Time-resolved Spectroscopy (time-resolved + spectroscopy)

Distribution by Scientific Domains


Selected Abstracts


Growth of GaN quantum dots on nonpolar A -plane SiC by molecular-beam epitaxy

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2006
S. Founta
Abstract We report on A -plane GaN quantum dots in AlN, grown on A -plane 6H SiC substrates by plasma-assisted molecular-beam epitaxy. AFM imaging revealed a strong alignment of the dots along the [100] direction that we correlated with the anisotropic morphology of the AlN buffer layer. A vertical correlation of these dots was evidenced by high resolution transmission electron microscopy on superlattice samples with an AlN spacer thickness of 5 nm. Time-resolved spectroscopy performed on both C -plane and A -plane samples revealed much shorter radiative lifetimes for the A -plane dots, indicating a strong reduction of the internal electric field with respect to the one present in their C -plane counterparts. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Time-resolved spectroscopy in an undoped GaN (1-101)

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2008
Eunhee Kim
Abstract Time-resolved photoluminescence spectroscopy was performed at 77 K in a GaN (1-101) grown on a 7 degree off-axis (001) Si substrate. The sample was grown by metal-organic-vapour-phase-epitaxy (MOVPE) and was un-intentionally doped with O, C and Si. By using photoluminescence intensity correlation method, the energy relaxation process of the photogenerated carriers near the band edge was investigated in pico-second regime. The correlation signal was represented by a single exponential decay curve and the energy relaxation time was determined, which depended strongly on the kinetic energy of the excess carriers. At low energies, the relaxation time was around 700 ps, while it was as short as a few ps at the highest energy under study. The correlation signals obtained for carriers of which kinetic energy was less than 80 meV showed an anti-correlation behaviour suggesting the occurrence of carrier accumulation. The time constants for the accumulation were of several picoseconds depending on the kinetic energy, which was nearly equal to the decay time constants determined at high energies. This fact shows that the energy relaxation at high energies is controlled by the emission of an LO phonon. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Dynamics of Al/Fe2O3 MIC Combustion from Short Single-Pulse Photothermal Initiation and Time-Resolved Spectroscopy

PROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 4 2009
Albert
Abstract Time-resolved spectroscopy was used to study the dynamics of the photothermal ignition of Al/Fe2O3 metastable intermolecular composites after single short-pulse laser initiation. The dynamics were recorded in several time domains from nanosecond to microsecond to quantify the dynamics from initial laser excitation to combustion. Time-averaged spectral data were also collected for the overall emission occurring during combustion. [source]


Metal Binding Properties of Fluorescent Analogues of Trichogin GA,IV: A Conformational Study by Time-Resolved Spectroscopy and Molecular Mechanics Investigations

CHEMBIOCHEM, Issue 1 2009
Mariano Venanzi Prof.
Abstract The metal ion binding properties of two fluorescent analogues of trichogin GA,IV, which is a natural undecapeptide showing significant antimicrobial activity, were studied by circular dichroism, time-resolved optical spectroscopy, and molecular mechanics calculations. Binding of CaII and GdIII to the peptides investigated was shown to promote a structural transition from highly helical conformations to folded structures characterized by formation of a loop that embedded the metal ion. Time-resolved spectroscopy revealed that peptide dynamics is also remarkably affected by ion binding: peptide-backbone motions slowed down to the microsecond time scale. Finally, molecular mechanics calculations emphasized the role of the central Gly5-Gly6 motif, which allowed for the twisting of the peptide segment that gave rise to the formation of the binding cavity. [source]


A ZZ Ceti white dwarf in SDSS J133941.11+484727.5

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006
B. T. Gänsicke
ABSTRACT We present time-resolved spectroscopy and photometry of the cataclysmic variable (CV) SDSS J133941.11+484727.5 (SDSS 1339) which has been discovered in the Sloan Digital Sky Survey (SDSS) Data Release 4. The orbital period determined from radial velocity studies is 82.524(24) min, close to the observed period minimum. The optical spectrum of SDSS 1339 is dominated to 90 per cent by emission from the white dwarf (WD). The spectrum can be successfully reproduced by a three-component model (white dwarf, disc, secondary) with TWD=12 500 K for a fixed log g= 8.0, d= 170 pc, and a spectral type of the secondary later than M8. The mass-transfer rate corresponding to the optical luminosity of the accretion disc is very low, , 1.7 × 10,13 M, yr,1. Optical photometry reveals a coherent variability at 641 s with an amplitude of 0.025 mag, which we interpret as non-radial pulsations of the white dwarf. In addition, a long-period photometric variation with a period of either 320 or 344 min and an amplitude of 0.025 mag is detected, which bears no apparent relation with the orbital period of the system. Similar long-period photometric signals have been found in the CVs SDSS J123813.73,033933.0, SDSS J204817.85,061044.8, GW Lib and FS Aur, but so far no working model for this behaviour is available. [source]


Time-Resolved and Steady-State Fluorescence Spectroscopy of Eumelanin and Indolic Polymers

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2007
Stephen P. Nighswander-Rempel
Eumelanin plays a variety of important physiological roles in human skin. However, its structure and fundamental properties still remain poorly understood. Although the absorbance of eumelanin is broad and reveals little about its structure, a variety of techniques have revealed the presence of a disordered array of chromophores within the melanin compound. In order to examine the fluorescence decay dynamics of these chromophores, time-resolved spectroscopy was applied to solutions of synthetic eumelanin and a melanin-like polymer of N-methyl,5-hydroxy,6-methoxyindole (N-Me-5H6MI). Solutions were excited with 80 fs laser pulses at 355, 370, 390 and 400 nm, and decay time courses were acquired at 20 nm intervals between 400 and 600 nm for each excitation wavelength. Decay profiles for both eumelanin and the polymer exhibited a characteristic multiexponential behavior with decay times between 0.5 and 15 ns, although steady-state spectra for the polymer exhibited only two peaks. The long-decay component in the polymer showed a significant decrease in both amplitude (30,5%) and decay time (14,6 ns) with increasing emission wavelength. In contrast, the amplitude and decay time in melanin increased slightly (10,15% and 7,10 ns, respectively) from 400 to 520 nm emission, at which point they leveled off. These trends were consistent for all excitation wavelengths. These results suggest that the multiexponential behavior of melanin fluorescence is characteristic of each oligomer within the eumelanin compound, and is consistent with the assertion that the diversity of constituents within eumelanin provides it with a robustness in spectral properties. [source]


Relationship between muscle oxygenation and electromyography activity during sustained isometric contraction

CLINICAL PHYSIOLOGY AND FUNCTIONAL IMAGING, Issue 4 2008
Eiji Yamada
Summary The purpose of this study was to clarify the relationship between electromyography (EMG) spectrum changes and muscle oxygenation measured by near-infrared time-resolved spectroscopy (TRS). Each subject performed sustained isometric knee extension at 50% of the maximal voluntary contraction load for 1 min. Surface EMG and TRS were simultaneously recorded from the right vastus lateralis muscle. Mean power frequency (MPF) of the power spectrum was calculated every 5 s during isometric contraction using fast Fourier transform, and decrease in the slope of MPF for 1 min was calculated using the least squares method. The maximal changes in oxygenated haemoglobin and myoglobin (Oxy Hb/Mb) and in deoxygenated haemoglobin and myoglobin (Deoxy Hb/Mb) from pre-contraction values of 1 min were calculated. There were significant relationships between the decrease in the slope of MPF and the maximal changes in Oxy Hb/Mb and Deoxy Hb/Mb (P < 0·05). These findings suggested that changes in Oxy Hb/Mb and Deoxy Hb/Mb indicate muscle fatigue assessed by EMG. [source]