Supersonic velocimetry in a shock tube using laser enhanced ionisation and planar laser induced fluorescence

A novel flow-tagging technique is presented which was employed to measure gas velocities in the free stream of a shock tube. This method is based on the laser spectroscopic techniques of Laser-Enhanced Ionisation (LEI) and Laser-Induced Fluorescence (LIF). The flow in the shock tube is seeded with small amounts of sodium, and LEI is used to produce a substantial depletion of neutral sodium atom concentration in a well-defined region of the flow, by using two wavelength-resonance excitation and subsequent collisional ionisation. At a specific time delay, single-laser-pulse planar LIF is utilised to produce a two-dimensional (2-D) inverse image of the depleted tagged region downstream of the flow. By measuring the displacement of the tagged region, free stream velocities in a shock tube were determined. Large variations in the concentration of sodium seeded into the flow were observed and even in the presence of these large variations accurate free-stream velocity measurements were obtained. The experimentally determined value for velocity compares very well with the predicted velocity.

Measurements of neutral atom diffusion and electron-ion recombination by laser enhanced ionisation and planar laser induced fluorescence in an air-acetylene flame

The spatial and temporal evolution of a depleted atomic distribution created by laser enhanced ionisation (LEI) was employed to determine both a diffusion coefficient for sodium (Na) and an electron (e(-)) and sodium ion recombination rate coefficient in an analytical air-C2H2 flame. A depleted distribution of neutral sodium atoms was produced in a flame by ionising approximately 80% of the irradiated sodium atoms in a well defined region using a two step LEI excitation scheme. Following depletion by ionisation, planar laser induced fluorescence (PLIF) images of the depleted region recorded the diffusion and decay of the depleted Na distribution for different depletion-probe delays. From measurements of the diffused width of the distribution, an accurate diffusion coefficient D = (1.19 +/- 0.03) x 10(-3) m(2) s(-1) for Na was determined in teh burnt gases of the flame. Measurements of the integrated fluorescence intensity in the depleted region for different depletion-probe delays were related to an increase in atomic sodium concentration caused by electron-ion recombination. At high concentrations (greater than or equal to 50 mu g ml(-1)), where the electron and ion concentrations in the depleted region were assumed equal, a recombination rate coefficient of 4.2 x 10(-9) cm(3) s(-1) was calculated. (C) 1997 Elsevier Science B.V.

Velocimetry and thermometry of supersonic flow around a cylindrical body

The supersonic flow around a cylindrical body has been studied using two optical techniques. For both sets of measurements, the cylinder was mounted from the side of the tunnel, allowing investigation of the bow shock region as well as in the wake. A new technique, laser-enhanced ionization flow tagging, was used for streamwise velocity determinations behind the body. From these measurements, it was found that the downstream velocity outside the wake was (1.90 +/- 0.06) km/s, whereas inside the wake the velocity was about 0-500 m/s in the upstream direction. Planar laser induced fluorescence of nitric oxide was employed for temperature determinations. It was established that the freestream temperature was (2120 +/- 100) K, decreasing to around (1550 +/- 400) K in the wake.

Ionic strontium fluorescence as a method for flow tagging velocimetry

A flow tagging technique based upon ionic fluorescence in strontium is investigated for applications to velocity measurements in gas flows. The method is based upon a combination of two laser based spectroscopic techniques, i.e. resonantly-enhanced ionisation and laser-induced ionic fluorescence. Strontium is first ionised and then planar laser-induced fluorescence is utilised to give 2D 'bright images' of the ionised region of the flow at a given time delay. The results show that this method can be used for velocity measurements. The velocities were measured in two types of air-acetylene flames - a slot burner and a circular burner yielding velocities of 5.1 +/- 0.1 m/s and 9.3 +/- 0.2 m/s, respectively. The feasibility of the method for the determination of velocities in faster flows than those investigated here is discussed.

Radiofrequency ablation of liver tumors - A systematic review

Objectives: To systematically review radiofrequency ablation (RFA) for treating liver tumors. Data Sources: Databases were searched in July 2003. Study Selection: Studies comparing RFA with other therapies for hepatocellular carcinoma (HCC) and colorectal liver metastases (CLM) plus selected case series for CLM. Data Extraction: One researcher used standardized data extraction tables developed before the study, and these were checked by a second researcher. Data Synthesis: For HCC, 1.3 comparative studies were included, 4 of which were randomized, controlled trials. For CLM, 13 studies were included, 2 of which were nonrandomized comparative studies and 11 that were case series. There did not seem to be any distinct differences in the complication rates between RFA and any of the other procedures for treatment of HCC. The local recurrence rate at 2 years showed a statistically significant benefit for RFA over percutaneous ethanol injection for treatment of HCC (6% vs 26%, 1 randomized, controlled trial). Local recurrence was reported to be more common after RFA than after laser-induced thermotherapy, and a higher recurrence rate and a shorter time to recurrence were dassociated with RFA compared with surgical resection (1 nonrandomized study each). For CLM, the postoperative complication rate ranged from 0% to 33% (3 case series). Survival after diagnosis was shorter in the CLM group treated with RFA than in the surgical resection group (1 nonrandomized study). The CLM local recurrence rate after RFA ranged from 4% to 55% (6 case series). Conclusions: Radiofrequency ablation may be more effective than other treatments in terms of less recurrence of HCC and may be as sale, although the evidence is scant. There was not enough evidence to determine the safety or efficacy of RFA for treatment of CLM.

Anodic-oxide-induced interdiffusion in GaAs/AlGaAs quantum wells

Enhancement of interdiffusion in GaAs/AlGaAs quantum wells due to anodic oxides was studied. Photoluminescence, transmission electron microscopy, and quantum well modeling were used to understand the effects of intermixing on the quantum well shape. Residual water in the oxide was found to increase the intermixing, though it was not the prime cause for intermixing. Injection of defects such as group III vacancies or interstitials was considered to be a driving force for the intermixing. Different current densities used in the experimental range to create anodic oxides had little effect on the intermixing. ©1998 American Institute of Physics.

Two-wave-mixing induced self-focusing in an active photorefractive oscillator

We derive a nonlinear wave equation for a signal beam which is coupled to a pump beam by two-wave-mixing in a photorefractive crystal. This equation describes self-focusing of the signal beam. We compare two-wave-mixing induced spatial self-focusing of single-pass experiments in a diffusion-type photorefractive crystal and of a photorefractive oscillator using the same crystal. We observe that the nonlinear refractive index change in the oscillator is decreased while increasing resonator losses.

Effect of quantum interference on a three-level atom driven by two laser fields

We study the effect of quantum interference on the population distribution and absorptive properties of a V-type three-level atom driven by two lasers of unequal intensities and different angular frequencies. Three coupling configurations of the lasers to the atom are analysed: (a) both lasers coupled to the same atomic transition, (b) each laser coupled to different atomic transition and (c) each laser coupled to both atomic transitions. Dressed stales for the three coupling configurations are identified, and the population distribution and absorptive properties of the weaker field are interpreted in terms of transition dipole moments and transition frequencies among these dressed states. In particular, we find that in the first two cases there is no population inversion between the bare atomic states, but the population can be trapped in a superposition of the dressed states induced by quantum interference and the stronger held. We show that the trapping of the population, which results from the cancellation of transition dipole moments, does not prevent the weaker field to be coupled to the cancelled (dark) transitions. As a result, the weaker field can be strongly amplified on transparent transitions. In the case of each laser coupled to both atomic transitions the population can be trapped in a linear superposition of the excited bare atomic states leaving the ground state unpopulated in the steady state. Moreover, we find that the absorption rate of the weaker field depends on the detuning of the strong field from the atomic resonances and the splitting between the atomic excited states. When the strong held is resonant to one of the atomic transitions a quasi-trapping effect appears in one of the dressed states. In the quasi-trapping situation all the transition dipole moments are different from zero, which allows the weaker field to be amplified on the inverted transitions. When the strong field is tuned halfway between the atomic excited states, the population is completely trapped in one of the dressed states and no amplification is found for the weaker field.

Exposure to automotive pollution increases plasma susceptibility to oxidation

Low-density lipoprotein oxidation is implicated in the development of atherosclerosis. Plasma susceptibility to oxidation may be used as a marker of low-density lipoprotein oxidation and thus predict atherosclerotic risk. In this study the authors investigated the relationship between plasma susceptibility to oxidation and exposure to automotive pollution in a group of automobile mechanics (n = 16) exposed to high levels of automotive pollution, vs. matched controls (n = 13). The authors induced plasma oxidation by a free radical initiator and they determined susceptibility to oxidation by (1) change in absorbance at 234 nm, (2) lag time to conjugated diene formation, and (3) linear slope of the oxidation curve. Mechanics had significantly higher values (mean standard error) for change in absorbance (1.60 +/- 0.05 vs. 1.36 +/- 0.05; p < .002), and slope (1.6 x 10(-3) +/- 0.1 x 10(-3) vs. 1.3 x 10(-3) +/- 0.1 x 10(-3); p < .001), compared with controls. These results indicate that regular exposure to automotive pollutants increases plasma susceptibility to oxidation and may, in the long term, increase the risk of developing atherosclerosis.

Nitrogen is a requirement for the photochemical induced 3-azabicyclo[3.3.1]nonane skeletal rearrangement!

Specific 3-azabicyclo[3.3.1]nonane derivatives undergo skeletal cleavage when subjected to light or Lewis acidic conditions affording novel heteratricycles, which is in stark contrast to 3-oxabicyclo[3.3.1]nonanes. (c) 2005 Elsevier Ltd. All rights reserved.

Nociceptive scores and endorphin-containing cells reduced by low-level laser therapy (LLLT) in inflamed paws of Wistar rat

Objective: This study aimed to investigate how local pain relief is mediated by laser therapy and how dose affects the relationship. Methods: Inflammation was induced in the hind-paws of Wistar rats. Two groups of rats received 780-nm laser therapy (Spectra-Medics Pty Ltd.) at one of two doses (2.5 and 1 J/cm(2)). One group acted as a control. Scores of nociceptive threshold were recorded using paw pressure and paw thermal threshold measures. Results: A dose of 1 J/cm(2) had no statistically significant effect on antinociceptive responses. A dose of 2.5 J/cm(2) demonstrated a statistically significant effect on paw pressure threshold (p < 0.029) compared to controls. There was no difference in paw thermal threshold responses and paw volumes at either dose. Immunohistochemistry in control animals demonstrated normal beta-endorphin containing lymphocytes in control inflamed paws but no beta-endorphin containing lymphocytes in rats that received laser at 2.5 J/cm(2). Conclusion: The results confirm previous findings that the effect of laser therapy is dose-related. The mechanism of effect may occur via a differentiated pressure-sensitive neural pathway rather than a thermal-sensitive neural pathway. The significance of the immunohistochemistry findings remains unknown.