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At least 991 records · Page 55Linked to original sources
PubMed1999

Relation of shear viscosity and self-diffusion coefficient for simple liquids.

A Stokes-Einstein-type relation is derived for the potential part of the shear viscosity of a simple liquid by means of statistical mechanics. The product of the shear viscosity and the self-diffusion coefficient is shown to be expressible in terms of the pair correlation function and the intermolecular force as well as the density. The shear viscosity formula, consisting of kinetic and potential parts and given in terms of the self-diffusion coefficient, is tested against experimental data with regard to the temperature and density dependence of the shear viscosity. Given the self-diffusion coefficient determined by experiment or simulations, the viscosity formula involving the Stokes-Einstein relation obtained produces the shear viscosity of argon, krypton, and xenon, in good agreement with experiment in the case of temperatures well away from the triple point. However, in the neighborhood of the triple point of argon examined, a cutoff parameter, which is a measure of the range of density variation, is needed to account for the experimental data. The applicability of the Stokes-Einstein relation to molecular particles is assessed, and it is found to remain applicable in the range of density and temperature examined.

Journal Article↗
PubMed2000

Extensive experimental mapping of sonoluminescence parameter space

Experimental phase diagrams are compared to theoretical predictions based on the Dissociation Hypothesis (DH) theory of single bubble sonoluminescence (SBSL). Ambient radius (R(o))- acoustic drive pressure (P(a)) phase diagrams are presented for krypton-nitrogen mixtures and for a collection of data with other noble gas-nitrogen mixtures. The data is also presented in terms of expansion ratio (maximum bubble radius R(max)/R(o)) vs P(a) and relative light intensity vs P(a). The results provide further evidence of the validity of DH and also provide a thorough mapping of SBSL at a single frequency.

Journal Article↗
PubMed2000

Stark broadening parameters of the Kr II and Kr III spectral lines

Stark widths and shifts of 14 singly (Kr II) and 11 doubly charged (Kr III) krypton ion spectral lines have been measured in the linear, low pressure, pulsed arc at 17 000 K electron temperature and 1. 65x10(23) m(-3) electron density. The measured width and shift values have been compared to the theoretical data calculated by us by using the modified semiempirical method.

Journal Article↗
PubMed2000

X-ray and extreme ultraviolet emission induced by variable pulse-width irradiation of Ar and Kr clusters and droplets

Measurements are presented of x-ray (>1.5 keV) and extreme ultraviolet (EUV, lambda=2-44 nm) emission from argon and krypton supersonic gas jets at room (T=300 K) and cryogenic (T=173 K) temperatures irradiated with constant energy (50 mJ), variable width laser pulses ranging from 100 fs to 10 ns. Two regimes of jet operation are explored: cluster formation (radius<100 nm) and droplet formation (radius>1 &mgr;m). The results for both clusters and droplets can be understood in terms of two time scales: a short time scale for optimal resonant absorption at the critical density layer in the expanding plasma, and a longer time scale for the plasma to drop below critical density.

Journal Article↗
PubMed2004

Phase-matching solutions for high-order harmonic generation in hollow-core photonic-crystal fibers.

Hollow-core photonic-crystal fibers are shown to allow phase-matched high-order harmonic generation by an isolated guided mode of pump radiation. Regimes of phase matching are analyzed for the fundamental guided mode of pump field with a wavelength around 800 nm, generating harmonics within the wavelength range of 25-50 nm in hollow photonic-crystal fibers filled with argon, krypton, and helium. Geometric parameters of the fiber structure and the pressure of the gas filling the fiber core are shown to serve as important, often orthogonal, control knobs, allowing a fine adjustment of the phase matching for high-order harmonic generation.

Journal Article↗
PubMed2006

Master singular behavior from correlation length measurements for seven one-component fluids near their gas-liquid critical point.

We present the master (i.e., unique) behavior of the correlation length, as a function of the thermal field along the critical isochore, asymptotically close to the gas-liquid critical point of xenon, krypton, argon, helium-3, sulfur hexafluoride, carbon dioxide, and heavy water. It is remarkable that this unicity extends to the correction-to-scaling terms. The critical parameter set, which contains all the needed information to reveal the master behavior, is composed of four thermodynamic coordinates of the critical point and one adjustable parameter which accounts for quantum effects in the helium-3 case. We use a scale dilatation method applied to the relevant physical variables of the one-component fluid subclass, in analogy with the basic hypothesis of the renormalization theory. This master behavior for the correlation length satisfies hyperscaling. We finally estimate the thermal field extent where the critical crossover of the singular thermodynamic and correlation functions deviates from the theoretical crossover function obtained from field theory.

Journal Article↗
PubMed2006

Three-body interactions and solid-liquid phase equilibria: application of a molecular dynamics algorithm.

The effect of three-body interactions on the solid-liquid phase boundaries of argon, krypton, and xenon is investigated via a novel technique that combines both nonequilibrium and equilibrium molecular dynamics. The simulations involve the evaluation of two- and three-body forces using accurate two-body and three-body intermolecular potentials. The effect of three-body interactions is to substantially increase the coexistence pressure and to lower the densities of liquid and solid phases. Comparison with experiment indicates that three-body interactions are required to accurately determine the total pressure. In contrast to vapor-liquid phase equilibria, the relative contribution of three-body interactions to the freezing pressure exceeds the contribution of two-body interactions at all temperatures.

Journal Article↗
PubMed2002

Depinning of atomically thin Kr films on gold.

We have employed the quartz-crystal microbalance technique to measure the sliding friction of krypton films physisorbed on gold. By slowly increasing the amplitude of the substrate oscillations, we have observed a sharp transition from a film locked to the substrate to a sliding one. This transition is characterized by hysteresis both in dissipation and inertial mass as the amplitude is decreased. Finally, the dependence of this transition on film coverage has been studied in some detail.

Journal Article↗
PubMed2002

Free volume and density and temperature dependence of diffusion coefficients of liquid mixtures.

A simple formula for the diffusion coefficient of liquid mixtures, expressed in terms of the work necessary to create a characteristic free volume in the liquid, is presented in the spirit of the Arrhenius activation theory and tested in comparison with available experimental data. If use is made of the generic van der Waals equation of state, the free volume appearing in the formula for the diffusion coefficient can be expressed in terms of the equilibrium pair correlation functions. The theoretical values for diffusion coefficients agree excellently with experimental values with regard to the density and temperature dependence of the diffusion coefficients of argon and krypton.

Journal Article↗
PubMed2002

Ramsey-type spectroscopy with high-order harmonics.

We present an experiment where Ramsey-type spectroscopy is applied to autoionizing states of krypton by using the ninth harmonic (88 nm) of a femtosecond Ti:sapphire laser. The ionization process, detected by an electron-energy spectrometer, shows the characteristic quantum interference pattern. The behavior of the fringe contrast compares favorably with a simple essential-state model, whose parameters have been taken from the literature. The experiment shows the feasibility of high-resolution spectroscopy in the extreme ultraviolet by using high-order laser harmonics.

Journal Article↗
PubMed2002

Demonstration of a Ni-like Kr optical-field-ionization collisional soft x-ray laser at 32.8 nm.

We report the first experimental demonstration of a Ni-like optical-field ionization collisional soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 760 mJ, 30 fs, 10-Hz Ti:sapphire laser beam in a few mm cell filled with krypton. We have measured a gain coefficient of 78 cm(-1) on the 3d(9)4d 1S0-3d(9)4p(1)P1 transition at 32.8 nm, which is here amplified for the first time. This radiation source represents the shortest wavelength optical-field ionization collisional soft x-ray laser ever produced. The influence of the gas pressure and the pumping energy on the lasing output are also presented.

Journal Article↗
PubMed2003

Distortion of a spherical gaseous interface accelerated by a plane shock wave.

The evolution of a spherical gaseous interface accelerated by a plane weak shock wave has been investigated in a square cross section shock tube via a multiple exposure shadowgraph diagnostic. Different gaseous bubbles, i.e., helium, nitrogen, and krypton, were introduced in air at atmospheric pressure in order to study the Richtmyer-Meshkov instability in the spherical geometry for negative, close to zero, and positive initial density jumps across the interface. We show that the bubble distortion is strongly different for the three cases and we present the experimental velocity and volume of the developed vortical structures. We prove that at late times the bubble velocities reach constant values which are in good agreement with previous calculations. Finally, we point out that, in our flow conditions, the gaseous bubble motion and shape are mainly influenced by vorticity and aerodynamic forces.

Journal Article↗
PubMed2006

X-ray microprobe of orbital alignment in strong-field ionized atoms.

We have developed a synchrotron-based, time-resolved x-ray microprobe to investigate optical strong-field processes at intermediate intensities (10(14) - 10(15) W/cm2). This quantum-state specific probe has enabled the direct observation of orbital alignment in the residual ion produced by strong-field ionization of krypton atoms via resonant, polarized x-ray absorption. We found strong alignment to persist for a period long compared to the spin-orbit coupling time scale (6.2 fs). The observed degree of alignment can be explained by models that incorporate spin-orbit coupling. The methodology is applicable to a wide range of problems.

Journal Article↗
PubMed1989

Subretinal new vessel recurrence in age-related macular degeneration.

Laser photocoagulation with argon or krypton is currently used to treat selected patients with age-related macular degeneration and sudden visual loss from subretinal new vessel formation. In 102 consecutive cases, treated over two years, closure of the vessels was achieved without additional major visual loss in 79% when assessed at two months. Early failure occurred in 21% and was due either to selection of cases with central location or failure to close the new vessel. Later failures due to recurrent subretinal new vessel formation or progression of other macular degenerative changes has occurred in a further 28% so far. Although controlled clinical trials show a significant benefit of treatment, the overall impact of these few successes on the population with macular degeneration is small. Further understanding of the mechanism of subretinal new vessel formation is urgently required. Areas under investigation include the response of injured retinal pigment epithelium, choroid and retina and cellular production of facilitatory factors including proteases and fibronectin.

Aging↗
PubMed1991

Diode laser trabeculoplasty.

Diode laser is a solid state Gallium Aluminium Arsenide (Ga AlAs) laser with a wavelength of approximately 800 nm (near infrared radiation). It has a similar penetration to, and absorption within the ocular tissues as, Krypton red. Diode laser trabeculoplasty (DLT) was carried out on 20 glaucomatous eyes resulting in a mean fall in intraocular pressure of 6.35 mmHg at one month and 6.45 mmHg at three months. Over this early period DLT has an effect similar to argon laser trabeculoplasty (ALT) in the management of open-angle glaucoma.

Follow-Up Studies↗
PubMed1993

Laser photocoagulation treatment of choroidal melanoma.

Ten cases of choroidal melanoma were treated with Argon and/or Krypton laser photocoagulation in this retrospective study. Each patient received an average of 9.6 laser treatment sessions. Three female and seven male patients were treated and followed for an average of 69.9 months. Clinical regression was observed in seven cases (70%). Tumour size in these successfully treated cases ranged from 4.5 x 5.0 x 1.5 mm to 7.0 x 7.0 x 3.5 mm. Continued growth occurred in one case (10%), tumour recurrence which failed to respond to additional laser treatment in another (10%), and although a recurrence appeared to respond to additional laser in a third case (the largest tumour in the series 11.0 x 9.0 x 2.5 mm) extraocular extension was found at enucleation (10%). Complications of laser treatment included cystoid macular oedema (Case 5), branch retinal vein occlusion (Cases 3 and 6), vitreous haemorrhage from neovascularisation at the edge of the scar, optic atrophy and thrombotic glaucoma (Case 8), and macular involvement by laser (Cases 4, 7 and 10). All patients remain alive without any clinical evidence of metastatic disease at the end of the follow-up period. These results suggest that laser photocoagulation treatment is a useful option in the management of small choroidal melanomas. It avoids the psychological trauma of enucleation, and maintains some vision, without compromising life expectancy.

Adult↗
PubMed2005

Eph/ephrin expression in the adult rat visual system following localized retinal lesions: localized and transneuronal up-regulation in the retina and superior colliculus.

Following unilateral optic nerve section in adult PVG hooded rat, the axon guidance cue ephrin-A2 is up-regulated in caudal but not rostral superior colliculus (SC) and the EphA5 receptor is down-regulated in axotomised retinal ganglion cells (RGCs). Changes occur bilaterally despite the retino-collicular projection being mostly crossed. Here we investigate the dynamics of Eph/ephrin expression using in situ hybridization and semi-quantitative immunohistochemistry after localized retinal lesions. Unilateral krypton laser lesions to dorso-nasal retina ablated contralaterally projecting RGCs (DN group); ventro-temporal lesions ablated contralaterally and ipsilaterally projecting RGCs (VT group). Lesions of the entire retina served as controls (Total group). Results are compared to normal animals in which tectal ephrin-A2 and retinal EphA5 are expressed, respectively, as shallow ascending rostro-caudal and naso-temporal gradients. In both SCs of DN and Total groups, tectal ephrin-A2 was up-regulated caudally; in the VT group, expression remained normal bilaterally. Unilateral collicular ablation indicated that bilateral changes in ephrin-A2 expression are mediated via intercollicular pathways. EphA5 expression in the VT group was elevated in the intact nasal region of experimental retinae. For each experimental group, EphA5 expression was also elevated in nasal retina of the opposite eye, resulting in uniform expression across the naso-temporal axis. Up-regulation of ephrin-A2 in caudal, but not rostral, SC suggests the enhancement of developmental positional information as a result of injury. Bilateral increases in retinal EphA5 expression demonstrate that signals for up-regulation operate interocularly. The study demonstrates that signals regulating guidance cue expression are both localized and relayed transneuronally.

Animals↗
PubMed2002

Laser treatment of rosacea.

Rosacea is a common condition often resulting in persistent erythema and telangiectasia as well as rhinophyma in a number of patients. Over the last two decades lasers have been increasingly used in the treatment of these permanent changes. The literature is reviewed in terms of the different laser systems, side-effects and comparison with other surgical techniques. Laser studies on rosacea-associated telangiectasia and erythema are limited. Copper-bromide, krypton and KTP lasers have been used with good to excellent results. However, the most commonly applied system is the flash lamp-pumped pulsed dye laser. Rhinophyma can be treated with a variety of different surgical methods, including laser resurfacing. CO(2) lasers are the most widely used lasers, others are the Er:YAG and Nd:YAG lasers. Cosmetic end results are comparable to partial excision with a scalpel or electrosurgery. There does not appear to be an increased risk of infection or scarring, but the conventional surgical methods are quicker to perform and more cost effective.

Humans↗