Structural varieties of polytypes

We present the results of theoretical analysis of structural varieties of compounds for which polytypism is important. As a result, the possibility of isomeric polytype varieties existing has been established, as well as regularities in their formation, depending on the period of repeatability of layers in packaging. In addition, the methods of density functional theory are used to calculate the structure and properties of various polytypic modifications of diamond. Dependences of the interlayer distances and the difference total energy on the hexagonality of diamond polytypes and their isomeric varieties have been found. The theoretical powder X-ray diffraction patterns of diamond polytypes are calculated and their comparative analysis is performed.     

Deformation change between184mAu and184gAu

Isotope shift and hyperfine structure measurements have been performed on the short lived¹⁸⁴Au nucleus using the COMPLIS apparatus installed at ISOLDE. The charge radius change and the magnetic and quadrupole moments of both isomeric and ground states have been deduced. The¹⁸⁴Au ground state appears to be slightly more deformed than the isomeric state.     

New isomeric states in 152,154,156Nd produced by spontaneous fission of 252Cf

Isomeric states have been observed in fission-fragments produced by spontaneous fission of ²⁵²Cf. These states are found in neutron rich nuclei of different structure and deformations. About 50 isomeric nuclei have been observed using coincidences between γ-rays identified in EUROGAM II and fission fragments detected in photovoltaic cells (SAPhIR). Lifetimes in the range from 20 ns to 2μs have been measured. Presented calculations based on HFB +D1S force on new measured isomeric states in the ^152,154,156Nd show evidence for K-isomers. Received: 3 November 1997 / Revised version: 15 December 1997     

Location of the181Os isomeric state

The decay of¹⁸¹Ir has been studied on-line with mass-separated sources from the ISOCELE facility. The level scheme of¹⁸¹ Os has been established. The isomeric state has been located at 48.9 keV above the ground state. Thus the isomeric and ground states are identified as the 7/2?(T1/2 =2.7 m) and 1/2?(T1/2=105 m) levels respectively.     

Identification and structure of yrast levels in theN=89 odd-odd158Tm nucleus

Excited levels have been identified in the odd-odd¹⁵⁸Tm nucleus produced in the¹⁵⁰Sm (¹⁴N,6n) reaction. The observed level structure based on an isomeric state (T_1/216±4 ns) is interpreted as a rotational aligned band.     

Dielectric and electro-optic behavior of pure ferroelectric liquid crystal material and the isomeric mixtures

We report here the dielectric relaxation spectroscopy and electro-optical properties of a ferroelectric liquid crystal (FLC) CB470 (R) and isomeric mixtures containing CB470 (R) and its optical isomer CB480 (S) in two different concentrations. We have studied with two isomeric mixtures: mixing CB470 (R) and CB480 (S) in the ratio 95:5 (w/w) and 90:10 (w/w). With increase of percentage of CB480 (S) in the mixture the spontaneous polarization (P_S) value has been decreased due to cancellation of a part of the dipolar contribution, since the direction of P_S of the two isomers are in opposite direction. The tilt angles of the pure FLC materials and the isomeric mixtures remain almost unchanged. The dielectric strength of Goldstone mode is almost proportional to the spontaneous polarization (P_S). An additional collective mode has been observed in the SmC¹ phase due to the formation of domain with large periodicity in pure FLC material and in one of the isomeric mixtures by unwinding the helical structures, which is known as domain mode. The existence of domain mode has been studied extensively with the variation of P_S in the isomeric mixtures. The result has been discussed.     

Calculation of the ratio of isomeric cross sections

A new, more rigorous method for calculating the ratio of isomeric cross sections has been developed. The method is particularly valuable when the energy and spin distribution are known before the gamma rays populating the isomeric and ground states start to cascade. The usefulness of the method was demonstrated by applying it to the analysis of the ratio of isomeric cross sections in the ¹⁹⁶Hg(n,γ)¹⁹⁷Hg reaction, which was initiated by thermal neutrons.     

Electronic spectroscopy of highly polar aromatics. X. Luminescence of the fluorobenzonitriles

The absorption and emission spectra of the three isomeric fluorobenzonitriles have been investigated. Quantum yields and decay lifetimes are reported for both fluorescence and phosphorescence processes. Kinetic parameters for energy-depletive processes have been generated and it is shown that this weakly polar subset of D-?-A molecules exhibits most of the characteristics which are peculiar to highly polar isomeric D-?-A triads.     

γ-Spectroscopy of 89 215 Ac126 and proton pairing inN=126 isotones

Theγ-decay of 5 excited states along the yrast line has been observed in²¹⁵Ac by inbeamγ-spectroscopy. Lifetimes andg-factors of 3 isomeric levels have been measured. The data show that pairing dominates the structure ofN=126 isotones.     

All Valence Electron Molecular Orbital Study of Electronic Strture and Spectra of the Three Isometric Xylenes

Abstract

The modified method of Del Bene and Jaffe' (CNDO-CI) has been applied to study the electronic structure and spectra of the three isomeric xylenes taking a three dimensional geometry for the CH₃ group. The spectral transition energies and the corresponding oscillator strengths have been computed and the results are compared with ?-electron calculations and experimental results. Quite satisfactory results have been obtained. Ionization potentials have also been computed by the INDO and extended Hückel methods.     

Erzeugung 21 neuer kurzlebiger Isomere durch Kernphotoreaktionen

Irradiation of 28 selected elements with 22.5 MeV bremsstrahlung led to the production of numerous short-lived isomeric states by photonuclear reactions. At least 21 of the observed isomeric transitions seem not to have been reported in the literature thus far. The energies and half-lives of the observed transitions were measured using a conventional scintillation spectrometer. The half-lives of the newly discovered isomeric states are in the region between 5·10^?6 and 6·10^?2 seconds. Isotopic assignments and multipolarities of the observed transitions have not yet been established.     

Proton emission, gamma deformation, and the spin of the isomeric state of Ho

The nonadiabatic quasiparticle model for triaxial shapes is used to perform calculations for decay of ¹⁴¹Ho, the only known odd-Z even-N deformed nucleus for which fine structure in proton emission from both ground and isomeric states was observed. All experimental data corresponding to this unique case namely, the rotational spectra of parent and daughter nuclei, decay widths and branching ratios for ground and isomeric states, could be well explained with a strong triaxial deformation γ20°. The recent experimental observation of fine structure decay from the isomeric state, can be explained only with an assignment of I=3/2⁺ as the decaying state, in contradiction with the previous assignment, of I=1/2⁺, based on adiabatic calculations. This study reveals that proton emission measurements could be a precise tool to probe triaxial deformations and other structural properties of exotic nuclei beyond the proton drip-line.     

High-resolution studies of the gamma rays from isomeric states with half-lives of 10 μs - 30ms in nuclei with Z = 63−83

Eight new isomeric states with half-lives in the range 10 μs-30 ms have been identified following bombardment with 17.5 MeV protons of the natural elements and separated isotopes of the elements Eu to Pb. Improved data on 14, previously reported, isomeric states have also been obtained. The energies, half-lives and multipolarities of the gamma rays from these states have been measured using an 8 cm³ Ge(Li) detector. In most cases, level schemes have been constructed, and the spin and parity of the isomeric level and intermediate levels populated in the decay have been determined. A group of hindered E2 transitions have been observed, all of which are retarded by approximately the same degree e.g. 3 × 10⁻³ Weisskopf units. The results are discussed in terms of the Nilsson model and in terms of the selection rules for transitions between Nilsson states.     

Nuclear-induced autoionization of radioactive atoms

In the experiment, where the atomic electron cloud serves as an indicator of nuclear isomeric transitions, measurements of the perturbation of the ionic charge-state distribution from the known isomeric transition of the ¹⁷³Hf nucleus obtained in a heavy-ion-induced reaction, were carried out. As a result, a definite experimental confirmation has been obtained for the effect of nuclear forbidden transitions on their atomic electron cloud. The importance and possibilities of this effect for the study of the interplay of nuclear and atomic excitations are discussed.     

Isomeric decay studies around 204Pt and 148Tb

Relativistic energy projectile fragmentation of ²⁰⁸Pb has been used to produce a range of exotic nuclei. The nuclei of interest were studied by detecting delayed gamma rays following the decay of isomeric states. Experimental information on the excited states of the neutron-rich N=126 nucleus, ²⁰⁴Pt, following internal decay of two isomeric states, was obtained for the first time. In addition, decays from the previously reported isomeric I=27ħ and I=(49/2)ħ states in ¹⁴⁸Tb and ¹⁴⁷Gd, respectively, have been observed. These isomeric decays represent the highest spin discrete states observed to date following a projectile fragmentation reaction, and opens further the possibility of doing `high-spin physics' using this technique.     

Chemically Induced Dynamic Magnetic Polarization. Part X. The Photoreduction of 2-Methyl-p-benzoquinone

While the photoreduction of quinones has been initially used by us as a model system to probe the role of excited triplet states in CIDEP¹ and CIDNP² phenomena, our continuing interest in the quinone photochemical systems is partly due to their photobiological importance. Alkyl substituted quinones have in fact been shown to be associated with chlorophyll³ and to be involved in photosynthesis⁴. The 2-methyl-p-benzoquinone (MQ) is the simplest unsymmetrical alkyl substituted p-quinone which, during photolysis, may lead to two isomeric semiquinone radicals. We are particularly interested in applying the CIDEP and CIDNP techniques to investigate the structure and reactivity of the intermediate semiquinone radicals as they may give some insight to the photochemical properties of the parent 2-methyl-p-quinone.     

Micellization of a cationic gemini surfactant in aqueous solutions with different alkanols and alkanediols as additives: Effect of nonpolar chain and position of hydroxyl groups

The gemini surfactant N-N′bis(dimethyldodecyl)-1,4-butanediammonium dibromide (12-4-12) was synthesized and its critical micelle concentrations (CMCs) and degree of counterion dissociation (α) in aqueous solutions in the presence of alkanols viz ethanol, isomeric butyl alcohols, 1-hexanol and alkanediols (ethanediol, 1,4-butanediol, 1,2-hexanediol, 2,5-hexanediol 1,6-hexanediol, and 1,8-octanediol) determined from electrical conductivity are reported. While ethanol, ethanediol, 1,4-butanediol showed an increase in CMC, a decrease was seen for isomeric butyl alcohols, 1-hexanol, isomeric hexanediols and 1,8-octanediol. For butyl alcohols the CMC decrease showed the trend 1° >2° > 3°; in C₆ diols, 1,2-hexanediol exerted more decrease as compared to 2,5-hexanediol and 1,6-hexanediol.The results are explained on the basis of the structure and hydrophobicity of alcohols that determine their effect as cosolvent or cosurfactant (partitioning in micelles). Two-dimensional nuclear overhauser enhancement spectroscopy (2D-NOESY) was used to examine the location for 1-butanol and 1,4-butanediol in micellar systems as representative additives from alkanols and alkanediols showing CMC decreasing and increasing effect respectively.     

Cross sections and isomeric ratios in the photoproduction of 44Sc from heavier nuclei

The yields and isomeric yield ratios in the photoproduction of⁴⁴Sc from ⁴⁵Sc, ⁵⁵Mn, Fe, ⁵⁹Co and ⁷⁵As have been measured by activation methods in the energy region 100–800 MeV. Mean cross sections and isomeric cross-section ratios in the energy region 250–800 MeV have been deduced and the number of valence neutrons available for the (γ, n) reaction estimated. The experimental cross sections are compared both to cascade-evaporation calculations based upon the free-nucleon photopion cross sections and to cross sections calculated with a semi-empirical formula. The isomeric cross-section ratios are compared to calculations based on the statistical formalism by Huizenga and Vandenbosch together with cascade-evaporation theory.     

Resonances in the isomeric and prompt fission probabilities of240Pu

The prompt and isomeric fission probabilities of²⁴⁰Pu have been studied using the²³⁹Pu(d,p) reaction. A number of resonances are observed in the subbarrier population of the 4 ns fission isomer for excitation energies between 4.0 and 6.2 MeV. Apart from a structure at 4.3 MeV, they do not coincide with transmission resonances found in prompt fission. Calculations with an extended doorway state model which simultaneously reproduce the measured delayed and prompt fission probabilities yield revised fission barrier parameters as well as spectroscopic information on fission and gamma widths of highly excited states in the second minimum.     

Simulation of the Processes of Isomerization of Complex Molecules and of Their Spectra with Time Resolution

Based on the method developed earlier to calculate the dynamic electronic-vibrational spectra of complex molecules with allowance for isomeric transformations, a more general method has been developed making it possible to model the spectra of several isomeric forms in any number depending on the problem posed. Population dynamics analysis of the energy levels of isomers is possible. Computational algorithms have been developed and optimized, and a specialized software has been developed for a PC and a supercomputer (of the type MVS 1000). Computer experiments were carried out, and the possibility of simulating dynamic spectra and kinetic intramolecular processes in real time for complex compounds with allowance for their intermolecular transformations is shown.