Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Krypton”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 937 records · Page 52Linked to original sources
PubMed2002

Time-resolved atomic inner-shell spectroscopy.

The characteristic time constants of the relaxation dynamics of core-excited atoms have hitherto been inferred from the linewidths of electronic transitions measured by continuous-wave extreme ultraviolet or X-ray spectroscopy. Here we demonstrate that a laser-based sampling system, consisting of a few-femtosecond visible light pulse and a synchronized sub-femtosecond soft X-ray pulse, allows us to trace these dynamics directly in the time domain with attosecond resolution. We have measured a lifetime of 7.9(-0.9)(+1.0) fs of M-shell vacancies of krypton in such a pump-probe experiment.

Journal Article↗
PubMed2003

Probing molecular dynamics with attosecond resolution using correlated wave packet pairs.

Spectroscopic measurements with increasingly higher time resolution are generally thought to require increasingly shorter laser pulses, as illustrated by the recent monitoring of the decay of core-excited krypton using attosecond photon pulses. However, an alternative approach to probing ultrafast dynamic processes might be provided by entanglement, which has improved the precision of quantum optical measurements. Here we use this approach to observe the motion of a D2+ vibrational wave packet formed during the multiphoton ionization of D2 over several femtoseconds with a precision of about 200 attoseconds and 0.05 ångströms, by exploiting the correlation between the electronic and nuclear wave packets formed during the ionization event. An intense infrared laser field drives the electron wave packet, and electron recollision probes the nuclear motion. Our results show that laser pulse duration need not limit the time resolution of a spectroscopic measurement, provided the process studied involves the formation of correlated wave packets, one of which can be controlled; spatial resolution is likewise not limited to the focal spot size or laser wavelength.

Journal Article↗
PubMed2005

Enhanced atmospheric loss on protoplanets at the giant impact phase in the presence of oceans.

The atmospheric compositions of Venus and Earth differ significantly, with the venusian atmosphere containing about 50 times as much 36Ar as the atmosphere on Earth. The different effects of the solar wind on planet-forming materials for Earth and Venus have been proposed to account for some of this difference in atmospheric composition, but the cause of the compositional difference has not yet been fully resolved. Here we propose that the absence or presence of an ocean at the surface of a protoplanet during the giant impact phase could have determined its subsequent atmospheric amount and composition. Using numerical simulations, we demonstrate that the presence of an ocean significantly enhances the loss of atmosphere during a giant impact owing to two effects: evaporation of the ocean, and lower shock impedance of the ocean compared to the ground. Protoplanets near Earth's orbit are expected to have had oceans, whereas those near Venus' orbit are not, and we therefore suggest that remnants of the noble-gas rich proto-atmosphere survived on Venus, but not on Earth. Our proposed mechanism explains differences in the atmospheric contents of argon, krypton and xenon on Venus and Earth, but most of the neon must have escaped from both planets' atmospheres later to yield the observed ratio of neon to argon.

Argon↗
PubMed2003

Bilateral uveal melanoma: a series of four cases.

AIM: To describe the occurrence of bilateral primary choroidal melanoma in four patients. METHODS: All patients attending the Liverpool Ocular Oncology Centre with uveal melanoma between January 1993 and February 2002 were identified, and those with bilateral primary choroidal melanoma were reviewed. Their presentation and management are described. RESULTS: Four patients, all female, were identified. Patient 1 presented with a right juxtapapillary melanoma at the age of 64, which was treated with krypton laser and endoresection, and then when aged 73 required proton beam radiotherapy for a melanoma in her left eye. Patient 2 presented at the age of 82 with bilateral choroidal melanomas and underwent simultaneous bilateral plaque radiotherapy. Patient 3 presented with bilateral choroidal melanomas at the age of 75 and was treated initially with bilateral proton beam radiotherapy. Patient 4 was treated at the age of 54 with right plaque radiotherapy for a choroidal melanoma, and 3 years later needed plaque radiotherapy for a melanoma in her other eye. CONCLUSION: Bilateral choroidal melanoma is possible and should be a consideration in the continuing management of patients with choroidal melanoma.

Aged↗
PubMed2004

Angiographic pattern of recurrent choroidal neovascularization in age-related macular degeneration.

PURPOSE: To evaluate the angiographic characteristics of recurrent choroidal neovascularization (R-CNV) in age-related macular degeneration (AMD). METHODS: A prospective investigation on 107 consecutive patients with exudative AMD and CNV not involving the fovea was conducted. Fluorescein angiography (FA) and indocyanine green angiography (ICGA) were planned before krypton laser treatment, and after 3 weeks, 2, 3, 4, 6, 9, 12, 18, and 24 months from photocoagulation. Laser treatment was FA-guided in eyes with classic CNV, and ICGA-guided in eyes with occult CNV on FA. RESULTS: At baseline on FA, 23.3% had classic CNV, whereas, 76.6% showed occult CNV. On ICGA, CNV assumed a focal and a plaque pattern in 81.3 and 18.6% of cases, respectively. Overall, post-laser CNVs occurred in 56 eyes. FA identified well-defined and ill-defined R-CNV in 25 and 75% of cases, respectively. ICGA identified three different R-CNV patterns: focal, annular, and plaque. Focal R-CNV was defined as a single dot-like hyperfluorescence, which was detected in 69.6% of cases, with subfoveal location in half of them. Annular R-CNV was identified by a hyperfluorescent lesion, partially or completely encircling treated area, which was visible in 19.6% of cases, all with subfoveal involvement. Plaque R-CNV was defined as a hyperfluorescent lesion larger than 1 disc diameter in size, and was seen in 10.7% of cases, all with subfoveal location. CONCLUSIONS: ICGA is able to improve R-CNV visualization identifying three different R-CNV patterns. Focal R-CNV is the most frequent pattern and can be re-treated in half of the cases.

Aged↗
PubMed2006

Laser-induced chorioretinal venous anastomosis using combined lasers with different wavelengths.

AIM: To evaluate the effectiveness of combined lasers with different wavelengths in creating chorioretinal venous anastomosis (CRVA) in nonischaemic central retinal vein occlusion (CRVO). METHODS: In this retrospective, noncomparative, consecutive case series, records of all patients with a diagnosis of nonischaemic CRVO who underwent combined laser treatment to induce CRVA and who were followed-up for 6 months or longer were reviewed. RESULTS: Records of 85 patients (85 eyes) were analysed. These patients were divided into two groups. In group 1 (24 cases, 24 eyes), combined lasers with two different wavelengths (argon blue-green laser combined with Nd-YAG laser were used. In group 2 (61 cases, 61 eyes), combined lasers with three different wavelengths (argon blue-green laser combined with krypton red laser and Nd-YAG laser) were used. Functional anastomosis formed in 13 out of 24 eyes (54%) in group 1 and 47 out of 61 eyes (77%) in group 2. In patients with successful anastomosis, visual acuity improved for two or more lines in 49 out of 60 eyes (82%), with the remaining eyes undergoing no change. There were no serious complications. CONCLUSION: The use of three laser wavelengths results in the creation of laser anastomosis and the success rate may be higher than when using two wavelengths alone.

Adolescent↗
PubMed2001

Theoretical investigations of proton-bound cluster ions.

Several proton-bound cluster ions have been studied by means of coupled cluster calculations with large basis sets. Among these are complexes of a krypton or xenon atom with the cations HCO+, HN2+ and HNCH+. Various spectroscopic properties have been calculated in all cases. Effects of vibrational anharmonicity are particularly pronounced for the intramolecular stretching vibrations of Kr...HN2+ and Xe...HN2+. The proton stretching vibration of (N2)H+(N2) is predicted around 800 cm-1, with a large transition dipole moment of 1.15 D. Both (N2)H+(N2) and (HCN)H+(NCH) have linear centrosymmetric equilibrium structures. Those of (OC)H+(CO) and (HCC-)H+(CCH-) are asymmetric with barrier heights to the centrosymmetric saddle points of 382 and 2323 cm-1, respectively. The dissociation energy of the anionic complex Cl-...HCCH is calculated to be Do = 3665 cm-1, 650 cm-1 larger than the corresponding value for Br-...HCCH. The complex between a fluoride ion and acetylene is more strongly bound and shows strongly anharmonic behaviour, similar to the bihalides FHF- or ClHCl-. Strong Fermi resonance interaction is predicted between nu 3 (approximately proton stretch) and 2 nu 4 (first overtone of intermolecular stretch).

Journal Article↗
PubMed1999

Photodynamic cystoscopy for detection of bladder tumors.

The use of a photodynamic fluorescence marker for diagnosis of tumors is an intriguing concept to improve thoroughness of surgical tumor resection. Complete surgical resection of multifocal bladder tumors and flat lesions as carcinoma in situ is known to be difficult, and thus a source of recurrencies. We report on the recent experience with the intravesical application of the photosensitizer prodrug 5-aminolevulinic acid (5-ALA), which is a nontoxic physiological heme substrate. Initial results from fluorescence diagnosis using krypton laser light and recent results with a modified incoherent light source constantly showed a 25% increase in urothelial tumor detection compared with white light endoscopy. Because of the high sensitivity, the number of biopsies could be decreased constantly compared with routine random biopsies taken under white light endoscopy. The results show about 25% to 30% of cases with fluorescent lesions, which are histologically benign. 5-ALA is a promising tool for diagnosis of bladder cancer. The outcome of the initial study of 5-ALA in urology in Germany is positive and is continued by prospective multicenter clinical studies to prove the hypothesis of reduction of tumor recurrence with this method. 5-ALA-based fluorescence endoscopy is strongly recommended for further clinical testing.

Adenocarcinoma, Papillary↗
PubMed1990

[Subretinal neovascularization: evolution of cicatrix of photocoagulation in the absence of recurrence].

Eleven patients successfully treated with the argon or krypton laser for macular subretinal neovascularization were followed up in order to determine the extent of long-term enlargement of the chorioretinal scars. The mean follow-up time was 51.5 months and the mean increase in choroidal scar size 46.5%. In 10 patients the scars enlarged more toward the fovea than toward the periphery (51% as opposed to 43%). The rate of enlargement per month decreased with time after treatment. Only one patient exhibited decreased visual acuity during the follow-up period, due to invasion of the foveola by the scar.

Adult↗
PubMed1988

[Concordance for myopia and discordance for optic disk cupping in a pair of monozygotic twins].

Of a pair of monozygotic twins who are both myopic, only twin B has an optic nerve pit in her left eye. At the age of 19 her visual acuity was reduced due to macular edema. Three years later, krypton laser coagulation treatment produced an appreciable improvement. Although both twins are myopic, the anisomyopia observed illustrates the influence of environment on ocular refraction.

Adult↗
PubMed1986

[Laser scotometry in macular diseases].

Retinal sensitivity is examined by projecting the aiming beam of an argon krypton laser photocoagulator onto the fundus through a contact lens. Even at normal intensity the brightness of the projected aiming on the retina is much greater than the maximum-intensity light target in ordinary scotometry. The discrepancy between scotomata when comparing the results of ordinary and laser scotometry suggests a potential for recovery of visual function, especially in macular diseases. This examination is practicable even if there is slight opacity of the media. By setting the laser beam at sufficiently low intensity, relative scotomata can be detected.

Adult↗
PubMed1993

[Photoablation using Excimer laser irradiation--a suitable concept for microneurosurgery?].

The suitability of Excimer laser beam for microneurosurgery was investigated in an animal experimental study. Cranial bones, cortex and the nervus ischiadicus of the rat were irradiated with 193 nm argon fluoride, 248 nm krypton fluoride, 308 nm xenon chloride and 351 nm xenon fluoride. After survival times of up to 30 days microscopic and electron optic findings of laser lesions at the tissues mentioned above, were studied. By means of the Excimer laser beam high precision tissue effects without or with only low thermal damage to the surrounding tissue were produced with any desired depth of penetration or extension. In analogy with the experiences gathered from animal experiments, a possible use is discussed for the removal of bone tissue around cranial nerves or vascular structures, of epileptogenic foci, or for cutting off pathways in pain surgery (e.g. the zone of entry of the dorsal root of spinal nerves).

Animals↗
PubMed1983

[Results of lung perfusion and lung ventilation scintigraphy following surgery of diaphragmatic defects].

To elucidate the question whether congenital hernias of the diaphragm and diaphragmatic defects are often combined with primary hypoplasia of the pulmonary vascular bed, we performed lung scintigraphy in twenty children after operation of a diaphragmatic defect. We found in 66% of the cases persisting perfusion disorders of the operated side. However, this does not prove primary vascular hypoplasia, because a postoperative ventilation disorder with secondary perfusion disorder cannot be excluded. Ventilation scintigraphy with krypton-81m showed good results in all cases, so that the combination of good ventilation and restricted perfusion indicates primary vascular hypoplasia.

Age Factors↗
PubMed1983

Postural effects on gas exchange in infants.

In adults with unilateral lung disease, pulmonary gas exchange is better when the patients is positioned with the good lung dependent. We studied the effects of body position on gas exchange in 10 infants with unilateral lung disease by measuring transcutaneous oxygen and carbon dioxide pressures in the supine and right and left lateral positions. We also performed krypton lung scans and measured changes in thoracic gas volumes in four of the infants. Transcutaneous oxygen pressure (mean +/- S.E.) was greater with the good lung uppermost (82 +/- 7.6 mm Hg) than with the good lung dependent (73 +/- 7 mm Hg) (P less than 0.02) or in the supine position (78 +/- 7 mm Hg). There were no changes in transcutaneous carbon dioxide pressure. The proportion of ventilation to the good lung was greater with the good lung uppermost than with the good lung dependent (P less than 0.01) or in the supine position (P less than 0.02) (64 +/- 3, 46 +/- 6, and 59 +/- 7 per cent, respectively). There were no significant changes with position in functional residual capacity, tidal volume, or dynamic lung compliance. We conclude that oxygenation in infants with unilateral lung disease is best with the good lung uppermost--the reverse of what has been observed in adults.

Carbon Dioxide↗
PubMed2004

Design and application of a multicoefficient correlation method for dispersion interactions.

A new multicoefficient correlation method (MCCM) is presented for the determination of accurate van der Waals interactions. The method utilizes a novel parametrization strategy that simultaneously fits to very high-level binding, Hartree-Fock and correlation energies of homo- and heteronuclear rare gas dimers of He, Ne, and Ar. The decomposition of the energy into Hartree-Fock and correlation components leads to a more transferable model. The method is applied to the krypton dimer system, rare gas-water interactions, and three-body interactions of rare gas trimers He3, Ne3, and Ar3. For the latter, a very high-level method that corrects the rare-gas two-body interactions to the total binding energy is introduced. A comparison with high-level CCSD(T) calculations using large basis sets demonstrates the MCCM method is transferable to a variety of systems not considered in the parametrization. The method allows dispersion interactions of larger systems to be studied reliably at a fraction of the computational cost, and offers a new tool for applications to rare-gas clusters, and the development of dispersion parameters for molecular simulation force fields and new semiempirical quantum models.

Algorithms↗
PubMed2004

On combining molecular dynamics and stochastic dynamics simulations to compute reaction rates in liquids.

An approach that combines molecular dynamics and stochastic dynamics calculations for obtaining reaction rates in liquids is investigated by studying the cis-->trans isomerization of HONO in liquid krypton. The isomerization rates are computed for several liquid densities by employing full-dimensional molecular-dynamics simulations. The rates are also computed by employing the stochastic dynamics method for a wide range of collision frequencies. Comparisons of the two sets of the computed rates show that for a wide range of liquid densities there is a simple linear relation between the liquid density rho and the collision frequency alpha, that is, alpha=crho. This suggests that once the constant c is determined from a molecular-dynamics calculation at a single density, the reaction rates can be obtained from stochastic dynamics calculations for the entire range of liquid densities where alpha=crho holds. The applicability of the combined molecular dynamics and stochastic dynamics approach provides a practical means for obtaining rate constants at considerable savings of computer time compared to that required by using full-dimensional molecular-dynamics simulations alone.

Journal Article↗
PubMed2004

Vibrational relaxation in methyl hydrocarbons at high temperatures: propane, isobutene, isobutane, neopentane, and toluene.

Vibrational relaxation has been seen in shock waves in propane, isobutene, isobutane, neopentane, and toluene dilute in krypton with the laser-schlieren technique. These experiments cover about 600-2200 K and post-shock pressures from 5 to 29 Torr. The process cannot be resolved in any for T<600 K, or in any for large molecule fraction. All the ultrasonic measurements of relaxation in these at room temperature show characteristic times in the 1-5 ns atm range, corresponding to fewer than five collisions, whereas the relaxation times in the shock waves range from 20 to 200 ns atm, with a clearly defined negative or "inverted" temperature dependence. It would seem the observed slowdown of relaxation with increasing T is simply a consequence of the much increased energy transfer required at high temperature in such large polyatomics when this is combined with a collision efficiency, here interpreted as down, already so large it cannot much increase. The simple method for the extraction of a down from relaxation data offered here by consideration of the energy relaxation equation for Evib=0 appears to be original and should prove quite useful in connecting thermal relaxation data to values obtained from spectroscopy and master-equation analyses. Here it is found that the derived down extrapolate well to room temperature ultrasonic measurements, showing a slight increase with temperature.

Journal Article↗
PubMed2005

A femtosecond study of the infrared-driven cis-trans isomerization of nitrous acid (HONO).

We investigate the dynamics and mechanism of the IR-driven cis-trans isomerization of nitrous acid (HONO) in a low-temperature krypton matrix applying ultrafast time resolved IR spectroscopy. After excitation of the OH-stretching mode the trans HONO state decays biexponentially on a 8 and 260 ps time scale. The initially excited cis HONO state decays on a 20 ps time scale. Cis HONO isomerizes with 10% quantum yield on a 20 ps time scale to trans HONO. The quantum yield we observe is significantly smaller than the previously reported 100%, which could imply that additional, much slower reaction channels exist. We furthermore developed a four-dimensional model of the system, which includes the three proton intramolecular degrees of freedom of HONO fully quantum mechanically and one intermolecular translational degree of freedom of the molecule in the crystal cage. We find that cis-trans isomerization necessarily is accompanied by a translation of the molecule as a whole in the crystal cage. The translational degree of freedom tunes the intramolecular proton states of HONO with respect to each other. When resonances occur, the proton states might couple and transfer population. We suggest a possible reaction pathway, where the cis OH-stretch excited state first couples to a high cis torsional mode, which then may transfer almost instantaneously to the trans side. The model qualitatively explains all experimental observations.

Journal Article↗