Bibliothek

Notizen: The physics of polarization spectroscopy (PS) is investigated by direct numerical integration of the time-dependent density matrix equations. The Zeeman structure of the upper and lower energy levels is included in a multistate formulation of the density matrix equations. The numerical solution of the time-dependent density matrix equations enables us to investigate the effects of strong saturation on PS signal levels and line shapes. Bath levels not directly coupled by the laser radiation are included in the numerical modeling to investigate the effects of collisional rates and different types of collisions on signal levels and line shapes. The effects of Doppler broadening are included by solving the density matrix equations for numerous velocity groups. At low laser power we find that the homogeneously broadened PS line shape is Lorentzian-cubed, as compared to the Lorentzian predicted in several previous low-power analytical solutions. In the low laser power regime, the line-center PS signal is proportional to (collision rate)−6, obviously greatly complicating the application of unsaturated PS for quantitative concentration measurements in flames and plasmas. As the transition begins to saturate at higher laser intensities, the dependences of the signal strength on the laser intensity and on the collision rate decrease drastically, although the line-center PS signal is still approximately proportional to (collision rate)−2. The dependence of the PS signal intensity on the ratio of the population-transfer collision rate to the dephasing collision rate is minimized for saturating pump beam intensities. For resonances that are both Doppler- and collision-broadened, the low-power PS line shape is Lorentzian with a linewidth equal to the collisional width for the case where the Doppler width is much greater than the collisional width. At low pump laser intensities, the PS signal is very dependent on the ratio of Doppler broadening to collisional broadening when the Doppler width is greater than the collisional width. However, at high intensity, the line-center PS signal intensity becomes nearly independent of collision rate when the collisional linewidth is less than the Doppler linewidth. Quantitative application of polarization spectroscopy for concentration measurements in flames and plasmas will almost certainly require resolution of the PS line shape and/or accurate measurement of the saturation curve. © 1998 American Institute of Physics.

Notizen: The anode hole injection model is based on a surface plasmon model in which the positive charge is generated by hole injection from the anode, where it is generated via a surface plasmon mechanism resulting finally in oxide breakdown. Attempts to detect the surface plasmons can rely only on indirect observations, such as electron-energy loss, the radiative decay of the surface plasmons, or d2I/dV2 measurements. These measurements show that the emission of surface plasmons is both a strong energy-loss mechanism and an electron-hole pair generation mechanism, particularly in poly-Si/SiO2 or poly-Si/vacuum interfaces. Calculation of the surface plasmon excitation threshold energy is shown to decrease with increasing temperature and is also confirmed by experiments. Thus, the positive charge density increases and the charge to breakdown decreases with increasing temperature. We have also measured and observed the surface plasmon excitation threshold energy at the poly-Si/SiO2 interface from the electron energy loss spectrum for the first time. The surface plasmon mechanism explains the oxide thickness and gate thickness dependence of the positive charge density and temperature dependence of the charge to breakdown. The calculated electron threshold energy to generate a positive oxide charge by the surface plasmon mechanism is EC–Si+2.24 eV. Also, the origin of substrate hole current can be explained by this proposed mechanism. Therefore, the anode hole injection model based upon surface plasmons is a reasonable thin oxide breakdown model that explains measured observations. © 1998 American Institute of Physics.

Notizen: We demonstrate the utilization of various transition metals in a biologically derived, nanometer-scale patterning process. This process created large arrays of nanometer-scale dots (nanodot arrays) with several of these metals. The correlation between nanodot array formation and the interaction energies between metal atoms, other metals atoms, and the surface of the sample was explored. The behavior of the metal films was then investigated with the aid of a Monte Carlo solid-on-solid simulation. © 1998 American Institute of Physics.

Notizen: Abstract: The ability of ethanol to interfere with insulin-like growth factor 1 (IGF-1)-mediated cell survival was examined in primary cultured cerebellar granule neurons. Cells underwent apoptosis when switched from medium containing 25 mM K+ to one containing 5 mM K+. IGF-1 protected granule neurons from apoptosis in medium containing 5 mM K+. Ethanol inhibited IGF-1-mediated neuronal survival but did not inhibit IGF-1 receptor binding or the neurotrophic action of elevated K+, and failed to potentiate cell death in the presence of 5 mM K+. Inhibition of neuronal survival by ethanol was not reversed by increasing the concentration of IGF-1. Significant inhibition by ethanol (15–20%) was observed at 1 mM and was half-maximal at 45 mM. The inhibition of IGF-1 protection by ethanol corresponded to a marked reduction in the phosphorylation of insulin receptor substrate 1, the binding of phosphatidylinositol 3-kinase (PI 3-kinase), and a block of IGF-1-stimulated PI 3-kinase activity. The neurotrophic response of IGF-1 was also inhibited by the PI 3-kinase inhibitor LY294002, the protein kinase C inhibitor chelerythrine chloride, and the protein kinase A inhibitor KT5720, but unaffected by the mitogen-activated protein kinase kinase inhibitor PD 98059. These data demonstrate that ethanol promotes cell death in cerebellar granule neurons by inhibiting the antiapoptotic action of IGF-1.

Notizen: Soil from a roadside ditch at Lake Waccamaw, North Carolina, was tested for cyst-forming free-living amoebae, and water from the same ditch was tested for fecal coliform bacteria. Soil samples incubated at room temperature (21-23° C) yielded Acanthamoeba polyphaga, Amoeba sp., Hyperamaeba sp., Mayorella penardi. Naegleria gruberi, Naegleria minor, and unidentified ciliates, testaceans and slime molds. Incubation at 37-39° C yielded Acanthamoeba jacobsi, Platyamoeba schaefferi, and a new amoeba to be described herein. Fecal contamination of the ditch was confirmed by a direct membrane filtration technique that yielded a mean of 1,410 + 134 bacteria/ ml. The new amoeba is assigned to the class Heterolobosea Page & Blanton, 1985 on the basis of promitotic nuclear division, and a flagellated stage with a cytostome as seen in the genus Tetramitus. Amoebae studied in hanging drop preparations were flattened and irregular as described for the genus Stachyamoeba in the family Gruberellidae but changed to a lobose cylindrical form as described for species of Vahlkampfia in the family Vahlkampfiidae. A new genus and species, Learamoeba waccamawensis, is proposed for the amoeba described herein.

Notizen: This research examines the implications of contingent-claims models for empirical research on default. We focus on the probability of default over a short horizon given the current state of the world, i.e., the conditional probability of default, which more closely resembles the estimates of empirical models. We highlight the differences between the conditional and unconditional approaches and provide guidance for empirical research by illuminating situations where the expected sign reverses over the shorter horizon or where the functional form is highly nonlinear.

Notizen: We hypothesized that pacing at two ventricular sites simultaneously would activate the myocardium more rapidly and improve ventricular function. We studied the effect of pacing at the right ventricular outflow tract (RYOT) and the RV apex (EVA) on systolic and diastolic function. In 14 patients with a reduced systolic ejection fraction 〈 40% (mean EF 32%±4%)we measured RV pressures, left ventricular pressures, EF, cardiac output, peak dP/dt, peak negative dP/dt, and the time constant of relaxation, Tau, during intrinsic rhythm, atrial pacing and DVI pacing at the RVA, the RVOT, and both RV sites combined in random order. Repeated measures analysis of variance showed no significant differences in any of these parameters. The highest absolute values of dP/dt were observed during sinus rhythm and the lowest with RVA pacing. This parameter tended to improve progressively with pacing in the RVOT and at both sites. Peak negative dP/dt showed a similar nonsignificant trend. Conclusion: These data suggest that in patients with poor LV function, there may be subtle improvements in diastolic and systolic function with pacing in the RVOT and at combined sites in the RV compared to traditional RVA pacing.

Notizen: Objective: The age at which a child becomes colonized with mutans streptococci (MS) is important for understanding early childhood caries. The aim of this study was to explore the relationship of age with MS colonization in infants. Methods: Inner-city children (n=149) between the ages of 8 months and 15 months, inclusive, who reportedly were still using a baby bottle, were sampled for MS. Results: Evidence of MS colonization was seen as early at 10 months of age. For children 12 months old or younger (n=80), 25 percent had detectable levels of MS; in the 15-month age group, 60 percent were colonized. Conclusion: This study suggests that prevention of MS colonization in some populations may need to be initiated prior to the child's first birthday.

Notizen: This study tested hypotheses from social comparison theory about adolescent condom use. Questionnaires were administered to 457 twelfth-grade students (284 sexually active). Three different operationalizations of social comparison were used—comparative ratings, affiliative preferences, and derogation; each produced different results. Low condom users who felt threatened by AIDS made more downward comparative ratings regarding condom use than did unthreatened low users. In contrast, low users preferred upward affiliations with high users. Adolescents with higher self-esteem derogated adolescents with AIDS less as threat increased. Adolescents with lower self-esteem who made downward comparative ratings were more satisfied with their own past condom use. Satisfied low condom users were less likely to intend condom use in the future. The results provide evidence that downward comparison may act indirectly to deter behavioral change.