Stability of high-density one-dimensional excitons in carbon nanotubes under high laser excitation

Through ultrafast pump-probe spectroscopy with intense pump pulses and a wide continuum probe, we show that interband exciton peaks in single-walled carbon nanotubes (SWNTs) are extremely stable under high laser excitations. Estimates of the initial densities of excitons from the excitation conditions, combined with recent theoretical calculations of exciton Bohr radii for SWNTs, suggest that their positions do not change at all even near the Mott density. In addition, we found that the presence of lowest-subband excitons broadens all absorption peaks, including those in the second-subband range, which provides a consistent explanation for the complex spectral dependence of pump-probe signals reported for SWNTs.     

Measurement of radial and circumferential modes in annular and circular fan ducts

The distribution of acoustic energy among the modes in a duct is important in determining the source distribution, the radiation characteristics and the effect of any acoustic linings. This distribution can be determined by processing the microphone signals obtained over planes perpendicular to the duct axis. In an annular or circular duct, the circumferential analysis is simply a Fourier transform, but the radial analysis is in terms of Bessel functions. Methods of determining the radial mode distribution are discussed and results of such analyses on idealized distributions are presented. The use of the most suitable methods of analysis is then demonstrated in measuring the modal distribution of the distortion generated noise of an isolated fan.     

Psychophysical tuning curves measured in simultaneous and forward masking

The level of a masker necessary to mask a probe fixed in frequency and level was determined as a function of masker frequency using a two-interval forced-choice technique. Both simultaneous- and forward- masking techniques were used. Parameters investigated include the level of the probe tone and the frequency of the probe tone. The general form of the psychophysical tuning curves obtained in this way is quite similar to that of single-neurone tuning curves, when low-level probe tones are used. However, the curves obtained to forward masking generally show sharper tips and steeper slopes than those found in simultaneous masking, and they are also generally sharper than neurophysiological tuning curves. For frequencies of the masker close to that of the probe a simultaneous masker was sometimes less effective than a forward masker. The results are discussed in relation to possible lateral suppression effects in simultaneous masking, and in relation to the observer's use of pitch cues in forward masking. It is concluded that neither the simultaneous-masking curves nor the forward-masking curves are likely to give an accurate representation of human neural tuning curves.     

Comparisons of frequency selectivity in simultaneous and forward masking for subjects with unilateral cochlear impairments

Two experiments are described in which frequency selectivity was estimated, in simultaneous and forward masking, for each ear of subjects with moderate (25-60 dB HL) unilateral cochlear hearing losses. In both experiments, the signal level was fixed for a given ear and type of masking (simultaneous or forward), and the masker level was varied to determine threshold, using an adaptive, two-alternative forced-choice procedure. In experiment I, the masker was a noise with a spectral notch centered at the signal frequency (either 1.0 or 1.5 kHz); threshold was determined as a function of notch width. Signal levels were chosen so that the noise level required at threshold for a notch width of zero was similar for the normal and impaired ear of each subject in both simultaneous and forward masking. The function relating threshold to notch width had a steeper slope for the normal ear than for the impaired ear of each subject. For the normal ears, these functions were steeper in forward masking than in simultaneous masking. This difference was interpreted as resulting from suppression. For the impaired ears, significant differences in the same direction were observed for three of the five subjects, but the differences were smaller. In experiment II, psychophysical tuning curves (PTCs) were determined in the presence of a fixed notched noise centered at the signal frequency (1.0 kHz). For the normal ears, the PTCs were sharper in forward masking than in simultaneous masking. For the impaired ears, the PTCs were similar in simultaneous and forward masking, but those in forward masking tended to be sharper at masker frequencies far removed from the signal frequency. Overall, the results suggest that suppression is reduced, but not completely absent in cases of moderate cochlear hearing loss.     

Crystal and molecular structures of two polymorphs of 4-methyl-2-nitroacetanilide (MNA)

Journal of Chemical Crystallography - The crystal and molecular structure of white (MNA-1) and yellow (MNA-3) forms of 4-methyl-2-nitroacetanilide have been determined by X-ray diffraction...     

Synthesis,characterization and photochemistry of some monometallic and bimetallic 2,2′-bipyrimidine complexes of chromium and tungsten carbonyls

Synthesis, electronic absorption spectra, ¹³C NMR and photochemistry are reported for the complexes M(CO)₄bpym (M = Cr or W) and [W(CO)₄]₂bpym. The electronic absorption spectra indicate, for these complexes, that the lowest lying metal-to-ligand (L) charge transfer (MLCT) excited state is lower in energy than the ligand field (LF) excited states. The ¹³C NMR spectra showed that the chemical shifts of C(5) and C(6) for the M-bpym complexes move downfield with respect to that of the free ligand, bpym, while C(4) moves upfield upon complexation. Small, wavelength-dependent quantum yields for loss of CO were obtained upon irradiation. These quantum yields were an order of magnitude larger for the Cr-bpym complex than for the W complexes (Φ = 2.4 x 10^?2 quanta/min for Cr-bpym, 2.5 x 10^?3 quanta/min for W-bpym and 1.1 x 10^?3 quanta/min for W-bpym-W, λ_irr = 366 nm).     

A method for the determination of the specific heat and heat of decomposition of composite materials

A method has been developed to determine the specific heat of a material during thermal decomposition using a combination of DSC and TGA data obtained at the same heating rate. The heat of decomposition is calculated simultaneously using the same data. This technique was used to determine the specific heat and heat of decomposition of a widely used fiberglass-filled phenol—formaldehyde resin and a fiberglass-filled acrylonitrile—butadiene (AB) copolymer. Experimental data are presented for the specific heat of both the virgin and char components for temperatures between 60 and 730°C. The calculated specific heat of the mixture during decomposition for both materials is also presented.     

The influence of surface coordination on field evaporation processes in tungsten

The sequence of evaporation of atoms on (111), (332), (433), and (411) surfaces of tungsten has been observed in the field ion microscope for the evaporation of successive layers. Atoms with many different types of surface coordination were observed to evaporate. The data were processed to give the relative probabilities of evaporation (ρ) for each type of atom using a statistical procedure adapted to give probability limits for the ρ values. The results showed that for (111) surfaces the ρ values were the same with helium or neon as image gas and that in general differences in ρ values between differently coordinated atoms at 78 K were observed to be greater than at 20 K. At both these temperatures however the atoms with high surface coordination generally had lower ρ values. With (111), (332) and (433) surfaces the atomic arrangement allowed prediction of surface diffusion paths and with atoms in (111) surfaces an easy diffusion path was essential before evaporation could occur. Amongst atoms which had a diffusion path available, ρ values were relatively lower when the path had a saddle point protruding from the surface and which allowed the atom to be subjected to a higher field. On (411) surfaces diffusion paths are less predictable and the correlation with ρ was less.     

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