Person: ALTUN, ZİKRİ
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ALTUN
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ZİKRİ
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Publication Open Access Dielectronic recombination data for dynamic finite-density plasmas - VI. The boron isoelectronic sequence(EDP SCIENCES S A, 2004-06) YUMAK YAHŞİ, AYŞE; Altun, Z; Yumak, A; Badnell, NR; Colgan, J; Pindzola, MSWe have calculated dielectronic recombination rate coefficients for 22 ions of the boron isoelectronic sequence, between C+ and Xe49+, within the generalized collisional-radiative framework, as outlined by Badnell et al. (2003). Calculations have been performed from both ground and metastable initial states, in both LS- and intermediate-coupling, allowing for Deltan = 0 and Deltan = 1 core-excitations. Results are presented and discussed for a selection of ions from the sequence. Results which are not presented here can be accessed from the Atomic Data and Analysis Structure (ADAS) database (Summers 2003) or from the Oak Ridge Controlled Fusion Atomic Data Center (http://www-cfadc.phy.ornl.gov). Comparison is made with the results of other existing theoretical calculations.Publication Open Access Stochastic enhancement of high-order harmonic generation(2012-01-01) ALTUN, ZİKRİ; YAVUZ, İLHAN; Yavuz İ., Bleda E. A., Altun Z.Publication Open Access Bonding Analysis of Compounds with Unusual Coordination of Carbon: Proposed Symmetric Systems with Six-Coordinate Carbon(MDPI, 2020-08-28) BLEDA, ERDİ ATA; Trindle, Carl; Altun, Zikri; Bleda, Erdi AtaThe possibility of carbon tetravalence in geometries other than tetrahedral and of carbon hypervalence has been taken seriously since the 1970s. Computational modeling and subsequent experimental validation have established the existence of molecules with carbon atoms with planar tetravalence and as many as six objects in carbon's coordination sphere. In this work, we develop insight into the nature of bonding to carbon in these unusual environs as provided by Bader's Atoms in Molecules (AIM) analysis of the electron density, along with the electron localization function (ELF) and the non-covalent index (NCI). We review several well-established systems (spiropentadiene dication, hexamethyl benzene dication, dimethanospiro[2.2]octaplane dication, and 1,8-dimethoxy-9-dimethoxyanthracene cation) and propose new D-2d-symmetric variants of a hexacoordinated species.Publication Metadata only Electron-impact ionization of atomic ions in the Na isoelectronic sequence(IOP PUBLISHING LTD, 2008) YUMAK YAHŞİ, AYŞE; Altun, Z.; Yumak, A.; Golcek, U.; Loch, S. D.; Pindzola, M. S.; Griffin, D. C.Electron-impact ionization cross-sections and rate coefficients for the Na-like ions Mg+, Ar7+, Kr25+, Sn39+, Xe43+ and W63+ are presented. The direct ionization cross-sections of 2s, 2p and 3s subshells are calculated within a configuration-average distorted-wave (CADW) method. In addition, excitation-autoionization contributions originating from the inner-shell excitations of the type 2s22p63s -> 2s22p53snl (n = 3-6) and 2s22p63s -> 2s2p63snl (n = 3-6) are calculated using both CADW and level-resolved distorted-wave (LLDW) methods. We include the branching ratios for the radiation damping of the autoionizing configurations and levels. The excitation-autoionization contributions are found to be quite large for the lighter ions. For the heavier ions the radiation damping of the excitation-autoionization contribution is found to be significant. We find that the distorted-wave results are in good agreement with available experimental measurements, which only exist for Mg+, Ar7+ and Xe43+. We also find that the computationally simple configuration-average calculations are in reasonable agreement with the computationally demanding level-resolved calculations for the excitation-autoionization contributions for all the Na-like ions studied. In addition, Maxwellian rate coefficients are produced from our cross-sections and are archived in a format useful for plasma modelers.Publication Metadata only Circular dichroism of some high-symmetry chiral molecules: B3LYP and SAOP calculations(SPRINGER, 2009) ALTUN, ZİKRİ; Trindle, Carl; Altun, ZikriComputational modeling of optical activity, circular dichroism (CD) and optical rotatory dispersion, is rapidly becoming a useful supplement to experimental studies of absolute configuration. Here, we investigate the predictions of two alternative formulations of the rotational strength based on time-dependent density functional theory (TD-DFT), for a series of high symmetry chiral systems. We employ the TD-DFT method as realized in Gaussian 03 suite with the hybrid functional B3LYP and as incorporated in the Amsterdam density functional (ADF) suite with PBE and SAOP functionals. The high-symmetry systems described here are somewhat larger than those used to evaluate the influence of basis sets and density functional choice, and for such large systems the very extensive basis sets recommended by most investigators may not be suitable for routine use. We observe that useful results for these systems can be obtained in modest bases, and in particular that diffuse functions may not be required for informative use of the ADF implementation. The statistical average of orbital potentials (SAOP) model developed by Baerends is essential to the success of the ADF implementation. In some cases chirality is defined by features of the molecular structure remote from the chromophore. This is a severe test of the TD-DFT theory, since high-lying excitations define the most prominent features of the CD spectra, and complicates the use of computations to guide the assignment of absolute configuration. Experimental investigation of the high symmetry systems described here is desirable.Publication Metadata only Level-resolved distorted-wave cross-sections of electron impact ionization of Ar5+(Institute of Physics Publishing, 2009-11-01) YUMAK YAHŞİ, AYŞE; Yumak A., Yavuz I., Altun Z.Electron impact ionization cross sections of Ar5+ were calculated using configuration-average (CADW) and level-resolved (LRDW) distorted-wave methods. Direct ionization cross-sections of 2s, 2p, 3s and 3p subshells were calculated within a CADW approximation. The contributions from the excitation autoionization channels were evaluated assuming single excitations from the 2s, 2p, and 3s subshells and in both CADW and LRDW methods. The radiative stabilization of the excitation autoionization channels were found to reduce the excitation autoionization by a small amount. © 2009 IOP Publishing Ltd.Publication Metadata only Complexes with Tunable Intramolecular Ferrocene to Ti-IV Electronic Transitions: Models for Solid State Fe-II to Ti-IV Charge Transfer(AMER CHEMICAL SOC, 2016) ALTUN, ZİKRİ; Turlington, Michael D.; Pienkos, Jared A.; Carlton, Elizabeth S.; Wroblewski, Karlee N.; Myers, Alexis R.; Trindle, Carl O.; Altun, Zikri; Rack, Jeffrey J.; Wagenknecht, Paul S.Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO2 by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and Ti-IV has been prepared and characterized by UV-vis spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp2Ti(C(2)Fc)(2), Cp*Ti-2(C(2)Fc)(2), and Cp2Ti(C(4)Fc)(2). Complexes with a single ferrocene utilize a titanocene with a trimethylsilyl derivatized Cp ring, Cp-TMS, and comprise the complexes (Cp2Ti)-Cp-TMS(C(2)Fc)(C2R), where R = C6H5, p-C6H4CF3, and CF3. The complexes are compared to Cp2Ti(C2Ph)(2), which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible Ti-IV/III reduction wave and an Fe-II/III oxidation that is irreversible for all complexes except (Cp2Ti)-Cp-TMS-(C(2)Fc)(C2CF3). All of the complexes with both Fc and Ti show an intense absorption (4000 M-1 cm(-1) < epsilon < 8000 M-1 cm(-1)) between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the difference between the Ti-IV/III and Fe-III/II reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, lambda, have been determined using band shape analysis (2600 cm(-1) < lambda < 5300 cm(-1)) and are in the range observed for other donor-acceptor complexes that have a ferrocene donor. Marcus-Hush-type analysis of the electrochemical and spectroscopic data are consistent with the assignment of the low-energy absorption as a MMCT band. TD-DFT analysis also supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at Ti-IV and the identity of the linker between the titanocene and the Fc. Complexes with a MMCT are photochemically stable, whereas Cp2Ti(C2Ph)(2) rapidly decomposes upon photolysis.Publication Open Access Determination of the recombination rate coefficients for Na-like SiIV forming Mg-like SiIII(EDP SCIENCES S A, 2006-11) ALTUN, ZİKRİ; Orban, I.; Glans, P.; Altun, Z.; Lindroth, E.; Kallberg, A.; Schuch, R.Aims. Absolute, total recombination rate coefficients for Si IV were determined using the CRYING heavy-ion storage ring. Calculated rate coefficients were used to estimate recombination into states that could not be detected in the experiment because of field ionization. Total, as well as separate, radiative and dielectronic plasma recombination rate coefficients were determined. Methods. Stored ions were merged with an expanded electron beam in the electron cooler section of the storage ring. Recombined ions were separated from the stored ion beam in the first dipole magnet after the electron cooler and were detected with unity efficiency. The absolute radiative and dielectronic recombination rate coefficients were obtained over a center-of-mass energy range of 0-20 eV, covering. Delta n = 0 core excitations up to the 3s -> 3d series limit. The results of an intermediate coupling AUTOSTRUCTURE calculation were compared with the experiment. The theoretical results were also used to estimate the contribution to dielectronic recombination by high Rydberg states, which were not detected because of field ionization. The spectra were convoluted with Maxwell-Boltzmann energy distributions in the 10(3)-10(6) K temperature range. Results. The resulting plasma recombination rate coefficients are presented and compared with theoretical results frequently used for plasma modeling. In the 10(3)-10(4) K range, a significant underestimation of the calculated dielectronic recombination plasma rate coefficients was observed. Above 3 x 104 K, the agreement between our dielectronic recombination plasma rate coefficients and two of the previously published rate coefficients is better than 20%. Conclusions. The observed differences between the experimental and calculated recombination rate coefficients at low temperatures reflect the need for benchmarking experiments. Our experimentally-derived rate coefficients can guide the development of better theoretical models and lead to more accurately-calculated rate coefficients.Publication Open Access Phosphorescence Tuning through Heavy Atom Placement in Unsymmetrical Difluoroboron beta-Diketonate Materials(WILEY-V C H VERLAG GMBH, 2018-02-06) ALTUN, ZİKRİ; Liu, Tiandong; Zhang, Guoqing; Evans, Ruffin E.; Trindle, Carl O.; Altun, Zikri; DeRosa, Christopher A.; Wang, Fang; Zhuang, Meng; Fraser, Cassandra L.Difluoroboron beta-diketonates (BF(2)bdks) show both fluorescence (F) and room-temperature phosphorescence (RTP) when confined to a rigid matrix, such as poly(lactic acid). These materials have been utilized as optical oxygen sensors (e.g., in tumors, wounds, and cells). Spectral features include charge transfer (CT) from the major aromatic donor to the dioxaborine acceptor. A series of naphthyl-phenyl dyes (BF(2)nbm) (1-6) were prepared to test heavy-atom placement effects. The BF(2)nbm dye (1) was substituted with Br on naphthyl (2), phenyl (3), or both rings (4) to tailor the fluorescence/phosphorescence ratio and RTP lifetimeimportant features for designing O-2 sensing dyes by means of the heavy atom effect. Computational studies identify the naphthyl ring as the major donor. Thus, Br substitution on the naphthyl ring produced greater effects on the optical properties, such as increased RTP intensity and decreased RTP lifetime compared to phenyl substitution. However, for electron-donating piperidyl-phenyl dyes (5), the phenyl aromatic is the major donor. As a result, Br substitution on the naphthyl ring (6) did not alter the optical properties significantly. Experimental data and computational modeling show the importance of Br position. The S-1 and T-1 states are described by two singly occupied MOs (SOMOs). When both of these SOMOs have substantial amplitude on the heavy atom, passage from S-1 to T-1 and emission from T-1 to S-0 are both favored. This shortens the excited-state lifetimes and enhances phosphorescence.Publication Metadata only Studies on spin state preferences in Fe(II) complexes(ELSEVIER, 2015) BLEDA, ERDİ ATA; Bleda, Erdi A.; Trindle, Carl; Altun, ZikriWe report modeling studies of high spin and low spin forms of the test set of problematic cases of pseudo-octahedral Fe(II) complexes defined by Ye and Neese. These include the most thoroughly studied spin crossover compound Fe(SCN)(2)(Phen)(2) and a more recently discovered Fe(II)-2,13-dimethyl-6,9-dio xa-3,12,18-triaza-bicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentene. Reliable estimates of the often small high-spin to low-spin energy gap require balanced treatment of electronic structure in very different bonding environments. Density functionals differ in their bias toward low spin or high spin configurations. For the test set defined by Ye and Neese, we describe the estimates of their energy gaps between high-spin and low-spin forms for 75 functionals; geometries are obtained by optimization with B3LYP with zero-order relativistic approximation (ZORA) in a triple-zeta quality basis. Fairly reliable results for high-spin to low-spin energy gaps are obtained by functionals IMP, HCTH, OPBE, and B3LYP*, and with the omega B97XD functional expressed in the cc-pVTZ basis, about half the size of the def2-QZVPP used by Ye and Neese. The best functionals are still biased systematically toward the high spin by about 20 kJ/mol. We apply these methods in a new study of a pseudo C-3v four coordinate crossover species reported by Scepaniak et al. Their four coordinate spin crossover complex of Fe with aryl-substituted scorpionate and N = P(Ph)(3) ligands is modeled by a truncated structure lacking aromatic substituents. These substituents appear to stabilize the lower spin. Reproducing the 80 K crossover temperature requires an empirical correction of about 12 kJ/mot in favor of the singlet for this truncated model system. (C) 2015 Elsevier B.V. All rights reserved.