Calculation Of Raman Scattering By Acoustic Phonons In Superlattices

We report on the calculation of low-frequency Raman scattering by longitudinal acoustic phonons in superlattices. The phonons are treated as elastic waves propagating in a continuum and their mode patterns are calculated by a transfer-matrix method. The electromagnetic propagation is calculated neglecting reflections and refractions at the interfaces, through the use of the same index of refraction for both constituent materials. The use of a complex refraction index allows for absorption effects to be included. The suitability of the numerical calculation for modeling superlattices is demonstrated. © 1994 The American Physical Society.5016118451184

Emission Spectra from Internal Shocks in Gamma-Ray-Burst Sources

Unsteady activity of gamma-ray burst sources leads to internal shocks in their emergent relativistic wind. We study the emission spectra from such shocks, assuming that they produce a power-law distribution of relativistic electrons and posses strong magnetic fields. The synchrotron radiation emitted by the accelerated electrons is Compton up-scattered multiple times by the same electrons. A substantial component of the scattered photons acquires high energies and produces e+e- pairs. The pairs transfer back their kinetic energy to the radiation through Compton scattering. The generic spectral signature from pair creation and multiple Compton scattering is highly sensitive to the radius at which the shock dissipation takes place and to the Lorentz factor of the wind. The entire emission spectrum extends over a wide range of photon energies, from the optical regime up to TeV energies. For reasonable values of the wind parameters, the calculated spectrum is found to be in good agreement with the burst spectra observed by BATSE.Comment: 12 pages, latex, 2 figures, submitted to ApJ

Evidence of anomalous dispersion of the generalized sound velocity in glasses

The dynamic structure factor, S(Q,w), of vitreous silica, has been measured by inelastic X-ray scattering in the exchanged wavevector (Q) region Q=4-16.5 nm-1 and up to energies hw=115 meV in the Stokes side. The unprecedented statistical accuracy in such an extended energy range allows to accurately determine the longitudinal current spectra, and the energies of the vibrational excitations. The simultaneous observation of two excitations in the acoustic region, and the persistence of propagating sound waves up to Q values comparable with the (pseudo-)Brillouin zone edge, allow to observe a positive dispersion in the generalized sound velocity that, around Q=5 nm-1, varies from 6500 to 9000 m/s: this phenomenon was never experimentally observed in a glass.Comment: 5 pages, 3 figures. To appear in Phys. Rev.

Nurses Alumni Association Bulletin, Fall 2010

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Fluorescence quantum efficiency of CdSe/CdS magic-sized quantum dots functionalized with carboxyl or hydroxyl groups

AbstractThe present letter reports the thermo-optical properties of functionalized CdSe/CdS magic-sized quantum dots (MSQDs) (sizes 1.9–2.3nm) with carboxyl (R–COOH) or hydroxyl (R–OH) groups in aqueous solutions. Atomic force microscopy and infrared transmittance measurements were used to determine the size of the QDs and to highlight the functionalized groups. Absolute nonradiative quantum efficiency (φ) and radiative quantum efficiency (η) values were determined by applying two techniques: thermal lens (TL) and an alternative method that analyzes the ring patterns generated in a laser beam due to thermally induced self-phase-modulation effects known as the conical diffraction. Fluorescence measurements corroborate the TL results