Phonon dispersion relations in semiconductor superlattices in the adiabatic bond-charge model

The phonon dispersion in semiconductor superlattices is studied in the adiabatic bond-charge model. The complex phonon dispersion relations of the constituent bulk materials are obtained via the eigenvalue method in a zeroth-order calculation, which includes only short range forces and Coulomb interactions between ions and bond charges in the same and neighboring layers. The eigenmode displacements of the superlattice are obtained by matching the eigenvectors associated with complex phonon branches at the interfaces. The effect of the remaining Coulomb interactions are then included in the first-order approximation. The results for the superlattice phonon frequencies compare favorably with the existing experimental data.     

Energy distributions of CO produced in an acetone-containing discharge

Tunable infrared diode laser absorption (TDLAS) and Fourier transform infrared absorption spectroscopies (FTIR) have been utilized to characterize the translational, rotational and vibrational distributions of CO in an acetone/argon DC plasma at total pressures ranging from 4 to 5 Torr and currents of 0.1–0.3 A. A broad vibrational distribution of CO was observed with gradually decreasing intensities from the fundamental band to v=12←11. When nitrogen was added to the plasma, the distribution is narrower, due to the efficient energy transfer between CO and N₂ molecules. The measured translational temperature in such plasmas ranged from 400–550 K. The rotational distribution can generally be fitted to a Boltzmann distribution within each vibrational level although the rotational temperature is highest for the lowest vibrational quantum number.     

Iridium-promoted deoxyglycoside synthesis: stereoselectivity and mechanistic insight

Herein, we devised a method for stereoselective O-glycosylation using an Ir(i)-catalyst which enables both hydroalkoxylation and nucleophilic substitution of glycals with varying substituents at the C3 position. In this transformation, 2-deoxy-α-O-glycosides were acquired when glycals equipped with a notoriously poor leaving group at C3 were used; in contrast 2,3-unsaturated-α-O-glycosides were produced from glycals that bear a good leaving group at C3. Mechanistic studies indicate that both reactions proceed via the directing mechanism, through which the acceptor coordinates to the Ir(i) metal in the α-face-coordinated Ir(i)-glycal π-complex and then attacks the glycal that contains the O-glycosidic bond in a syn-addition manner. This protocol exhibits good functional group tolerance and is exemplified with the preparation of a library of oligosaccharides in moderate to high yields and with excellent stereoselectivities.

Ir(i)-catalyzed α-selective O-glycosylation of glycals provided an access to both 2-deoxyglycosides and 2,3-unsaturated glycosides with a broad substrate scope. The underlying rationale of α-selectivity has been illustrated by the DFT study.     

Applications of the channel flow cell for UV-visible spectroelectrochemical studies. Part 2: Transient signals

The use of the channel flow cell for ultraviolet-visible (UV-vis) spectroelectrochemical experiments has been developed to determine the diffusion coefficients of electrogenerated species by means of monitoring the transient absorbance response resulting from a potential step at a working electrode immediately upstream of the incident spectrophotometric beam. The technique is applied to measure the diffusion coefficient of tris(4-bromophenyl)amine (TBPA) radical cation in acetonitrile at 25°C. It is shown that the diffusion coefficient of electrogenerated TBPA radical cation (1.64 ± 0.02 × 10⁻⁵ cm² s⁻¹) is very close to that of the parent molecule (1.57 ± 0.03 × 10⁻⁵ cm² s⁻¹).     

FTIR studies on the gas phase laser-induced decomposition of CF3CH2ONO

The unimolecular decomposition of CF₃CH₂O (2,2,2-trifluoroethoxy) radical generated from 355 nm pulsed nanosecond laser photolysis of CF₃CH₂ONO (2,2,2-trifluoroethylnitrite) in the gas phase has been studied using Fourier transform infrared absorption spectroscopy. The radical preferentially dissociates via its C–H bond cleavage to yield CF₃CHO (trifluoroacetaldehyde) as the major product. The infrared spectrum of formaldehyde, one of the products of C–C bond dissociation of CF₃CH₂O was not observed under a range of nitrite and argon buffer gas pressures. Similar results were obtained when thermal heating and broadband xenon lamp irradiation of the nitrite were carried out. The addition of high pressures of NO further decreased the production of CF₃CHO since recombination of NO with the trifluoroethoxy radical competes with the unimolecular dissociation process. Surprisingly, CF₃CDO was also the only product observed when the deuterated species CF₃CD₂ONO was photolysed by the 355 nm laser. These observations contradicted MP2/aug-cc-pVTZ calculations which were found to favour the C–C bond dissociation channel. However, 355 nm photolysis of CF₃CH₂ONO in the presence of O₂ yielded trifluoroethylnitrate, CF₃CH₂ONO₂ as the main product while CF₃CHO and CF₂O were also observable at much lower yields.     

A passive three-branch optical power divider in LiNbO3

A passive three-branch waveguide power divider has been fabricated by titanium diffusion into a Y-cut LiNbO₃ substrate and characterized. Control of power distribution was achieved by modifying index values in the branches through the deposition of a dielectric cladding over the outer branches. Experimental results are presented and compared with previous theoretical predictions based on single-mode propagation.     

Nonuniqueness in a Free Boundary Problem from Combustion

We study a parabolic free boundary problem with a fixed gradient condition which serves as a simplified model for the propagation of premixed equidiffusional flames. We give a rigorous justification of an example due to J.L. Vázquez that the initial data in the form of two circular humps leads to the nonuniqueness of limit solutions if the supports of the humps touch at the time of their maximal expansion. A. Petrosyan was supported in part by NSF grant DMS-0701015. N.K. Yip was supported in part by NSF grant DMS-0406033.