Bibliothek

Notizen: The particle reflection coefficient is determined experimentally and by computer simulation for the bombardment of two different kinds of carbon with deuterium at normal and oblique incidence in the energy range from 1 keV down to 33 eV. Highly oriented pyrolytic graphite (HOPG) and EK98 graphites served as targets, the former as an example with a relatively flat surface, the latter with a rough surface topography. The experimental technique is based on the measurement of the trapped amount by nuclear reaction analysis using the reaction d(3He,p)α; protons are detected with a surface barrier detector. It is found that the usual assumption of complete trapping at low fluences is not fulfilled at low energies (≤100 eV). This is demonstrated by measuring the decrease of the implanted amount of deuterium with further bombardment of protons at the same energy. This loss of implanted atoms can be described by an exponential function which can be used to determine the correct trapping coefficient and from this the correct particle reflection coefficient. The experimentally determined particle reflection coefficients for HOPG agree reasonably well with data calculated with the Monte Carlo program TRIM.SP (version TRVMC); only at the lowest energy of 50 eV the experimental values are somewhat higher at intermediate angles of incidence than the calculated ones. The rough surface of EK98 is investigated with a scanning tunneling microscope. It is found that this surface can be described by a fractal surface of dimension 2.05. For this surface agreement of the experimental values with those calculated with the program VFTRIM (based on TRIM.SP, assuming a fractal surface) is found. © 1995 American Institute of Physics.

Notizen: The X 2A2' band of the photoelectron spectrum of NO−3 is theoretically calculated in the framework of a multimode vibronic coupling model. Linear coupling between and within the X 2A2' and the B 2E' electronic states of NO3 is found to be the important mechanism governing the dynamics. The necessary coupling constants are obtained from the ionization potentials of NO−3 calculated at different geometries using ab initio Green's functions. Comparison of the theoretical with experimental results [A. Weaver, D. W. Arnold, S. E. Bradforth, and D. M. Neumark, J. Chem. Phys. 94(3), 1740 (1991)] for the PE spectrum shows good agreement if a temperature of the NO−3 anion of 455 K in the experiment is assumed. A general theoretical treatment of thermal effects in vibronically coupled electronic states is presented. The ground electronic state potential surface of NO3 is discussed in the framework of the vibronic coupling model. A shallow minimum at a C2v geometry with a barrier height of 0.006 eV relative to the minimum energy D3h configuration is found. It is too weak to deform the effective geometry of the molecule from D3h to C2v. Mode-mode coupling is found to play a relevant role in the ground electronic state of NO3, in general.

Notizen: Dispersed domains in two-phase systems often exhibit complex and intriguing morphologies. For many of these systems, it is possible to predict the shape of such a domain through an evaluation of the free energy. In this paper, we present a numerical technique to calculate domain shapes through a variational approach. This is a significant extension to previous work which has primarily involved examining free energies of specific classes of shapes. The solution diagram is presented in the neighborhood of the first four bifurcations from a circle and the affect of domain size is examined. We find the only stable domain shapes are circular or bilobed. More importantly, we find that the transition between these two shapes is discontinuous, contrary to the findings of previous shape studies. Examining the free energy functional in the neighborhood of stable solutions, we find that, in general, domain shape calculations are best applied to small nonelongated domains. Finally, we present graphical differentiation, a numerical technique we developed to enable solution of the problem.

Notizen: Dispersed domains in two-dimensional two-phase systems often exhibit complex and intriguing morphologies. For many of these systems, it is possible to predict the shape of such a domain through an evaluation of the free energy. In this paper, we extend our previous numerical technique for calculating domain shapes by allowing for titled dipoles; the original technique assumed dipole moments oriented normal to the system plane. The solution diagram is presented in the vicinity of the primary shape transition for varying tilt angles. We find that circular domains are only stable in the absence of dipole tilt. Moreover, we find that the discontinuous transition found when the dipoles are vertical gives way to continuous elongation with increasing dipole tilt. Interestingly, it appears that the critical angle is independent of domain size. © 1995 American Institute of Physics.

Notizen: We report material gain measurements of bulk 1.55 μm InGaAsP/InP performed at constant temperature and carried out over a large range of carrier densities and a large spectral region. In addition, a polynomial model for the material gain is proposed that not only fits the experimental data well over the whole measured range but also shows stable convergence in simulation tools when carrier densities exceed the usual range. To get reliable parameters for the model we eliminated temperature effects arising from different current biases and performed material-gain measurements over the largest possible carrier density range. The material-gain model realized is used in semiconductor optical amplifier simulation tools. © 2000 American Institute of Physics.

Notizen: Strained single- and triple-quantum-well (SQW and TQW), large optical cavity GaInAsSb/AlGaAsSb/GaSb laser diodes emitting at 2.26 μm are investigated. Internal loss coefficients as low as 5 and 7.7 cm−1 for the SQW and TQW, respectively, and relatively high internal quantum efficiencies of 65% (SQW) and 69% (TQW) were obtained. Extrapolated threshold current densities for infinite cavity lengths of 55 and 150 A/cm2 have been deduced for the SQW and TQW, respectively. These values scale very well with the number of QWs and are among the lowest reported for diode lasers in this wavelength range. A differential quantum efficiency as high as 50% and a total power efficiency of 23% were achieved at 280 K. The temperature dependence of the threshold current density revealed a high characteristic temperature of 110 K. Single-ended output powers of 240 mW in continuous-wave mode and exceeding 0.5 W in pulsed operation were obtained for a TQW laser with high-reflection/antireflection coated facets at 280 K, mounted substrate-side down. © 2000 American Institute of Physics.

Notizen: 1 A fructose (Fru)-enriched diet induces a mild increase in blood pressure associated with hyperglycaemia, hypertriglyceridaemia, and insulin resistance, resembling the human ‘syndrome X’, being an useful model to study hypertension and type 2 diabetes. 2 A sustained elevation of blood pressure is associated with cardiovascular structural modifications such as left ventricular hypertrophy and increased wall thickness:lumen diameter ratio in blood vessels. 3 Prostanoids (PR), metabolites of arachidonic acid through the cyclooxygenase pathway, include vasoactive substances synthesized and released by the vessel walls. 4 The aim of the present study was to analyse, in Fru-treated rats: (i) the morphology of mesenteric vessels and; (ii) the PR production in aorta and mesenteric vessels, in order to assess whether these parameters are related with the haemodynamic alterations observed in this experimental model. 5 Blood pressure, glycaemia and triglyceridaemia, were significantly elevated in both (4 and 22 weeks) Fru-treated groups. Meanwhile body and heart weight as well as insulinaemia were similar between experimental animals and controls. 6 The mesenteric vessels of Fru-treated rats (22 weeks) showed an increased thickness and area of the media when compared with the controls; meanwhile, the lumen diameter was similar in both groups. 7 The Fru treatment for 4 weeks did not modify PR production in aorta, whereas in the mesenteric bed it diminished prostaglandin (PG) E2 release significantly compared with the controls. However, in the group treated for 22 weeks, Fru reduced PGI2 production in the aorta, as assessed by 6-keto-PGF1α measurements. Meanwhile, in the mesenteric bed, the chronic Fru treatment decreased PGE2 release but, rather surprisingly, increased the output of PGI2 when compared with its corresponding controls. 8 In conclusion, the present study shows the existence of an alteration in the morphology of mesenteric vessels in Fru-treated rats, which could be related to an increase in peripheral resistance and the consequent mild hypertension observed in this model. However, a diminished release of vasodilator PRs, such as PGE2 in mesenteric vessels at 4 and 22 weeks and PGI2 in aorta at 22 weeks could further impair the vessel response. The increase in PGI2 observed in the chronic group in mesenteric vessels could be attributed to a compensatory mechanism.

Notizen: The rovibronic spectrum of the Na3 B system is computed taking into account full rovibronic coupling between different vibronic states. The rovibronic coupling matrix elements are calculated using the vibronic states following from a pseudo Jahn–Teller model for the vibronic dynamics in the B system. Comparison with results of optical–optical double resonance measurements [W. E. Ernst and S. Rakowsky, Can. J. Phys. 72, 1307 (1994)] shows good agreement. In particular, certain properties of the Coriolis splittings, so far explained by an ad hoc spin–rotation interaction, are now well understood as the consequence of rovibronic coupling between two vibronic states. A general discussion of rovibronic dynamics on electronic potential energy surfaces with three equivalent minima is presented. © 1996 American Institute of Physics.

Notizen: A new general method to compute rovibronic energy levels and eigenstates in molecules with possibly strong vibronic coupling is presented. The method can be applied to arbitrary coupling mechanisms of electronic potential energy surfaces and is not limited to a special type of molecule. Explicit criteria for the applicability of the method are derived. To investigate different types of rovibronic interactions, parameter studies for linear and bilinear E×ε Jahn–Teller systems are performed. As an application to a real molecule we investigate the rotational structure of the Li3 electronic A state. A variety of possible nontrivial effects is found which cannot be explained by rotational–vibrational coupling on a single electronic potential energy surface. Attention is paid to construct explicit effective Hamiltonians describing the findings. The rovibronic motion on potential surfaces with equivalent minima is also discussed in this context. © 1996 American Institute of Physics.