Search

We investigate the reduction in the efficiency of band-edge radiative recombination in InN by two carrier recombination processes via mid-gap states: radiative recombination via deep states and nonradiative recombination (NR). Because of the small band-gap energy value and the existence of the surface electron accumulation layer, the carrier transition processes via deep states cannot be observed easily. We address this problem by using mid-infrared photoluminescence (PL) measurements, and observe an emission peak around 0.32 eV at room temperature, which we interpret as being caused by transition processes via deep-defect states. Since this emission is weaker than the band-edge emission, the dominant carrier recombination process is concluded to be NR by phonon emission. The NR rate is known to be determined by the NR defect density, carrier transport processes to NR defects, and thermal activation processes of carriers. Carrier transport and capture processes by NR defects are investigated using p-type samples for various carrier mobility values. It is concluded that the NR rate is highly affected by the carrier transport, and that the candidates for the NR defect species are point defects and complexes of acceptor nature. We have also observed the correlation between the thermal conductivity and the band-edge PL intensity. As a result, we have found that the NR rate is highly affected by the carrier transport and thermal activation processes in InN. [ABSTRACT FROM AUTHOR]

We present a simulation model for a stream of photon counts that can be detected in fluorescence fluctuation spectroscopy systems from diffusing molecules undergoing spontaneous transitions between radiative and nonradiative states. Using our model, we can obtain various characteristics of a random stream of photon counts which allows us to test fluorescence fluctuation spectroscopy methods and to predict the results of full-scale experiments. [ABSTRACT FROM AUTHOR]

We report on preliminary results from the experiment performed at Legnaro National Laboratories aiming at the investigation of the shape evolution of the 88Mo nucleus. The GDR spectra obtained for two data sets measured at 48Ti beam energy of 300 and 450 MeV corresponding to 3 and 3.8 MeV temperatures, respectively, are presented. The low energy component in GDR strength functions, similar to the theoretical predictions of LSD model, possibly indicates the presence of Jacobi shape transition. [ABSTRACT FROM AUTHOR]

The ASACUSA collaboration at CERN has previously measured the optical transition frequencies of antiprotonic helium to a precision of <1 part in 108 by laser spectroscopy. We describe some recent theoretical and experimental developmental work carried out by our collaboration to further improve the experimental precision. Utilisant la spectroscopie laser, la collaboration ASACUSA au CERN a déjà mesuré les fréquences de transition optique de l'hélium antiprotonique à mieux qu'une partie dans 108. Nous décrivons des travaux récents, théoriques et expérimentaux, effectués par nos collaborateurs, afin d'améliorer la précision expérimentale. [ABSTRACT FROM AUTHOR]

We study the spectral evolution of 13 short-duration gamma-ray bursts (GRBs) detected by the Gamma Burst Monitor onboard Fermi. We study spectra resolved in time at the level of 2-512 ms in the 8 keV-35 MeV energy range. We find a strong correlation between the observed peak energy and the flux within individual short GRBs. The slope of the correlation for individual bursts ranges between ∼0.4 and ∼1. There is no correlation between the low-energy spectral index and the peak energy or the flux. Our results show that in our 13 short GRBs, evolves in time tracking the flux. This behaviour is similar to what found in the population of long GRBs and it is in agreement with the evidence that long GRBs and (the still few) short GRBs with measured redshifts follow the same rest-frame - correlation. Its origin is most likely to be found in the radiative mechanism that has to be the same in both classes of GRBs. [ABSTRACT FROM AUTHOR]

Abstract: The paper introduces general considerations on structural properties of aromatic, antiaromatic and non-aromatic conjugated systems in terms of potential energy along bond length alternation and distortion coordinates, taking as examples benzene, cyclobutadiene and cyclooctatetraene. Pentalene, formally derived from cyclooctatetraene by cross linking, is also considered as a typical antiaromatic system. The main interest is concerned with [n]annulenes and model [n]annulene molecular systems, n ranging from 10 to 18. The rich variety of conformational and configurational isomers and of dynamical processes among them is described. Specific attention is devoted to bridged [10]- and [14]annulenes in the ground and lowest excited states as well as to s-indacene and biphenylene. Experimental data obtained from vibrational and electronic spectroscopies are discussed and compared with ab initio calculation results. Finally, porphyrin, tetraoxaporphyrin dication and diprotonated porphyrin are presented as annulene structures adopting planar/non-planar geometries depending on the steric hindrance in the inner macrocycle ring. Radiative and non-radiative relaxation processes from excited state levels have been observed by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy. A short account is also given of porphycene, the structural isomer of porphyrin, and of porphycene properties. [ABSTRACT FROM AUTHOR]

In recent years, terahertz (THz) sources between 0.1 THz and 10 THz have attracted much attention for imaging and sensing applications. THz emission from radiative transitions in impurity states has been demonstrated in Si and Ge devices by either electrical or optical pumping. Compared to Si as the material for THz emission, the wide-band-gap material SiC exhibits several advantages such as a higher dopant ionization energy, which allows a higher device operating temperature. Combining with its superior material qualities such as high breakdown field and high thermal conductivity, SiC is a promising material for high-temperature and high-power THz emitting devices. This article describes recent progress in using SiC materials to increase the operating temperature and output power of dopant-based THz sources. [ABSTRACT FROM AUTHOR]

We measured the spontaneous-emission branching ratios of 69 levels in Sm II selectively populated via single-frequency laser excitation of a 10 keV ion beam. The levels studied had term energies up to 29 600 cm–1, and decay branches with spontaneous emission in the range 250–850 nm were detected. The experimental accuracy was in the range of 10%. We used these branching ratios along with our previously determined radiative lifetimes to infer transition probabilities and oscillator strengths for 608 transitions in the wavelength range 363–771 nm, which are useful for stellar abundance determinations.PACS Nos.: 32.70.Cs, 95.30.Ky [ABSTRACT FROM AUTHOR]

Radiative parameters for transitions depopulating the levels belonging to the 4d85s2, 4d96s and 4d95d configurations of Ag ii have been obtained from a combination of theoretical lifetimes and experimental branching fractions. On the experimental side, a laser-produced plasma was used as a source of Ag+ ions. The light emitted by the plasma was analysed by a grating monochromator coupled with a time-resolved optical multichannel analyser system. Spectral response calibration of the experimental system was performed using a deuterium lamp in the wavelength range from 200 to 400 nm, and a standard tungsten lamp in the range from 350 to 600 nm. The transition probabilities were obtained from measured branching ratios and theoretical radiative lifetimes of the corresponding states calculated with a relativistic Hartree–Fock approach including core-polarization effects and configuration interaction in an extensive way. Theoretical and experimental data have been compared and the new data have also been compared with the few previous results available in the literature. [ABSTRACT FROM AUTHOR]

It has been shown that the low-frequency periodic processes in spectroscopy and chemical transformations that proceed without a change in the total energy of a complex molecular system can be explained as a result of synchronized radiationless transitions between the resonant ground states of quantum systems of a different structure. [ABSTRACT FROM AUTHOR]

Abstract: Spectroscopic properties of the Sm3+-doped LiNbO3 crystal were investigated at various temperatures. Polarised absorption spectra, as well as emission and lifetime measurements were performed. The energies of crystal-field levels have been presented. The Judd–Ofelt theory, extended to anisotropic system, has been applied to evaluate the Ω t=2,4,6 intensity parameters, radiative transition rates A r, branching ratios β, and radiative lifetime τ r of the fluorescent 4G5/2 level. The emission cross-sections of potential laser lines between 603 and 652nm were estimated. [Copyright &y& Elsevier]

Absorption and emission as well as emission decay times of Ho3+ ions embedded in LaAlO3 single crystal were measured. The experimental results were analysed in the frame of the Judd–Ofelt theory. Energy levels, oscillator strengths and probabilities of radiative transitions were determined. The results indicate on nonradiative transitions due to cross-relaxation down and up-conversions processes. [Copyright &y& Elsevier]

Optical spectra of Dy3+:LiYF4 (Dy3+:YLF) single crystal were analyzed using Judd–Ofelt theory and discrete variational multielectron method (DVME). Oscillator strengths (OS) of transitions up to 39 000 cm-1 in the absorption spectrum, branching ratios and nonradiative transition rates were calculated; assignment of the absorption transitions was given. A good agreement between calculated and experimental values of branching ratios for transitions from quartet 4F(3)9/2 to the low-lying sextet levels is demonstrated. Ab initio calculations of OS were performed. [Copyright &y& Elsevier]

Mixed alkali effect (MAE) in xNa2O·(30-x)K2O·70B2O3 (x=5, 10, 15, 20 and 25) glasses doped with 0.5Ho2O3 has been investigated by measuring the optical properties of Ho3+. From the optical absorption spectra, optical band gaps (Eopt) for both direct and indirect transitions have been calculated using Davis and Mott theory and are found to exhibit a minimum when the two alkalies are in equal concentration (due to mixed alkali effect). Spectroscopic parameters like Racah (E¹, E², E³), spin-orbit (ξ4f), configuration interaction (α, β) and Judd-Ofelt intensity parameters (Ω2, Ω4 and Ω2) have been calculated as a function of x. Also radiative and non-radiative transition probabilities (AT and WMPR), radiative lifetimes (τR), branching ratios (β) and integrated absorption cross sections (Σ) have been obtained. The spectra! profile of the hypersensitive transition has been correlated to the site symmetry of the rare earth ion. The trends observed in the intensity parameters, radiative lifetimes and stimulated emission cross sections as a function of x in these borate glasses have been discussed, keeping in view the mixed alkali effect. [ABSTRACT FROM AUTHOR]

The article presents the author's comment of the article "Hot gases: the transition from the line to spectra to thermal radiation," by M. Vollmer. According to the author, the article has several errors that invalidate the explanation on how the line of spectrum of atoms is transformed into the continuous spectrum of thermal radiation. He observed that the transformation of line spectrum to thermal radiation at discrete frequencies associated with the transitions has been discussed often, but the idea on how discrete atomic transitions can give rise to black-body radiation has been ignored.

In the era of large stellar spectroscopic surveys, there is an emphasis on deriving not only stellar abundances but also the ages for millions of stars. In the context of Galactic archeology, stellar ages provide a direct probe of the formation history of the Galaxy. We use the stellar evolution code MESA to compute models with atomic diffusion—with and without radiative acceleration—and extra mixing in the surface layers. The extra mixing consists of both density-dependent turbulent mixing and envelope overshoot mixing. Based on these models we argue that it is important to distinguish between initial, bulk abundances (parameters) and current, surface abundances (variables) in the analysis of individual stellar ages. In stars that maintain radiative regions on evolutionary timescales, atomic diffusion modifies the surface abundances. We show that when initial, bulk metallicity is equated with current, surface metallicity in isochrone age analysis, the resulting stellar ages can be systematically overestimated by up to 20%. The change of surface abundances with evolutionary phase also complicates chemical tagging, which is the concept that dispersed star clusters can be identified through unique, high-dimensional chemical signatures. Stars from the same cluster, but in different evolutionary phases, will show different surface abundances. We speculate that calibration of stellar models may allow us to estimate not only stellar ages but also initial abundances for individual stars. In the meantime, analyzing the chemical properties of stars in similar evolutionary phases is essential to minimize the effects of atomic diffusion in the context of chemical tagging. [ABSTRACT FROM AUTHOR]

The article presents the author's response to the comment on his article "Hot gases: the transition from the line to spectra to thermal radiation." According to the author, he agreed on the comment that his approach of using a simple two-level system was oversimplified and that a proper application of Albert Einstein's derivation is not possible if only a two-level system with well-defined energies is assumed. On the other hand, he argued on the critics opinion that his explanation of how a line spectrum is transformed to the continuous spectrum of thermal radiation.