Iodine laser production of highly charged Ta ions

The results of systematic studies of multiply charged Ta ion production with the fundamental frequency of an iodine laser (=1.315m), and its 2nd (0.657m) and 3rd (0.438m) harmonics are summarized and discussed. Short laser pulse (350 ps) and a focus spot diameter of 100m allowed for the laser power densities in the range of 5×10¹³–1.5×10¹⁵ W/cm². Corpuscular diagnostics were based on time-of-flight methods; two types of ion collectors and a cylindrical electrostatic ion energy analyzer were used. The Ta ions with charge state up to 55+ were registered in the distance of 210 cm; the maximum amplitude of the signal of a high energy ion group was found to belong to the ions with the charge state around 43+, depending on the laser power density. The ion energy distribution was measured for all three wavelengths, however, in a different energy range; the maximum registered ion energy was 8.8 MeV. The occurrence of highly charged ions in the far expansion zone is discussed in view of the mechanism of charge distribution freezing during two-temperature isothermal plasma expansion.The work was performed in a partial fulfillment of the research grant project No. A1010525 sponsored by the Academy of Sciences of the Czech Republic and grant project No. 202/95/0039 sponsored by the Grant Agency of the Czech Republic.     

Aperiodic stochastic resonance in a hysteretic population of cardiac neurons

Aperiodic stochastic resonance (ASR) is studied for a densely interconnected population of excitatory and inhibitory neurons that exhibit hysteresis. Switching between states in the presence of noisy external forcing is represented as a "competition between averages" and this is further explained through a semianalytical model. In contrast to energy-based approaches where only the timing of a switch between states is represented, the competition between averages also identifies the input history responsible for a switch. This last point leads to some interesting conclusions regarding cause and effect in the presence of noisy forcing of a hysteretic system. For example, at subthreshold inputs, it is found that the input history causing a switch between states is primarily dependent upon the noise level even though the corresponding time to switch is sensitive to both the distance from the threshold and the noise level. Since the application considered here is to cardiac neuronal control, control performance is considered over the full input range. Noise tuning for adequate control performance is found to be unnecessary if the noise level is high enough. This is consistent with studies of ASR for sensory neurons. Another observation made here that may be of clinical significance is that at higher noise levels, constraints placed upon inputs to ensure adequate control performance are likely to depend upon the switching direction.     

Further investigation of the effect of framework catenation on hydrogen uptake in metal-organic frameworks

Hydrogen-sorption studies have been carried out for the catenation isomer pairs of PCN-6 and PCN-6' (both have the formula of Cu(3)(TATB)(2), where TATB represents 4,4',4'-s-triazine-2,4,6-triyl-tribenzoate with a formula of C(24)H(12)N(3)O(6)). Inelastic neutron scattering (INS) studies reveal that the initial sites occupied by adsorbed H(2) are the open Cu centers of the paddlewheel units with comparable interaction energies in the two isomers. At high H(2) loadings, where the H(2) molecules adsorb mainly on or around the organic linkers, the interaction is found to be substantially stronger in catenated PCN-6 than in noncatenated PCN-6', leading to much higher H(2) uptake in the isomer with catenation. Hydrogen sorption measurements at pressures up to 50 bar demonstrate that framework catenation can be favorable for the enhancement of hydrogen adsorption. For example, the excess hydrogen uptake of PCN-6 is 72 mg/g (6.7 wt %) at 77 K/50 bar or 9.3 mg/g (0.92 wt %) at 298 K/50 bar, respectively, and that for PCN-6' is 42 mg/g (4.0 wt %) at 77 K/50 bar or 4.0 mg/g (0.40 wt %) at 298 K/50 bar. Importantly, PCN-6 exhibits a total hydrogen uptake of 95 mg/g (8.7 wt %) (corresponding to a total volumetric value of 53.0 g/L, estimated based on crystallographic density) at 77 K/50 bar and 15 mg/g (1.5 wt %) at 298 K/50 bar. Significantly, the expected usable capacity of PCN-6 is as high as 75 mg/g (or 41.9 g/L) at 77 K, if a recharging pressure of 1.5 bar is assumed.     

Syntheses and fluorescence of salicylaldimine schiff base derivatives of 1,1′‐di(aminoethylaminocarbonylalkyl)‐2,2′‐biimidazole

The amide‐amine, 1,1′‐di(aminoethylaminocarbonylethyl)‐2,2′‐biimidazole (DAEPB) ( 1 ), and subsequent Schiff base imine product, 1,1′‐di(salicylaldiminoethylaminocarbonylethyl)‐2,2′‐biimidazole (DSEB) ( 2a ), have been synthesized from the ester, 1,1′‐di(ethoxycarbonylethyl)‐2,2′‐biimidazole (DEPB). Additionally, 1,1′‐di(salicylaldiminoethylaminocarbonylmethyl)‐2,2′‐biimidazole (DSMB) ( 2b ), was prepared from its corresponding amide‐amine. All compounds were characterized with FTIR, NMR and elemental analyses. The salicylaldimines, compounds ( 2a ) and ( 2b ), exhibit fluorescence at 540 and 520 nm, respectively, over a broad range of excitation wavelengths.     

Producing Slow Antihydrogen for a Test of CPT Symmetry with ATHENA

The ATHENA experiment at the Antiproton Decelerator facility at CERN aims at testing CPT symmetry with antihydrogen. An overview of the experiment, together with preliminary results of development towards the production of slow antihydrogen are reported. This revised version was published online in August 2006 with corrections to the Cover Date.     

18F‐Labeling of Aryl‐SCF3, ‐OCF3 and ‐OCHF2 with [18F]Fluoride

We report that halogenophilic silver(I) triflate permits halogen exchange (halex) nucleophilic ¹⁸F‐fluorination of aryl‐OCHFCl, ‐OCF₂Br and ‐SCF₂Br precursors under mild conditions. This Ag^I‐mediated process allows for the first time access to a range of ¹⁸F‐labeled aryl‐OCHF₂, ‐OCF₃ and ‐SCF₃ derivatives, inclusive of [¹⁸F]riluzole. The ¹⁸F‐labeling of these medicinally important motifs expands the radiochemical space available for PET applications.     

Estimating linear functionals of a sparse family of Poisson means

Assume that we observe a sample of size n composed of p-dimensional signals, each signal having independent entries drawn from a scaled Poisson distribution with an unknown intensity. We are interested in estimating the sum of the n unknown intensity vectors, under the assumption that most of them coincide with a given “background” signal. The number s of p-dimensional signals different from the background signal plays the role of sparsity and the goal is to leverage this sparsity assumption in order to improve the quality of estimation as compared to the naive estimator that computes the sum of the observed signals. We first introduce the group hard thresholding estimator and analyze its mean squared error measured by the squared Euclidean norm. We establish a nonasymptotic upper bound showing that the risk is at most of the order of \(\sigma ^2(sp+s^2\sqrt{p}\log ^{3/2}(np))\). We then establish lower bounds on the minimax risk over a properly defined class of collections of s-sparse signals. These lower bounds match with the upper bound, up to logarithmic terms, when the dimension p is fixed or of larger order than \(s^2\). In the case where the dimension p increases but remains of smaller order than \(s^2\), our results show a gap between the lower and the upper bounds, which can be up to order \(\sqrt{p}\).     

Crystal structures of 1,1′-di(2-propanone)-2,2′-biimidazole dihydrazone and 2,6-diacetylpyridine dihydrazone

The nucleophilic addition reactions between 1,1′-di(2-propanone)-2,2′-biimidazole or 2,6-diacetylpyridine and hydrazine hydrate afford 1,1′-di(2-propanone)-2,2′-biimidazole dihydrazone (1) and 2,6-diacetylpyridine dihydrazone (2), respectively. Compound1 crystallizes in the orthorhombic space groupP2₁2₁2₁, witha=9.042(2),b=9.731(3),c=15.683(4)Å, V = 1379.9(6)Å^? andZ=4. Compound2 crystallizes in the orthorhombic space groupPnma, witha=10.948(2),b=19.742(6),c=4.566(1)Å, V=986.9(4)Å^? andZ=8. A pseudo center of inversion is present at the midpoint of the C?C bond joining the imidazole rings of1, whose substituents crystallize in atrans configuration. The imidazole rings are rotated 2.5(3)° about the C?C bond. In contrast to the essentially planar structure of2, the hydrazone substituent groups of1 are at angles of 89.9(1)° and 88.4(1)° with respect to the plane of the adjacent imidazole moiety.