Significant nonlinear‐optical switching capacity in atomic clusters built from silicon and lithium: A combined ab initio and density functional study

Starting from a hypothetical but fundamental charge/discharge sequence, the topic of nonlinear optical switching in atomic clusters built from silicon and alkali metals is opened up. The outcomes presented in this work, obtained with ab initio methods of exceptional predictive capabilities, offer strong evidences that sizable hyperpolarizability contrasts between neutral and charged alkali metal doped cluster forms might be simultaneously accomplished. The observed switching procedure involves redox polyatomic clusters formed by Si atoms. These centers function as electron acceptors at the ground state and as electron donors at the excited states facilitating low energy charge transfer transitions upon electronic excitation. © 2014 Wiley Periodicals, Inc.     

Radiocesium sorption-desorption processes in lake sediments

The kinetics of radiocesium sorption and desorption by cation solutions and the corresponding radiocesium distribution profiles in sediment cores were investigated. The results have shown that a significant percentage of radiocesium is adsorbed in the sediments during the first 3 d. Radiocesium reaches rapidly (< than 5 d) at a depth which does not exceed 3.5 cm. The desorption of radiocesium was found to depend on cation concentrations. Empirical laws are derived both for cesium adsorption and desorption.     

Correlations between bonding, size, and second hyperpolarizability (gamma) of small semiconductor clusters: ab initio study on Al(n)P(n) clusters with n=2, 3, 4, 6, and 9

A comprehensive investigation of the correlation between the second hyperpolarizability and the bonding and structural characteristics of stoichiometric aluminum phosphide clusters up to 18 atoms is presented. Several aluminum phosphide species displaying different types of configurations and bonding have been studied. The obtained ab initio and density functional finite field results suggest that the ionic AlP clusters are considerably less hyperpolarizable than the covalent bonded species. Other structural features such as symmetry, atoms' arrangement, and shape also play an important role on the hyperpolarizabilities of those species. However, they are only noticeable among clusters characterized by the same bonding patterns. Furthermore, the results of this study demonstrate that the bonding which is determined by the atoms' arrangement of a cluster has a more profound effect on the second hyperpolarizability than the cluster's composition or size. In addition, the mean second hyperpolarizability increases with the increasing number of atoms, assuming that the bonding characteristics among the clusters of increasing size are similar. On the other hand, the hyperpolarizability per atom rapidly decreases with the increase of atoms' number in the cluster and converges to values of approximately 900e(4)a(0)(4) and approximately 1300e(4)a(0)(4)E(h) (-3) at the HF/cc-pVDZ and MP2/cc-pVDZ levels of theory respectively. Lastly, this work provides the first systematic study on the hyperpolarizabilities of small aluminum phosphide clusters which, in their covalent forms, exhibit larger second order hyperpolarizabilities than the well studied small gallium arsenide clusters.     

Measurements of222Rn migration in soil

The concentration of radon (²²²Rn) was measured in the soil near the ground surface, using CR/39 solid state nuclear track detectors. The measurements were carried out in PVC tubes at 0.25 m intervals up to 1.25 m. The detectors were etched in 7N NaOH solutions at 80°C. The -tracks from radon's decay were counted using a microscope. A microscope-camera-computer system developed for automatic counting was also used. The results provide evidence for the non-diffusive transport of radon in soils. A transport length of (46.9±3.2) cm was estimated for radon transport near ground surface. Also the variation of soil's radon concentration was correlate to humidity and atmospheric pressure.     

Molecular geometry and polarizability of small cadmium selenide clusters from all-electron ab initio and Density Functional Theory calculations

We have calculated molecular geometries and electric polarizabilities for small cadmium selenide clusters. Our calculations were performed with conventional ab initio and density functional theory methods and Gaussian-type basis sets especially designed for (CdSe)(n). We find that the dipole polarizability per atom converges rapidly to the bulk value.     

Selective exchange of strontium in an aluminum pillared montmorillonite

The selective uptake of strontium by a specially tailored aluminum pillared montmorillonite was investigated. Selective exchange kinetics revealed a fast component of the order of less than one hour, while exchange isotherms revealed the preference for strontium uptake over sodium and calcium with separation factors α _Na ^Sr =50 and α _Ca ^Sr =2. In an environment of rumen liquid taken from slaughtered sheep, 26% of strontium was removed by 1 g and 40% by 3 g of material.     

Deconvolution of liquid scintillation alpha spectra of mixtures of uranium and radium isotopes

A method for the determination of uranium and radium isotopes in water samples is proposed. Liquid scintillation techniques were used for collecting alpha spectra, which were then analyzed by fitting the alpha peaks with overlapping Gaussians. The analysis can quantify the observed isotopes with accuracy depending on the activity of each isotope.In order to simulate the peaks with Gaussian normal distribution functions, the centroid of each peak as well as the full width at half maximum (FWHM) are required, as they depend on the quenching of the sample. For this purpose, samples with known activities of ²²⁶Ra and its decay products and also of the uranium isotopes ²³⁸U and ²³⁴U, at various quenching levels, were used to establish the correlation of the peaks’ shift with the quench effect. In addition, the correlation of the FWHM with the centroid of a peak was determined, using the same procedure.Following the above analysis technique, an average of 97 ± 2% of detection efficiency and a lower limit of detection of 8.2 mBq kg⁻¹ for alpha isotopes were achieved.     

Hyperpolarizability of GaAs dimer is not negative

We present a systematic study of the static electric hyperpolarizability of Ga(2)As(2). The authors rely on finite-field high-level ab initio calculations with carefully optimized basis sets. Their best values for the mean and the anisotropy of the dipole polarizability are alpha=158.57 and Deltaalpha=130.33e(2)a(0) (2)E(h) (-1). For the hyperpolarizability we propose an estimate gamma=(155+/-15)x10(3)e(4)a(0) (4)E(h) (-3), which does not agree with the negative value predicted by Lan et al. [J. Chem. Phys. 124, 094302 (2006)]. Density functional theory based methods yield values close to those predicted by conventional ab initio methods. The (hyper)polarizability components are particularly enhanced along the direction defined by the Ga-Ga axis.     

Ab initio finite field (hyper)polarizability computations on stoichiometric gallium arsenide clusters GanAsn (n=2-9)

We report reliable ab initio finite field (hyper)polarizability values at Hartree-Fock and second order Moller-Plesset perturbation theory (MP2) levels of theory for different geometrical configurations of small gallium arsenide clusters Ga(n)As(n) with n=2-5. We relied on all-electron basis sets and pseudopotentials suitable for (hyper)polarizability calculations. In each case, we used structures that have been established in the literature after we optimized their geometries at B3LYP/cc-pVTZ-PP level of theory. Our results suggest that the first order hyperpolarizability (beta) is much more sensitive to the special geometric features than the second order hyperpolarizability (gamma). For the most stable configurations up to ten atoms the second order hyperpolarizability at MP2 level of theory varies between 15 x 10(4) and 32 x 10(4) e(4)a0 (4)Eh(-3). In addition, we examined the polarizability per atom evolution versus the cluster size for Ga(n)As(n) with n=2-9. Our work extends earlier theoretical studies which were limited to eight atoms and exposes that the polarizability/atom of the most stable stoichiometric configurations up to Ga(9)As(9) continues the monotonic downward trend with increasing size. Lastly, from the methodological point of view, our analysis shows that apart from polarizabilities, augmented pseudopotentials yield reliable first and second hyperpolarizability values as well.