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Publication Metadata only Effects of the range of the potential on the structure and dynamics of two-dimensional Coulomb clusters(Taylor & Francis Ltd, 2008) Calvo, Florent; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The structure, dynamics, and thermodynamics of finite systems of two-dimensional particles interacting mutually through a shielded Coulomb potential, V(r) = e((-kappa r))/r, and bound together by an isotropic parabolic trap, are investigated by means of global optimization and simulation methods. As the range of the potential decreases, or as kappa increases, the number of distinct minima rapidly increases for a given size and the cluster generally shrinks. Looking more specifically at selected sizes below 50, non-monotonic cluster size effects seem to dominate the energetics, chaotic dynamics, and thermodynamics, even though the melting point is weakly affected by the range of the potential.Publication Metadata only Finding the global minima of (Ne)(n)(+) clusters with non-empirical models: a comparison of results(Elsevier, 2003) Sebastianelli, Francesco; Gianturco, Franco A.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The energetics and the structural properties of some of the smaller ionic clusters of neon atoms (Ne-n(+), with n up to 25) are examined using different modeling for the interactions within each cluster moiety. A new, very simple model for the global interaction, the polygon growth model or PGM, is presented and discussed in detail. The results of the calculations, and the physical reliability of the methods, are then examined in comparison with earlier theoretical results and with some available experimental data. In spite of the simplicity of the PGM scheme, the present treatment shows it to be able to reproduce all the important features known for these clusters from earlier calculations and from measurements.Publication Metadata only Neutral and ionic dopants in helium clusters: interaction forces for the Li-2(a(3)sigma(+)(u))-He and Li-2(+)(x-2 sigma(+)(g))-He complexes(Taylor and Francis Ltd, 2005) Bodo, E; Gianturco, F. A.; Yurtsever, M.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The potential energy surface (PES) describing the interactions between Li-2((1)Sigma(u)(+)) and He-4 and an extensive study of the energies and structures of a set of small clusters, Li-2(He)(n), were reported in a previous series of publications. In the present work, the same analysis is extended to the case of the excited Li-2(a(3)Sigma(u)(+)) and of the ionized Li-2(+) (X(2)Sigma(g)(+)) moiety. Calculated interaction potentials for the two title systems and the corresponding fitting of the computed points are shown. For both surfaces the MP4 method with cc-pV5Z basis sets has been used to generate an extensive range of radial/angular coordinates of the two-dimensional PESs which describe rigid rotor molecular dopants interacting with one He partner.Publication Metadata only Quantum chemical view on the growth mechanisms of odd-sized nitrogen cluster anions(Amer Chemical Soc, 2019) Calvo, Florent; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The stable structures of odd-numbered anionic nitrogen clusters, N-2n+3(-), have been theoretically investigated in the size range n = 1-9 using a variety of quantum chemistry methods that include perturbation theory, coupled cluster, and density-functional theory with different exchange correlation functionals. We generally find that the clusters are composed of an azide chromophore N-3(-) surrounded by essentially neutral nitrogen molecules. The growth initially proceeds by placing the neutral molecules parallel to the azide anion, completing a first shell at N-13(-), above which the extra molecules arrange on the side but with a significantly lower binding energy. Comparison with the cyclic N-5(-) anionic core shows that the latter is unfavorable, the spectral signatures of both N-5(-) and N2N3- being provided in both the infrared and ultraviolet ranges. The trend of these clusters to be highly stable as (N-2)(n)N-3(-) agrees with recent mass spectrometry experiments under the cryogenic environment of helium droplets. The issues associated with the successful development of a nonreactive force field for such clusters are also highlighted.Publication Metadata only Quenching vibrations of cesium dimers by He at low and ultralow temperatures: quantum dynamical calculations(Springer, 2011) Caruso, D.; Tacconi, M.; Bovino, S.; Gianturco, F. A.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129This paper analyses in detail the energy redistribution from the upper vibrational levels of Cs dimers, thought to be obtained from initial recombination processes that generate excited internal states of the triplet configuration (3) I pound (u) (+) . Their quenching is examined as they are made to further collide with (4)He buffer gas at temperatures below and around 100 mK. The relevant cross sections are computed by using a multichannel quantum dynamical approach and employ a full, ab initio potential energy surface. Due to their smallness (see Ref. [R.B. Ross, J.M. Powers, T. Atashroo, W.C. Ermler, I.A. LaJohn, P. Christiansen, J. Chem. Phys. 93, 6654 (1999)]) the fine structure effects have not been explicitly included in this study. The final, cumulative cross-sections are discussed and analyzed in terms of the overall quenching behavior shown by different initial states of the dimer and in terms of the changing ratios between collisional cooling and vibrational quenching cross sections as a function of trap temperature. The corresponding quenching rates are also computed and analysed.Publication Metadata only Solvation of K+ in helium droplets(Springer, 2007) Yıldırım, Erol; Yurtsever, Mine; Bodo, Enrico; Gianturco, Franco A.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Solvation of K+ in helium droplets is studied by classical simulation methods. We have previously shown that additive potentials can be used to describe structures of helium droplets when an ionic species is present. Here, we present an accurate ab-initio potential for the K+ - He interaction. Global minima of KHen+ for up to n=70 are searched for employing Basin Hopping Monte Carlo simulations with a random growth scheme. The extent of the solvation is analyzed. A clear formation of two shells with 15 and 23 atoms is detected.Publication Metadata only Spin-polarized Rb-2 interacting with bosonic He atoms: potential energy surface and quantum structures of small clusters(Amer Chemical Soc, 2012) Rodriguez-Cantano, R.; Lopez-Duran, David; Gonzalez-Lezana, T.; Delgado-Barrio, G.; Villarreal, P.; Gianturco, F. A.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129A new hill-dimension potential energy surface of the three-body He-Rb-2(E-3(u)+) complex and a quantum study of small (H-4(e))(N)-Rb-2(E-3(u)+) clusters, 1 <= N <= 4, are presented. We have accurately fitted the ab initio points of the interaction to an analytical form and addressed the dopant's vibration, which is found to be negligible. A Variational approach and a Diffusion Monte Carlo technique have been applied to yield energy and geometric properties of the selected species. Our quantum structure calculations show a transition in the arrangements of the helium atoms from N = 2, where they tend to be separated across the diatomic bond, to N = 4, in which a closer packing of the rare gas particles is reached, guided by the dominance of the He-He potential over the weaker interaction of the latter adatoms with the doping dimer. The deepest well of the He-Rb-2 interaction is placed at the T-shape configuration, a feature which causes the dopant to be located as parallel to the helium "minidroplet". Our results are shown to agree with previous findings on this and on similar systems.Publication Metadata only Structuring a quantum solvent around a weakly bound dopant: the He-Cs2((3)sigma u) complex(2009) Prosmiti, Rita; Delgado-Barrio, Gerardo; Villarreal, Pablo; Coccia, Emanuele; Gianturco, Franco A.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The structure and energetics of 3,4HeCs2(3Σu) molecules are analyzed from first principles. Fixing the cesium dimer at its equilibrium distance, the electronic structure was determined through ab initio methods at the CCSD(T) level of theory using a large basis set to compute the interaction energies. At the T-shaped geometry, there is a shallow well with a depth of ∼2 cm−1 placed at R ∼6.75 Å, R being the distance from the center of mass of Cs2 to He. That depth gradually decreases to ∼0.75 cm−1, while R increases to about 11.5 Å at linear arrangements. A simple model of adding atom−atom Lennard-Jones potentials with well-depth and equilibrium distance parameters depending on the angular orientation was found to accurately reproduce the ab initio points. Using this analytical form, variational calculations at zero total angular momentum are performed, predicting a single bound level at ∼−0.106 (∼−0.042) cm−1 for the boson (fermion) species. Further calculations using Quantum Monte Carlo methods are carried out and found to be in good agreement with the variational ones. On the basis of the present results, such analytical expression could in turn be used to describe the structure and binding of larger complexes and therefore opens the possibility to further studies involving such aggregates.Publication Metadata only Ultraviolet upconversion spectra of sonochemically synthesized doped NaYF4 crystals(Elsevier, 2014) N/A; Department of Chemistry; Yıldırım, Cansu; Birer, Özgür; Master Student; Researcher; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; College of SciencesA facile sonochemical method was developed for the production of alpha- and beta-forms of Yb3+(20%)/Tm3+(2%) or Er3+(2%) doubly doped NaYF4 crystals. The reactions took place in aqueous solutions at low temperatures and employed oxide or salt precursors. Micrometer sized beta-NaYF4 and nanometer sized alpha-NaYF4 were obtained with upconversion property. The required activation energy for the solid-solid phase transition was overcome with the energy provided by cavitations. Upconversion emission properties of the Yb3+(20%), Er3+(2%) or Tm3+(2%) doubly doped, alpha- and beta-NaYF4 were investigated. The alpha-form with emission in the ultraviolet region had higher efficiency contradicting the literature. Several new peaks in the spectra, most likely due to crystal defects, were temporarily assigned.Publication Metadata only Vibrational spectroscopy of structural defects in oligothiophenes(Taylor & Francis Ltd, 2003) Department of Chemistry; N/A; Yurtsever, İsmail Ersin; Kırmızıaltın, Serdal; Faculty Member; Undergraduate Student; Department of Chemistry; College of Sciences; College of Sciences; N/A; N/AVibrational spectra of oligothiophenes with structural defects are calculated within the density-functional-theory methodology. The effects of the defective ab linkages on the infrared (IR) and Raman spectra are characterized from calculations of all isomers up to the hexamer. The signatures of ab linked monomers can be found in IR spectra from broken symmetry arguments which result in absorptions localized in the defective region. The positions of the absorption peaks in the Raman spectra seem to be unaffected by the presence of such defects; however, strong reductions in the intensities are observed because of the shortening of the conjugation length.