Reactions de photoaddition de benzo[b]selenophenes avec l'acetylene dicarboxylate de methyle et avec le dichloro-1,2 ethylene

Photoirradiated in presence of acetophenone, benzo[b]selenophene and its 3-methyl derivative add to dimethyl acetylenedicarboxylate. In each ease, the primary reaction product is unstable and has not been isolated. Photoexeited in its triplet state (the energy of which is in the neighbourhood of 69 $\text{kcal}\text{mole}$) benzo[b]selenophene and its 2- and 3-methyl, 2,3 dimethyl, 3 acetoxy and 2-methyl-3-acetoxy derivatives add to 1,2 dichloroethylene leading to cyclobutanes. Neither cyclo-addition occurs in absence of photosensitiser. Single-crystal X-ray analysis gave the structures of the two adducts of 3-acetoxybenzo[b]selenophene with trans-1,2-dichloroethylene. In both compounds the chlorine atoms are trans.     

Stereoselective photoinduced electrocyclic ring closure of aromatic enehydrazides. Asymmetric synthesis of 3-aryl dihydroisoquinolones and tetrahydroisoquinolines

A flexible route for the stereoselective synthesis of a variety of 3-aryl dihydroisoquinolones and tetrahydroisoquinolines has been developed. The key step is a diastereoselective photoinduced 6π-electrocyclic ring closure of enantiopure aromatic enehydrazides via a 1,4-remote asymmetric induction. N–N bond cleavage to release the chiral appendage from the preliminary annulated compounds and/or concomitant reduction of the lactam carbonyl group completed the synthesis of the title compounds.     

Control of defect concentrations within a semiconductor through adsorption

The technologically useful properties of a crystalline solid depend upon the concentration of defects it contains. Here we show that defect concentrations as deep as 0.5 microm within a semiconductor can be profoundly influenced by gas adsorption. Self-diffusion rates within silicon show that nitrogen atoms adsorbed at less than 1% of a monolayer lead to defect concentrations that vary controllably over several orders of magnitude. The results show that previous measurements of diffusion and defect thermodynamics in semiconductors may have suffered from neglect of adsorption effects.     

Multiple resonance heteronuclear decoupling under MAS: dramatic increase of spectral resolution at moderate magnetic field and MAS frequencies

The effects of multiple-resonance heteronuclear decoupling under magic angle spinning (MAS) on the resolution of one-dimensional ¹⁹F and ³¹P and various two-dimensional MAS NMR spectra and on the residual non-refocusable coherence lifetimes in fluorinated aluminophosphate AlPO₄-CJ2, i.e. a compound that contains numerous highly abundant nuclei but no homonuclear spin bath, has been investigated. The design of the four-channel (¹H, ¹⁹F, ²⁷Al, ³¹P) MAS probe used for this study is first described. ¹H and ¹H–²⁷Al double-resonance decouplings allows lengthening the optimized transverse relaxation and increasing the resolution in the ¹⁹F and ³¹P dimensions. Under the application of multi-nuclear decoupling, a two-dimensional ¹⁹F–³¹P CP-HETCOR correlation spectrum for AlPO₄-CJ2 is recorded with unprecedented high-resolution in the two dimensions. Moreover, because ¹H-decoupling increases the ¹⁹F , it has been applied during the entire duration of the 2D NMR experiments, allowing the direct use of residual small interactions to generate ¹⁹F–¹⁹F and ¹⁹F–²⁷Al 2D NMR correlation spectra in AlPO₄-CJ2.     

Temporal onset-order discrimination through the tactual sense

Tactual temporal-onset order thresholds were measured for two sinusoidal vibrations of different frequencies delivered to two separate locations (thumb and index finger) of a multi-finger tactual stimulating device. The frequency delivered to the thumb was fixed at 50 Hz and that to the index finger at 250 Hz. The amplitude and duration of each of the two sinusoidal vibrations were roved independently from trial to trial in a 1-interval, 2AFC procedure. Performance, measured as a function of stimulus-onset asynchrony (SOA), indicated that the temporal-onset-order threshold averaged 34 ms across four subjects. The data were further classified into subsets according to both the amplitude and duration of the two stimuli in each trial of the roving-discrimination paradigm. The results indicated that the amplitude differences of the two stimuli in each trial had a substantial effect on onset-order discrimination, while duration differences generally had little effect. The effects of amplitude differences are explained qualitatively in terms of amplitude latency relationships and stimulus interactions such as temporal masking. Overall, the results not only contribute to an enhanced understanding of the temporal sensitivity of the tactual system but also provide guidelines for the design of tactual aids for hearing-impaired persons.     

An improved automatic time-of-flight picker for medical ultrasound tomography

Objective and motivationTime-of-flight (TOF) tomography used by a clinical ultrasound tomography device can efficiently and reliably produce sound-speed images of the breast for cancer diagnosis. Accurate picking of TOFs of transmitted ultrasound signals is extremely important to ensure high-resolution and high-quality ultrasound sound-speed tomograms. Since manually picking is time-consuming for large datasets, we developed an improved automatic TOF picker based on the Akaike information criterion (AIC), as described in this paper.MethodsWe make use of an approach termed multi-model inference (model averaging), based on the calculated AIC values, to improve the accuracy of TOF picks. By using multi-model inference, our picking method incorporates all the information near the TOF of ultrasound signals. Median filtering and reciprocal pair comparison are also incorporated in our AIC picker to effectively remove outliers.ResultsWe validate our AIC picker using synthetic ultrasound waveforms, and demonstrate that our automatic TOF picker can accurately pick TOFs in the presence of random noise with absolute amplitudes up to 80% of the maximum absolute signal amplitude. We apply the new method to 1160 in vivo breast ultrasound waveforms, and compare the picked TOFs with manual picks and amplitude threshold picks. The mean value and standard deviation between our TOF picker and manual picking are 0.4 μs and 0.29 μs, while for amplitude threshold picker the values are 1.02 μs and 0.9 μs, respectively. Tomograms for in vivo breast data with high signal-to-noise ratio (SNR) (∼25 dB) and low SNR (∼18 dB) clearly demonstrate that our AIC picker is much less sensitive to the SNRs of the data, compared to the amplitude threshold picker.Discussion and conclusionsThe picking routine developed here is aimed at determining reliable quantitative values, necessary for adding diagnostic information to our clinical ultrasound tomography device - CURE. It has been successfully adopted into CURE, and allows us to generate such values reliably. We demonstrate that in vivo sound-speed tomograms with our TOF picks significantly improve the reconstruction accuracy and reduce image artifacts.     

Passive signal enhancement in spatially offset Raman spectroscopy

We demonstrate experimentally, for the first time, the feasibility of enhancing signals in Spatially Offset Raman Spectroscopy (SORS) using a dielectric bandpass filter, building on our earlier experimental work on the enhancement of transmission Raman signals. The method is shown to lead to the enhancement of both the surface and subsurface Raman layer signal improving the signal‐to‐noise ratio of Raman spectra from the deep areas of samples, thus enhancing the technique's sensitivity and penetration depth. The filter is placed over the laser illumination zone, on the sample surface acting as a ‘unidirectional’ mirror transmitting the collimated laser beam on one side and reflecting photons escaping from the sample back into it. This enhances the degree of coupling of laser radiation into the medium and associated generated Raman signal. The feasibility study was performed on a two‐layer sample with the second layer located at the limit of the penetration depth of the method for this sample. The sample consisted of a 2.2‐mm over‐layer of a thinned paracetamol tablet followed by a 2‐mm layer of trans‐stilbene powder. The Raman signal was collected from a spatially offset region through a hole fabricated within the filter. The experiments demonstrate the presence of an enhancement of the Raman signal from both the layers by a factor of 4.4–4.5 and the improved signal‐to‐noise ratio of sublayer signal by a factor of 2.2, in agreement with photon shot noise dominated signal. Copyright © 2008 John Wiley & Sons, Ltd.     

Integration of auditory and vibrotactile stimuli: effects of frequency

Perceptual integration of vibrotactile and auditory sinusoidal tone pulses was studied in detection experiments as a function of stimulation frequency. Vibrotactile stimuli were delivered through a single channel vibrator to the left middle fingertip. Auditory stimuli were presented diotically through headphones in a background of 50 dB sound pressure level broadband noise. Detection performance for combined auditory-tactile presentations was measured using stimulus levels that yielded 63% to 77% correct unimodal performance. In Experiment 1, the vibrotactile stimulus was 250 Hz and the auditory stimulus varied between 125 and 2000 Hz. In Experiment 2, the auditory stimulus was 250 Hz and the tactile stimulus varied between 50 and 400 Hz. In Experiment 3, the auditory and tactile stimuli were always equal in frequency and ranged from 50 to 400 Hz. The highest rates of detection for the combined-modality stimulus were obtained when stimulating frequencies in the two modalities were equal or closely spaced (and within the Pacinian range). Combined-modality detection for closely spaced frequencies was generally consistent with an algebraic sum model of perceptual integration; wider-frequency spacings were generally better fit by a Pythagorean sum model. Thus, perceptual integration of auditory and tactile stimuli at near-threshold levels appears to depend both on absolute frequency and relative frequency of stimulation within each modality.