One-dimensional zigzag chains of Cs-: the structures and properties of Li+ (cryptand[2.1.1])Cs- and Cs+ (cryptand[2.2.2])Cs-

The crystal structure and properties of lithium (cryptand[2.1.1]) ceside, Li+ (C211)Cs-, are reported. Li+ (C211)Cs- is the second ceside and third alkalide with a one-dimensional (1D) zigzag chain of alkali metal anions. The distance between adjacent Cs- anions, 6 A, is shorter than the sum of the van der Waals radii, 7 A. Optical, magic angle spinning NMR, two-probe alternating and direct current conductivity, and electron paramagnetic resonance measurements reveal unique physical properties that result from the overlap of adjacent Cs- wave functions in the chain structure. The properties of cesium (cryptand[2.2.2]) ceside, Cs+ (C222)Cs-, were also studied to compare the effects of the subtle geometric changes between the two 1D zigzag chain structures. Li+ (C211)Cs- and Cs+ (C222)Cs- are both low-band-gap semiconductors with anisotropic reflectivities and large paramagnetic 133Cs NMR chemical shifts relative to Cs- (g). An electronic structure model consistent with the experimental data has sp2-hybridized Cs- within the chain and sp-hybridized chain ends. Ab initio multiconfiguration self-consistent field calculations on the ceside trimer, Cs3(3-), support this model and indicate a net bonding interaction between nearest neighbors. The buildup of electron density between adjacent Cs- anions is visualized through an electron density difference map constructed by subtracting the density of three cesium atoms from the short Cs3(3-) fragment.     

Neutrino mixing discriminates geo-reactor models

Geo-reactor models suggest the existence of natural nuclear reactors at different deep-earth locations with loosely defined output power. Reactor fission products undergo beta decay with the emission of electron antineutrinos, which routinely escape the earth. Neutrino mixing distorts the energy spectrum of the electron antineutrinos. Characteristics of the distorted spectrum observed at the earth's surface could specify the location of a geo-reactor, discriminating the models and facilitating more precise power measurement.     

Observer weighting strategies in interaural time-difference discrimination and monaural level discrimination for a multi-tone complex

Two experiments measured listeners' abilities to weight information from different components in a complex of 553, 753, and 953 Hz. The goal was to determine whether or not the ability to adjust perceptual weights generalized across tasks. Weights were measured by binary logistic regression between stimulus values that were sampled from Gaussian distributions and listeners' responses. The first task was interaural time discrimination in which listeners judged the laterality of the target component. The second task was monaural level discrimination in which listeners indicated whether the level of the target component decreased or increased across two intervals. For both experiments, each of the three components served as the target. Ten listeners participated in both experiments. The results showed that those individuals who adjusted perceptual weights in the interaural time experiment could also do so in the monaural level discrimination task. The fact that the same individuals appeared to be analytic in both tasks is an indication that the weights measure the ability to attend to a particular region of the spectrum while ignoring other spectral regions.     

Direct sample fraction deposition using electrospray in narrow-bore size-exclusion chromatography/matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for polymer characterization

A direct sample fraction deposition method was developed for off-line size-exclusion chromatography (SEC)/matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. By using electrospray, the SEC eluent, together with a suitable matrix solution added coaxially, was directly deposited on the MALDI plate. Owing to the formation of very small droplets in electrospray, solvent evaporation is much faster. The fractionation volume in narrow-bore SEC, which can directly be collected in one MALDI spot, can easily be optimized in the range of a few microlitres. In addition, fairly homogeneous sample spots were obtained. The possible influence of composition variation of the SEC effluent on the analytical results using direct fraction deposition was investigated; no substantial effects were observed. The applicability of the method was demonstrated by characterizing a broad poly(methyl methacrylate) sample. Copyright 2000 John Wiley & Sons, Ltd.     

Detection of interaural differences of intensity in trains of high-frequency clicks as a function of interclick interval and number

Listeners were asked to detect interaural differences of intensity in trains of 4000-Hz clicks as the interclick interval (ICI) was varied from 10 to 1 ms and the number of clicks in a train (n) was varied from 1 to 32. As has previously been shown for differences of time [Hafter and Dye, J. Acoust. Soc. Am. 73, 644-651 (1983)], plots of log interaural threshold versus log n produced slopes that decrease with ICI. These results are explained in terms of a saturation model which argues that as the click rate increases, the evoked neural activity changes from what is essentially a tonic response toward one that is more phasic.     

The effects of intensity on the detection of interaural differences of time in high-frequency trains of clicks

Threshold values of interaural differences of time (delta IDTs ) were measured for trains of dichotic clicks whose levels were 20, 40, or 60 dB SPL. All clicks were bandpass filtered at 4 kHz, and the number of clicks in the train (n) was 1, 2, 4, 8, 16, or 32. The interclick interval (ICI) was 5, 2, or 1 ms. Performance was compared to that of an ideal integrator of information, which produces slopes of - 0.5 when log delta IDT versus log n is plotted. The results showed that increases in level had no effect on the slopes of the log-log functions regardless of the ICI but did decrease the intercepts. Shortening the ICI caused the slopes to go from nearly - 0.5 towards 0.0. The improvement with level could be explained by either a decrease in the temporal variability of neural discharges, or by an increase in the number of samples of IDT at higher intensities brought on by increased firing rates or the activation of more auditory units. A review of the physiological literature found the most parsimonious explanation to be that the decline in threshold IDT was mediated by an increase in the number of active units, each possessing the same degree of adaptation.     

New Families of Complete Caps, and the Asymptotic Size of the Largest Caps, of Quadrics over Prime Fields

A cap of a quadric is a set of its points whose pairwise joins are all chords. Such a cap is complete if it is not part of a larger one. Few examples of complete caps are known except for quadrics in low dimensions. In this paper, we consider the case when the coordinate field is GF(p), with p an odd prime, and construct, in each projective space GF(n,p) with n p – 1 and n – 2(mod p), a cap on one of its nonsingular quadrics. We use this in two ways. Firstly, we combine its size with the recent Blokhuis–Moorhouse upper bound for quadric caps to show that the size of the largest cap of any nonsingular quadric in PG(N,p) is asymptotic to Np – 1/(p – 1) ! as N tends to infinity. Secondly, by establishing situations when our cap is complete, we produce various infinite families of complete quadric caps over GF(p) for each p. Earlier work determined all complete caps of all nonsingular quadrics over GF(2).