Search results for " Optics"
showing 10 items of 5880 documents
As(III)/Sb(III)/Bi(III)–halide distances and stretching vibrations. An application of the Varshni relationship upon hypervalent group 15 compounds
1998
Abstract A Varshni treatment between stretching frequencies and bond lengths is given for 16 heterocycles of the types Hal–M(SCH 2 CH 2 ) 2 X and Hal–M(CH 2 CH 2 CH 2 ) 2 X (M=As/Sb/Bi, Hal=Cl/Br/I, X=O, S, NR). The intensities of the Raman emissions for the M–Hal bonds increase in the following order: Cl≪Br −1/2 , decrease in the following order: Cl>Br≫I. Practically, the Varshni constants are used to estimate the complementary quantity, stretching frequency or bond length, in 35 cases.
DFT study of polarizabilities and dipole moments of water clusters
2005
Density functional theory (DFT) calculations with different exchange- correlation functionals, Becke's three-parameter exchange functional and the gradient- corrected functional of Lee, Yang, and Paar (B3LYP) and Becke's three-parameter functional with Perdew-Wang correlational functional (B3PW91), are performed to study the dielectric properties of small and medium-sized water clusters. For these H- bonded systems, we optimize the geometries and compute the dipole moments and polarizabilities using a supermolecule approach. The corresponding properties of the individual water molecules in the clusters are extracted from the molecular properties using the Hirshfeld expansion of the electron…
Hypervalent tin-organic compounds: Vibrational spectroscopy in the solid as a tool for structure determination
1997
Abstract A full set of IR- and Raman spectra down from 600 cm1 has been determined and assigned for 60 penta- or hexacoordinated tin-compounds in the groups RSn(CH2CH2CH2)3N (R = Cl, Br, I, Me), RR'Sn(MCH2CH2)2X (R,R′ = Cl, Br, I, Me, Ph, M = CH2 S; X = NMe, O, S), RPh2SnCH2CH2CH2NMe2 (R = Cl, Br, I) and RMe2Sn—CHD—COOMe (R = F, Cl, Br, I, Me). In these hypervalent compounds an approach of the donor atom X to the central atom tin outlines a ‘path’ of nucleophilic attack from a tetrahedron to a trigonal bipyramid. Along this ‘reaction path’ the bond lengths of the axial ligand increase continuously while the bond lengths of the equatorial ligands slightly decrease. The number of similar hype…
Improved embedded molecular cluster model
2002
We demonstrate that boundary effects (i.e., displacements of the cluster boundary atoms from their lattice sites and the difference between effective charges of the perfect crystal atoms and those of the cluster atoms in the case of a cluster with no point defect in it) in an embedded molecular cluster (EMC) model can be radically reduced. A new embedding scheme is proposed. It includes search for the structural elements (SE) of which perfect crystal is composed, use of corresponding to these SE expression for the total energy, and application of the degree of localization of equations consistent with the wave functions of the cluster. To get equations for the cluster wave functions, the pr…
Atomic structure solution of the complex quasicrystal approximant Al77Rh15Ru8 from electron diffraction data
2014
The crystal structure of the novel Al77Rh15Ru8phase (which is an approximant of decagonal quasicrystals) was determined using modern direct methods (MDM) applied to automated electron diffraction tomography (ADT) data. The Al77Rh15Ru8E-phase is orthorhombic [Pbma,a= 23.40 (5),b= 16.20 (4) andc= 20.00 (5) Å] and has one of the most complicated intermetallic structures solved solely by electron diffraction methods. Its structural model consists of 78 unique atomic positions in the unit cell (19 Rh/Ru and 59 Al). Precession electron diffraction (PED) patterns and high-resolution electron microscopy (HRTEM) images were used for the validation of the proposed atomic model. The structure of the E…
Localized molecular orbitals for excited states of polyenals, polyendials, and polyenones
2003
The work is focused on the generation of localized molecular orbitals for excited states. A recently developed a priori method based in a CAS-SCF–type algorithm is applied. The method generates directly localized orbitals and can be applied to multireference wavefunctions. A detailed description of the performance of the method as well as the locality of the MOs for the example of the singlet nπ* (CO) excited state is given. It is in general possible to obtain local orbitals for the doubly occupied and virtual valence orbitals. The partial delocalization of the π* (CO) orbital is discussed, as is the effect of the use of different CAS spaces. The systems under study are polyenals, polyendia…
Large Quadratic Hyperpolarizabilities with Donor–Acceptor Polyenes Exhibiting Optimum Bond Length Alternation: Correlation Between Structure and Hype…
1997
Donor-acceptor polyenes of various lengths, and that combine aro- matic electron-donating moieties with powerful heterocyclic electron-withdraw- ing terminal groups, have been synthe- sized and characterized as efficient non- linear optical (NLO) chromophores. Their linear and nonlinear optical proper- ties have been investigated, and variations in these properties have been related to ground-state polarization (dipole p) and structure. In particular, unprecedented quadratic hyperpolarizabilities (p) have been achieved (up to p(0) =I500 x 10-30esu) by reduction of the bond- length alternation (BLA) in the polyenic chain. In each series of homologous com-
Up-conversion processes in NaLaF4:Er3+
2009
abstract Structural and spectroscopic investigation of NaLaF 4 :Er 3+ material at different doping concentrations ispresented. X-ray diffraction patterns, up-conversion luminescence spectra and decay curves for 2 H 9/2 ? 4 I 15/2 , 4 S 3/2 ? 4 I 15/2 and 4 F 9/2 ? 4 I 15/2 optical transitions in the material are shown and possibleexcitation routes are discussed. Raman spectrum for the undoped material is presented and the effectivephonon energy of the material is estimated. Based on the obtained results application of rare-earth dopedNaLaF 4 in the field of up-conversion phosphors is evaluated. 2009 Elsevier B.V. All rights reserved. 1. IntroductionFor many years rare-earth (RE) doped materi…
The use of X-ray-induced and thermostimulated visible and UV luminescence for understanding X-ray poling of silica glasses
2007
In this paper, we studied the X-ray-induced visible and UV luminescence (XRL) during X-ray irradiation, afterglow (after X-ray was switched off) and thermostimulated luminescence (TSL). Comparisons were performed between with and without applying an electric field of 1.5 kV/mm on the sample. The typical TSL curve for X-ray-irradiated samples present two peaks centered at 90 and 280 °C. The relative intensity of these two peaks is X-ray dose dependent. An external electric field enhances the XRL, but has no effect on the afterglow. The TSL and afterglow spectra are identical with one peak at 3.25 eV, whereas XRL spectrum displays an additional peak at 4.25 eV. A model of electron exchange be…
Donor-acceptor substituted polyenes : orientation in mono- and multilayers
1992
Large molecules containing different chemical units whose interactions within the molecule result in new macroscopically observable effects, have become increasingly important.The organization of molecules of this type in ordered structures leads to functional molecular materials.Their use in molecular electronics requires that the units exhibit specific electronic properties. Recently, we reported on the intramolecular energy transfer through terminally substituted conjugated polyenes. An intramolecular electron transfer within donor-acceptor substituted polyenes can be achieved by introducing suitable terminal groups.