Search results for "Alkali metal"
showing 10 items of 190 documents
Ferroelastic phase transitions of alkali halide-alkali cyanide mixed crystals
1990
Abstract The alkali cyanides and their CN-rich mixtures with alkali halides are proper ferroelastics with transitions from the cubic high temperature phase into the various non-cubic low temperature phases. As the CN concentration is reduced, the transformations temperature decreases and eventually the ferroelastic low temperature state is suppresse in favour of a glass-like state. The structural and elastic properties of these compounds are discussed with a special emphasis on the martensitic aspect of the transitions.
Polymorphism and the glasslike state of cyanide mixed crystals: A mesoscopic problem.
1990
The martensitic aspect of the structural phase transitions of the alkali cyanides and their solid solutions with the alkali halides is discussed. It is suggested that the polymorphism is intimately connected with the problem of mesoscopic transformation strains. At intermediate CN concentrations a competition between glasslike inhomogeneous strains, modulated strains, and ferroelastic strains is observed.
UV cathodoluminescence of crystalline α-quartz at low temperatures
2004
Two luminescence bands in the UV range were detected in crystalline a-quartzunder electron beam excitation (6 kV, 3–5mA). One band is situated at 5 eV and could be observed in pure samples. Its intensity increases with cooling below 100 K and undergoes saturation below 40 K alongside a slow growth with the time of irradiation at 9 K. The decay curve of the band at 5 eV contains two components, a fast (o10 ns) and a slow one in the range of 200ms. The photoluminescence band at 5 eV with a similar temperature dependence was found in previously neutron-irradiated crystalline a-quartz. Therefore, the band at 5 eV was attributed to host material defects in both irradiation cases. The creation me…
Photostimulable energy transfer in doped nanostructures of alkali halides
1998
Abstract In nanostructures of doped alkali halides, unrelaxed H-centres produced in the exciton decay are able to escape from the surface thus producing in the grains of polycrystalline film an excess concentration of F-centres, which has a great tendency to form an interacting F-centre chain or a canal. Role of this F-centre chain in photostimulable energy transfer is analysed. It is concluded that specific interaction of unrelaxed H-centres and electrons with the F-centre chain (or the canal) leads to the temperature-independent exciton or electron transport at any distance. Hence, the temperatureindependent photostimulated luminescence (PSL) response production and retrieval are experime…
Circular dichroism in atomic vapors: magnetically induced transitions responsible for two distinct behaviors
2021
Atomic transitions of alkali metals for which the condition $F_e-F_g = \pm2$ is satisfied have null probability in a zero magnetic field, while a giant increase can occur when an external field is applied. Such transitions, often referred to as magnetically-induced (MI) transitions, have received interest because their high probabilities in wide ranges of external magnetic fields which, in some cases, are even higher than that of usual atomic transitions. Previously, the following rule was established: the intensities of MI transitions with $\Delta F=\pm2$ are maximum when using respectively $\sigma^\pm$ radiation. Within the same ground state, the difference in intensity for $\sigma^+$ and…
Density functional study of alkali-metal atoms and monolayers on graphite (0001)
2007
Alkali metal atoms (Li, Na, K, Rb, Cs), dimers and (2$\times$2) monolayers on a graphite (0001) surface have been studied using density functional theory, pseudopotentials, and a periodic substrate. The adatoms bind at the hollow site (graphite hexagon), with Li lying closest to (1.84 ��) and Cs farthest (3.75 ��) from the surface. The adsorption energies range between $0.55-1.21$ eV, and the energy ordering of the alkali adatoms is Li$>$Cs$\ge$Rb$\ge$K$>$Na. The small diffusion barriers (0.02-0.21 eV for the C-C bridge) decrease as the atom size increases, indicating a flat potential energy surface. The formation (cohesion) energies of (2$\times$2) monolayers range between 0.55-0.81 …
COLLECTIVE SPIN EXCITATIONS OF ALKALI-METAL CLUSTERS
1993
The response function of alkali-metal clusters, modeled as jellium spheres, to dipole (L=1) and quadrupole (L=2) spin-dependent fields is obtained within the time-dependent local-spin-density approximation of density-functional theory. We predict the existence of low-energy spin modes of surface type, which are identified from the strength function. Their collectivity and evolution with size are discussed.
Calculations of the ground and excited states ofF-type centers in corundum crystals
1994
The semiempirical intermediate neglect of differential overlap method was used for calculating optical properties of ${\mathit{F}}^{+}$ and F centers (oxygen vacancy trapped one and two electrons, respectively) embedded into large quantum clusters, ${\mathrm{Al}}_{26}$${\mathrm{O}}_{39}$. The geometry was optimized for both the ground and excited states of defects. Calculated absorption and luminescence energies obtained for ${\mathit{F}}^{+}$ and F centers are in good agreement with experimental data. Their energy levels lie in the gap between the upper valence band and conduction band, like for similar centers in MgO and alkali halides. It is shown that the oxygen vacancy in corundum crys…
First observation of the blue optical lines of francium
1987
We report here the first wave-length measurements in the second resonance doublet of francium, D1'(7s2S1/2-8p2P 1/2) and D2'(7s2S1/2-8p2P 3/2), carried out by collinear fast-beam laser spectroscopy. The transition wave numbers are D1' = 23112.9603(50) cm-1 and D2' = 23658.3058(40) cm-1, corresponding to a 8p fine-structure splitting of δW8p = 545.3454(70) cm-1. In addition the hyperfine structure in both lines and the isotope shift in the D2' line for the isotopes 212,213,220,221Fr have been measured. The results are discussed with special emphasis on the analysis of the atomic structure in the heaviest alkali element and compared with theoretical predictions, as well as the only earlier sp…
Nuclear orientation of radon isotopes by spin exchange optical pumping
1988
This paper reports the first demonstration of nuclear orientation of radon atoms. The method employed was spin exchange with potassium atoms polarized by optical pumping. The radon isotopes were produced at the ISOLDE isotope separator of CERN. The nuclear alignment of /sup 209/Rn and /sup 223/Rn has been measured by observation of ..gamma..-ray anisotropies and the magnetic dipole moment for /sup 209/Rn has been measured by the nuclear-magnetic-resonance method to be chemically bond..mu..chemically bond = 0.838 81(39)..mu../sub N/.