1s Threshold Research Articles

Exploring fully heavy scalar tetraquarks [formula omitted

The masses, current couplings and widths of the fully heavy scalar tetraquarks X4Q=QQQ‾Q‾, Q=c,b are calculated by modeling them as four-quark systems composed of axial-vector diquark and antidiquark. The masses m(′) and couplings f(′) of these tetraquarks are computed in the context of the QCD sum rule method by taking into account a nonperturbative term proportional to the gluon condensate 〈αsG2/π〉. Results m=(6570±55)MeV and m′=(18540±50)MeV are used to fix kinematically allowed hidden-flavor decay channels of these states. It turns out that, the processes X4c→J/ψJ/ψ, X4c→ηcηc, and X4c→ηcχc1(1P) are possible decay modes of X4c. The partial widths of these channels are evaluated by means of the couplings gi,i=1,2,3 which describe strong interactions of tetraquark X4c and mesons at relevant vertices. The couplings gi are extracted from the QCD three-point sum rules by extrapolating corresponding form factors gi(Q2) to the mass-shell of a final meson. The mass of the scalar tetraquark X4b is below the ηbηb and ϒ(1S)ϒ(1S) thresholds, therefore it does not fall apart to these bottomonia, but transforms to conventional particles through other mechanisms. Comparing m=(6570±55)MeV and Γ4c=(110±21)MeV with parameters of structures observed by the LHCb, ATLAS and CMS collaborations, we interpret X4c as the resonance X(6600) reported by CMS. Comparisons are made with other theoretical predictions.

Doubly hidden 0++ molecules and tetraquarks states from QCD at NLO

Motivated by the LHCb-group discovery of exotic hadrons in the range (6.2 ∼ 6.9) GeV, we present new results for the masses and couplings of 0++ fully heavy (Q‾Q)(QQ‾) molecules and (QQ)(QQ‾) tetraquaks states from relativistic QCD Laplace Sum Rule (LSR) within stability criteria where Next-to-Leading Order (NLO) Factorized (F) Perturbative (PT) corrections is included. As the Operator Product Expansion (OPE) usually converges for d ⩽ 6–8, we evaluated the QCD spectral functions at Lowest Order (LO) of PT QCD and up to 〈G3〉. We also emphasize the importance of PT radiative corrections for heavy quark sum rules in order to justify the use of the running heavy quark mass value in the analysis. We compare our predictions in Table 3 with the ones from ratio of Moments (MOM). The broad structure arround (6.2 ∼ 6.9) GeV can be described by the η‾cηc, J/ψ‾J/ψ and χc1‾χc1 molecules or/and Sc‾Sc, Ac‾Ac and Vc‾Vc tetraquarks lowest mass ground states. The narrow structure at (6.8 ∼ 6.9) GeV if it is a 0++ state can be a χc0‾χc0 molecules or/and its analogue Pc‾Pc tetraquark. The χc1‾χc1 predicted mass is found to be below the χc1χc1 threshold while for the beauty states, all of the estimated masses are above the ηbηb and ϒ(1S)ϒ(1S) threshold.

Photoionization dynamics of Ar trapped in a fullerene anion: Coulomb confinement resonances in 2s subshell and its impact on the 3s subshell

The photoionization dynamics of the inner subshell of an endohedral anion, using relativistic random phase approximation (RRPA), is investigated. The charged nature of the fullerene induces Coulomb confinement resonances (CCRs) not only on the 2s cross-section but also on other dynamical attributes such as angular distribution asymmetric parameter, phase shift, and time delay of the same subshell. More importantly, we find that the 2s CCRs resurrect confinement resonances into the photoionization dynamics of 3s ionization channels at energies far away from the 3s threshold. The impacts of the revivification of these resonances on the 3s cross-section, phase shift, and time delay are explored in this work.

Nonstatistical behavior of the photoionization of spin–orbit doublets

The photoionization branching ratios of spin–orbit doublets are studied both experimentally and theoretically at energies several keV above threshold. The results show significant relativistic effects for Ar 2p in the autoionizing region below the 1s threshold, and large many-body effects for Xe 3d and 4d in the vicinity of the L-shell thresholds. The branching ratios in Xe are also found to vary significantly over very broad multi-keV energy regions both above and below the inner-shell thresholds. In addition, the Ar 2p study confirms experimentally the decades-old theoretical prediction that the nonresonant branching ratio does not approach the statistical (nonrelativistic) value, and, in fact, progressively diverges from statistical with increasing photon energy.

Compton scattering of keV photons at helium atom near the He$$^+$$(1s) threshold for small momentum transfer

We study Compton scattering at helium atom exposed to an electromagnetic field with a central frequency of 80 a.u. ( $$\sim 2.18$$ keV). We consider the situation where the incident light and scattered light are polarized along the same direction with small relative propagation angle $$\beta $$ . The energy of the emitted electron ranges from 0.2 to 2.1 a.u. ( $$\sim 5.44$$ –57.14 eV). The approach is based on previous works on stimulated Compton scattering (see Bachau et al. Phys Rev Lett 112:073001, 2014). We consider a field intensity of $$3.51\times 10^{16}$$ W/cm $$^2$$ , where stimulated Compton scattering can be treated in perturbative regime. In lowest order perturbation theory, the process results from the contribution of $$\mathbf{A\cdot \mathbf{P}}$$ in second order and $$\mathbf{A}^2$$ in first order ( $$\mathbf{P}$$ is the electron momentum operator and $$\mathbf{A}$$ the vector potential of the field); both terms induce two-photon transitions. The Compton matrix element $$|{{\mathcal {M}}}_{fg}|^2$$ is extracted numerically resolving the time-dependent Schrodinger equation, and in perturbation theory, emphasis is put on the calculation of the second-order amplitude associated with $$\mathbf{A\cdot \mathbf{P}}$$ . We investigate the cases of relative propagation angle $$\beta =0$$ and 10 degrees. The photoelectron energy distributions are dominated by the nondipole term $$\mathbf{A}^2$$ ; they increase by orders of magnitude when $$\beta $$ grows from 0 to 10 degrees. At $$\beta =0$$ degree, both $$\mathbf{A\cdot \mathbf{P}}$$ and $$\mathbf{A}^2$$ are at play and we show that the electrons are emitted in the (forward) direction of the momentum transfer $${{\mathcal {Q}}}$$ and to a lesser extent in the backward direction. When $$\beta $$ increases, the nondipole contribution $$\mathbf{A}^2$$ tends to dominate and the forward/backward asymmetry vanishes.

Thermal induced depletion of cationic vacancies in NiO thin films evidenced by x-ray absorption spectroscopy at the O 1s threshold

The effects of thermal annealing on the concentration of cationic vacancies in p-type semiconducting NiO thin films have been studied by x-ray absorption spectroscopy at the O 1s threshold. This technique proves to be very sensitive to the amount of Ni vacancies through the intensity of a prepeak observed below the absorption threshold, associated with Ni ions in a high oxidation state. Samples with different vacancy concentrations were obtained by radio frequency magnetron sputtering with different O2/Ar ratios in the plasma. Thermal effects have been studied both during thin film growth and after postprocessing annealing. In both cases, the observed effects were very similar, showing a depletion of cationic vacancies with temperature. By changing the surface sensibility of the x-ray absorption spectroscopy measurements, the authors could find out that the transition to stoichiometric NiO begins at the surface.

Ionization and Fragmentation of a Global Warming Gas by EUV and X-Ray Photons

An experimental investigation of the processes leading tothe fragmentation of the singly ionized 1,1,1,2-tetrafluoroethane (HFC-134a, CH2FCF3) by EUV and soft X-rays is presented. HFC-134ais taken into consideration as the most convenient replacement for CFC-12 in refrigeration applications due to the fact that it has null ozone depletion factor. Dissociation of the singly ionized HFC-134amolecule was induced by valence, direct and indirect C 1s core photoionization or photoexcitation and the ionic fragments were detected in coincidence (PEPICO mode) with the ejected electrons without energy analysis. The singly ionized parent ion CF4CH2+can be detected even at photon energies above the C 1s threshold.

Rearrangement of electron shells and interchannel interaction in the K photoabsorption of Ne

A detailed theoretical analysis of the 1s photoionization of neon is presented. It is found that the most significant many-electron correlation in computing photoionization of inner shells is the rearrangement of the outer shells caused by the inner vacancy. Further noticeable effects are: (i) the polarization of the ion core by the outgoing photoelectron and (ii) the coherent effect of double excitation/ionization. The core polarization increases the photoionization cross section by about 10% at the 1s threshold, and the coherent excitation results in further increases by about 5%. Incoherent excitation of the satellite channel leads to an additional 10% increase in the photoabsorption cross section in the double-ionization threshold region.

Local electronic structure and nanolevel hierarchical organization of bone tissue: theory and NEXAFS study

Theoretical and experimental investigations of native bone are carried out to understand relationships between its hierarchical organization and local electronic and atomic structure of the mineralized phase. The 3D superlattice model of a coplanar assembly of the hydroxyapatite (HAP) nanocrystallites separated by the hydrated nanolayers is introduced to account the interplay of short-, long- and super-range order parameters in bone tissue. The model is applied to (i) predict and rationalize the HAP-to-bone spectral changes in the electronic structure and (ii) describe the mechanisms ensuring the link of the hierarchical organization with the electronic structure of the mineralized phase in bone. To check the predictions the near-edge x-ray absorption fine structure (NEXAFS) at the Ca 2p, P 2p and O 1s thresholds is measured for native bone and compared with NEXAFS for reference compounds. The NEXAFS analysis has demonstrated the essential hierarchy induced HAP-to-bone red shifts of the Ca and P 2p-to-valence transitions. The lowest O 1s excitation line at 532.2 eV in bone is assigned with superposition of core transitions in the hydroxide OH−(H2O)m anions, Ca2+(H2O)n cations, the carboxyl groups inside the collagen and [PO4]2− and [PO4]− anions with unsaturated P–O bonds.

Negative-Ion/Positive-Ion Coincidence Yields of Core-Excited Water.

We report yields of mass-resolved negative ions and positive ions measured in coincidence after core excitation of water molecules. The analysis of negative-ion/positive-ion and negative-ion/positive-ion/positive-ion coincidence events provides new information on pathways leading to negative ion production, enhancing the present understanding of the dissociation processes of the water molecule. Dissociation following (resonant) Auger decay dominates negative ion production, but radiative decay is shown to contribute above the O 1s ionization threshold. A peak in the H(-)/O(+) yield above the O 1s threshold is attributed to decay from doubly excited states.

Inner-shell photodetachment from a Si− negative ion: strong effect of many-electron correlations

The first theoretical investigation of the inner-shell single-photodetachment from the Si− (1s22s22p63s23p3 4So) negative ion is presented. The partial and total cross sections, the photoelectron phaseshifts, and the parameters of angular anisotropy are calculated in the framework of the many-body theory for L-shell photodetachment from Si− ion in the experimentally accessible range of photon energies (7.5–14 Ry). Comparison is made between the calculations of the response of the ionic many-electron system Si− to an electromagnetic field at the different levels of approximation: the ‘frozen-field’ random phase approximation with exchange (RPAE), and the static relaxation approximation. The optimal analysis is made when the dynamic relaxation and polarization are included within the Dyson equation method (DEM) simultaneously with the RPAE corrections (the RPAE&DEM approach). It is predicted that the photoexcitation to a resonance state of complex ‘shape-Feschbach’ nature in the open p-shell reveals itself as a prominent resonance structure in the photodetachment cross sections in the energy range of the 2s and 2p inner shell thresholds similar to that in 1s inner-shell photodetachment from C− (2006 J. Phys. B: At. Mol. Opt. Phys. 39 1379). The photodetachment dynamical characteristics clearly demonstrate the significance of all the considered many-electron correlations within the RPAE&DEM approach, however the total photodetachment cross section is dominated by a strong resonance peak just after the 2s threshold. Dynamical relaxation (screening) is identified as a decisive factor in the formation of this resonance.

K-shell Photoionization of Atomic Cl

SynopsisRecent measurements of the photoionization of atomic Cl in the vicinity of the 1s thresholds have motivated the present R-matrix calculation which takes into account relativistic effects via the Breit-Pauli operator. The computer code CIV3 of Hibbert and Glass and Hibbert, which also includes relativistic effects, is used to obtain the discrete wavefunctions. These are constructed with orbitals generated from a carefully-chosen large scale configuration interaction expansion. The open-shell nature of the Cl atom translates into the existence of actually four 1s thresholds, 3P°0,1,2 and 1P°1. The results are analyzed with particular focus on the resonances leading up to the four thresholds, and the various effects that dominate the cross sections in this energy range are unraveled.

Photodesorption and Photostability of Acetone Ices: Relevance to Solid Phase Astrochemistry

Acetone, one of the most important molecules in organic chemistry, also a precursor of prebiotic species, was found in the interstellar medium associated with star-forming environments. The mechanisms proposed to explain the gas phase abundance of interstellar acetone are based on grain mantle chemistry. High energy photons coming from the stellar radiation field of the nearby stars interact with the ice mantles on dust grains leading to photoionization, photodissociation, and photodesorption processes. In this work we investigate the photodesorption and the photostability of pure acetone ices due to soft X-ray impact. Absolute desorption yields per photon impact for the main positive ionic fragments were determined at the O 1s resonance energy (531.4 eV). The photostability of acetone ice was studied by exposure to different irradiation doses with a white beam of synchrotron radiation. The degradation of the ice was monitored by NEXAFS around the O 1s threshold. From this study we determine the photodissociation cross-section to be about 1.5 × 10–17 cm2 which allowed us to estimate the half-life for acetone ice in astrophysical environments where soft X-rays play an important role in chemical processes.

Autoionization resonances in the partial photoionization cross sections of Al V

The photoionization cross sections for 2p or 2s sub-shell ionization of Al V are calculated using the R-matrix method in LS-coupling approximation. In the present work, the distorted-wave method is employed to calculate the orbital wave-functions, and the R-matrix method is employed to calculate the photoionization cross sections from ground state 2s22p5 2P and excited state 2s2p6 2S. The 12 series of autoionization resonances between the 2s22p4 3P and 1s22p6 1S threshold are identified.

Ionization and Fragmentation of Methane Induced by 40 eV to 480 eV Synchrotron Radiation: From Valence to Beyond Core Electron Ionization

Photoionization and fragmentation of gaseous methane induced by tunable synchrotron radiation were investigated in a wide energy range, from 40 eV up to 480 eV. We report electron-ion coincidence experiments by measuring the relative partial-ion yields and precursor-specific relative yields for individual fragment ions and for ion fragment pairs as a function of photon energy. The fragmentation patterns are discussed with emphasis on the transition behavior of the bond breaking reactions and of the hydrogen rearrangements from valence to core electron ionization. Below the C 1s threshold, a comparison between photon induced dissociation and electron impact data showed that the ionic fragments formation depends for both projectiles on the same final electronic state reached upon ionization.

O 1s excitation and ionization processes in the CO2 molecule studied via detection of low-energy fluorescence emission

Oxygen 1s excitation and ionization processes in the CO2 molecule have been studied with dispersed and non-dispersed fluorescence spectroscopy as well as with the vacuum ultraviolet (VUV) photon–photoion coincidence technique. The intensity of the neutral O emission line at 845 nm shows particular sensitivity to core-to-Rydberg excitations and core–valence double excitations, while shape resonances are suppressed. In contrast, the partial fluorescence yield in the wavelength window 300–650 nm and the excitation functions of selected O+ and C+ emission lines in the wavelength range 400–500 nm display all of the absorption features. The relative intensity of ionic emission in the visible range increases towards higher photon energies, which is attributed to O 1s shake-off photoionization. VUV photon–photoion coincidence spectra reveal major contributions from the C+ and O+ ions and a minor contribution from C2 +. No conclusive changes in the intensity ratios among the different ions are observed above the O 1s threshold. The line shape of the VUV–O+ coincidence peak in the mass spectrum carries some information on the initial core excitation.

TEXTING WHILE DRIVING: EVALUATION OF GLANCE DISTRIBUTIONS FOR FREQUENT/INFREQUENT TEXTERS AND KEYPAD/TOUCHPAD TEXTERS.

The threat that cell-phones pose to driving has been a well researched topic. There are fewer studies of the threat that texting creates for drivers, but the risks are obvious and the few existing studies confirm this. What is not obvious is whether frequent texters will expose themselves to the same risks as infrequent texters. This is important to know because many texters, especially teens who text frequently, may consider themselves immune to the dangers of texting while driving. As such, a comparison of frequent and infrequent texters was undertaken on a driving simulator. It is also not immediately clear what effects the different types of interfaces have on driving performance while text messaging. The interfaces under evaluation included keypad or "qwerty" phones (e.g., Blackberries) and touchpad phones (iPhone). It was found that the frequent and infrequent texters were equally likely to glance at least once for more than 2s inside the vehicle while sending a text message. It was also found that touchpad texters had a larger number of glances above the 2s threshold than keypad users, though this difference was not significant. The implications of this for future public policy are discussed.

Inner-shell photofragmentation of Cl2

We report an extensive study on partial-ion-yield spectroscopy around the Cl 1s and 2p ionization thresholds for Cl2. All positive ion channels, several with the same mass/charge ratio, which could be distinguished by taking the advantage of the 37Cl isotope, have been measured at a photon resolution of nearly 6500. At the Cl 1s ionization threshold, no significant differences are reported between the absorption and the partial-ion yields. In contrast, near the 2p ionization thresholds, we detect large variations in the fragmentation patterns following excitations to the Rydberg series when comparing the atomic fragment ions to the molecular fragment ions. We attribute the different behaviours to the more-or-less diffuse nature of Rydberg states with different angular momenta.

Electronic structure and chemical bonding of nanocrystalline-TiC/amorphous-C nanocomposites

The electronic structure of nanocrystalline (nc-) TiC/amorphous C nanocomposites has been investigated by soft x-ray absorption and emission spectroscopy. The measured spectra at the Ti 2p and C 1s thresholds of the nanocomposites are compared to those of Ti metal and amorphous C. The corresponding intensities of the electronic states for the valence and conduction bands in the nanocomposites are shown to strongly depend on the TiC carbide grain size. An increased charge-transfer between the Ti 3d-eg states and the C 2p states has been identified as the grain size decreases, causing an increased ionicity of the TiC nanocrystallites. It is suggested that the charge-transfer occurs at the interface between the nanocrystalline TiC and the amorphous C matrix and represents an interface bonding which may be essential for the understanding of the properties of nc-TiC/amorphous C and similar nanocomposites.

Fragmentation properties of three-membered heterocyclic molecules by partial ion yield spectroscopy: C2H4O and C2H4S

We investigated the photofragmentation properties of two three-membered ring heterocyclic molecules, C(2)H(4)O and C(2)H(4)S, by total and partial ion yield spectroscopy. Positive and negative ions have been collected as a function of photon energy around the C 1s and O 1s ionization thresholds in C(2)H(4)O, and around the S 2p and C 1s thresholds in C(2)H(4)S. We underline similarities and differences between these two analogous systems. We present a new assignment of the spectral features around the C K-edge and the sulfur L(2,3) edges in C(2)H(4)S. In both systems, we observe high fragmentation efficiency leading to positive and negative ions when exciting these molecules at resonances involving core-to-Rydberg transitions. The system, with one electron in an orbital far from the ionic core, relaxes preferentially by spectator Auger decay, and the resulting singly charged ion with two valence holes and one electron in an outer diffuse orbital can remain in excited states more susceptible to dissociation. A state-selective fragmentation pattern is analyzed in C(2)H(4)S which leads to direct production of S(2+) following the decay of virtual-orbital excitations to final states above the double-ionization threshold.