole and Si L23VV Auger decay, and the second being Si L1VV Auger decay. The branching ratio of the Si L1L23V Coster–Kronig transition to the Si L1VV Auger decay is estimated to be 96.7% ± 0.4% to 3.2% ± 0.4%.

Search for microscopic black holes and string balls in final states with leptons and jets with the ATLAS detector at s $$ \sqrt{s} $$ = 8 TeV

A search for an excess of events with multiple high transverse momentum objects including charged leptons and jets is presented, using 20.3 fb^-1 of proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2012 at a centre-of-mass energy of sqrt(s) = 8 TeV. No excess of events beyond Standard Model expectations is observed. Using extra-dimensional models for black hole and string ball production and decay, exclusion contours are determined as a function of the mass threshold for production and the fundamental gravity scale for two, four and six extra dimensions. For six extra dimensions, mass thresholds of 4.8-6.2 TeV are excluded at 95% confidence level, depending on the fundamental gravity scale and model assumptions. Upper limits on the fiducial cross-sections for non-Standard Model production of these final states are set.

Back-to-back black holes decay signature at neutrino observatories

We propose a decay signature for non-thermal small black holes with masses in the TeV range which can be discovered by neutrino observatories. The black holes would result due to the impact between ultra high energy neutrinos with nuclei in water or ice and decay instantaneously. They could be produced if the Planck scale is in the few TeV region and the highly energetic fluxes are large enough. Having masses close to the Planck scale, the typical decay mode for these black holes is into two particles emitted back-to-back. For a certain range of angles between the emitted particles and the center of mass direction of motion, it is possible for the detectors to measure separate muons having specific energies and their trajectories oriented at a large enough angle to prove that they are the result of a back-to-back decay event.

Photodegradation of contaminants using Ag@AgCl/rGO assemblages: Possibilities and limitations

Plasmonic photocatalysts of the form Ag@AgCl/rGO were synthesized by a deposition–precipitation–photoreduction method and the composite materials characterized using Raman spectroscopy, attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The as-prepared plasmonic photocatalysts exhibited enhanced photoactivity for the degradation of formic acid (FA) under visible light irradiation, however, both the Ag@AgCl and the graphene oxide support underwent continuous transformation during photolysis. The high activity was attributed principally to the ability of the composite Ag0/AgCl system to absorb light in the visible wavelength region and to retard the recombination of electron–hole pairs. While the presence of the reduced graphene oxide (rGO) support may also have contributed to a reduction in rate of electron–hole pair recombination, the effect was not found to be particularly significant. HO trapping experiments with phthalhydrazide as well as the influence of t-butanol on FA degradation suggested that either photo-formed holes or surface chlorine atoms were the main reactive species inducing degradation of FA under visible light irradiation. Total free chlorine, but not hydrogen peroxide, could be detected by N,N-diethyl-p-phenylenediamine (DPD) spectrophotometry suggesting that while photogenerated holes decay via reaction with chloride ions, photogenerated electrons decay via a pathway other than reaction with oxygen. Possible reaction mechanisms that conform to these observations are discussed. Although this system has potential for the oxidative degradation of contaminants, greater understanding of the factors that promote excessive Ag(I) reduction, concomitant formation of Ag(0) and eventual deactivation of the catalyst is needed.

Nonviolent nonlocality

If quantum mechanics governs nature, black holes must evolve unitarily, providing a powerful constraint on the dynamics of quantum gravity. Such evolution apparently must in particular be nonlocal, when described from the usual semiclassical geometric picture, in order to transfer quantum information into the outgoing state. While such transfer from a disintegrating black hole has the dangerous potential to be violent to generic infalling observers, this paper proposes the existence of a more innocuous form of information transfer, to relatively soft modes in the black hole atmosphere. Simplified models for such nonlocal transfer are described and parameterized, within a possibly more basic framework of networked Hilbert spaces. Sufficiently sensitive measurements by infalling observers may detect departures from Hawking's predictions, and in generic models black holes decay more rapidly. Constraints of consistency---internally and with known and expected features of physics---restrict the form of information transfer, and should provide important guides to discovery of the principles and mechanisms of the more fundamental nonlocal mechanics.

Theory of Core Hole Decay Dynamics of Adsorbate on Metal Surfaces

A comparative study of a core hole decay dynamics is made for an adsorbate on a metal surface. On the basis of the available experimental results for a CO molecule chemisorbed on Cu(110) as a prototype system, we investigate the elementary processes of the core hole decay via various new Auger channels open for adsorbates, i.e., the crossed Auger transition and the participation of an electron in the initially unoccupied level of the adsorbate created by charge transfer from the substrate before core hole decay or resonant core excitation. We calculate the valence Auger spectrum and deexcitation spectrum following resonant excitation. These spectra are compared with the direct valence photoemission, in particular with a single valence hole state screened by charge transfer from the metal. Absence of shake-up satellite in the deexcitation spectrum following resonant core to bound (e.g., 2π* of a CO molecule) level excitation and the mismatch of the binding energy of the screened final state between direct photoemission or normal Auger spectrum and resonantly excited Auger spectrum are explained in terms of the relaxation processes of the excited state before the Auger decay of the core hole.

Core hole screening and decay rates of double core ionized first row hydrides

Because of the high intensity, X-ray free electron lasers allow one to create and probe double core ionized states in molecules. The decay of these multiple core ionized states crucially determines the evolution of radiation damage in single molecule diffractive imaging experiments. Here we have studied the Auger decay in hydrides of first row elements after single and double core ionization by quantum mechanical ab initio calculations. In our approach the continuum wave function of the emitted Auger electron is expanded into spherical harmonics on a radial grid. The obtained decay rates of double K-shell vacancies were found to be systematically larger than those for the respective single K-shell vacancies, markedly exceeding the expected factor of two. This enhancement is attributed to the screening effects induced by the core hole. We propose a simple model, which is able to predict core hole decay rates in molecules with low Z elements based on the electron density in the vicinity of the core hole.