Showing papers on "Excimer published in 1995"

Photodecomposition of anthracene on dry surfaces: products and mechanism

Abstract: The photochemistry of anthracene ( 1 ) was studied on dry surfaces of silica, CabOSil (fumes silica) and alumina (neutral) at low coverages (less than 11% of a monolayer). The adsorption of 1 onto these surfaces from cyclohexane obeys a Freundlich adsorption isotherm, demonstrating a distribution of adsorption sites for interactions between 1 and the surface. Photolysis of 1 ( λ ex =350 nm) adsorbed on silica, CabOSil or alumina, under deaerated conditions, proceeds slowly to give the anthracene-9,10-photodimer ( 2 ) as the only product. Diffuse reflectance and fluorescence spectroscopy show that a novel process, involving the formation of a stable ground state pair between two molecules of 1 takes place on all three surfaces at coverages of more than 1% of a monolayer. We propose a dimerization mechanism involving a singlet excimer formed from the stable ground state pairs. Photolysis at the solid-air interface, on the other hand, is considerably faster and proceeds to give 2 and photo-oxidation products of 1 . The primary oxidation product is anthracene-9,10-endoperoxide ( 3 ) which undergoes thermal decomposition on the surface to give 9,10-anthraquinone ( 4 ), 9,10-dihydro-9,10-dihydroxyanthracene ( 5 ), bianthronyl ( 6 ), 9-hydroxyanthrone ( 7 ) and 10,10′-dihydroxy-9,9′,10,10′-tetrahydro-9,9′-bianthryl ( 8 ) as the secondary products. Photo-oxidation is mediated by the addition of singlet molecular oxygen to ground state 1 .

Excimer laser in situ keratomileusis

Abstract: We have been using a 193-nm excimer laser under protocol to ablate the corneal stromal bed after creating a hinged corneal flap with a microkeratome in order to reduce or eliminate refractive error. Thirty-one consecutive eyes are reviewed with a preoperative mean spherical equivalent refraction of -6.25 diopters (D) (range -3.50 to -11.75 D). Preoperative mean astigmatism was +0.87 D (range +0.25 to +2.75 D). At 6 months postoperatively, the mean spherical equivalent refraction was -0.50 D (range -3.50 to +2.00 D). Mean postoperative astigmatism was 0.64 D (range 0.25 to 3.50 D). A result within 1.00 D of attempted correction was achieved in 74.2% of eyes. Uncorrected visual acuity after a single procedure was 20/40 or better in 81% of eyes. Spectacle corrected visual acuity was unchanged in 26 (84%), decreased 1/2 line in 2 (6%), and increased 2 lines in 1 (3%). The incidence of postoperative complications was minimal. Excimer laser ablation in the stromal bed in combination with a hinged corneal flap seems to offer many advantages over excimer laser surface ablation with fewer complications.

Ultraviolet-radiation-induced chemical reactions through one-and two-photon absorption processes in GeO 2 –SiO 2 glasses

Abstract: Photochemical reactions in 10 GeO(2)-90 SiO(2) glass induced by irradiation with excimer lasers (KrF, 5.0 eV, XeCl, 4.0 eV) and a Hg lamp (4.9 eV) were examined. The irradiation with excimer lasers generated two types of paramagnetic defect, an electron-trapped center associated with fourfold coordinated Ge ions and a self-trapped hole center on bridging oxygen. Taking the optical band gap (~7.1 eV) of the glass obtained in this work and the power density of laser pulses [10-90 mJ/(cm(2) pulse), pulse duration 20 ns] into account, we concluded that these centers were formed by band-to-band excitation by two-photon absorption process. On the other hand, the lamp illumination ( 16 mW/cm(2)) caused the formation of Ge E' centers from preexisting oxygen-vacancy-type defects by the one-photon absorption process. These two kinds of reaction proceed independently, depending on the power densities of UV beams, at least in our experimental condition.

Excimer laser : a new tool for precision micromachining

Abstract: The unique features of the pulsed UV light emitted by excimer lasers lead to a variety of innovative applications of this relatively new industrial laser type. The excimer laser is used for micromachining, surface treatment, microlithography and high-quality marking. The resolution of excimer laser processing is very high: typical structures of 100 μm down to 0.2 μm are produced. Moreover, typical ablation rates of about 0.1–1 μm per laser pulse also allow extremely high precision in the depth of material removal.

Dual‐laser ablation for particulate‐free film growth

Abstract: A novel dual laser ablation process that leads to particulate‐free film growth is presented. A pulsed CO2 laser and an excimer (KrF) laser have been spatially overlapped on a Y2O3 target with a temporal delay between the pulses. The particulate density of the films grown by this method are at least three orders of magnitude smaller than the particulate density of a single excimer laser ablated film of similar thickness. In addition, a time‐of‐flight ion probe study indicates a sixfold enhancement of the plume species kinetic energies under dual‐laser ablation. The degree of the plume excitation is observed to depend strongly on the delay between the laser pulses.

Surface microarchitectural design in biomedical applications: preparation of microporous polymer surfaces by an excimer laser ablation technique.

Abstract: In this report we demonstrate a microprocessing method to prepare microporous polymer films by an excimer laser ablation technique, which may enable the fabrication of functional biomedical devices such as advanced artificial organs. The irradiation of a KrF excimer laser pulses (wavelength 248 nm ; fluence 1 J/cm 2 pulse) onto several polymer films was achieved by passing a laser pulse through an optical microscope, resulting in the formation of an etched pit on the irradiated surface due to ablative photodecomposition. The number of pulses and the micropositioning of the irradiation were precisely controlled by a computer-aided control unit. Minimal ablation was observed for polyethylene with very small absorption coefficient (α) at 248 nm. For polymers which absorbed the laser photons, the etch depth increased linearly with number of pulses. The etch depth per pulse decreased with an increase in α values. An excellent structural quality, with micron-order precision of an etched pit, was found for those polymers with larger α values, such as polyimide, segmented polyurethane, and polycarbonate. © 1995 John Wiley & Sons, Inc.

Excited state relaxation channels of liquid-crystalline cyanobiphenyls and a ring-bridged model compound. Comparison of bulk and dilute solution properties

Abstract: The photophysical properties of 4-alkyl- and 4-alkoxy-4′-cyanobiphenyls with different chain lengths were compared in the dilute solution phase and bulk phases (crystalline, liquid-crystalline and isotropic) with respect to Stokes shifts and decay times. In solution, highly polar states are formed with decay times around 1 ns and subnanosecond rise times indicative of a molecular rearrangement occurring prior to emission. In the bulk phase, the major proportion of the emission derives from an excimer-type state with a red-shifted spectrum and very long decay time (10–25 ns depending on temperature and chain length). Possible excimer conformations are discussed. An excimer with a twisted geometry (twisted intramolecular charge transfer (TICT) excimer) is excluded by comparison with a bridged biphenyl system (fluorene derivative). The same compound also demonstrates that excited state intramolecular relaxation towards a 90° twist angle is unlikely. In polar solvents, relaxation occurs towards an increased planarity in accordance with the Rapp model. This conclusion is discussed in relation to results from quantum chemical calculations.

Image forming method

Abstract: PURPOSE:To improve the resolution of a projection optical system with a reflection system as a main body by exciting at least one type of gas which is selected from xenon, krypton, and argon, using a dielectric barrier discharge lamp utilizing excimer emission which is discharged from the excitation gas for exposing photo resist. CONSTITUTION:An annular inner space 1 is a discharge space, xenon, argon, krypton, or a mixture gas of krypton and chlorine are filled inside, and discharge excitation is performed by an AC power supply 6, thus obtaining excimer light from the filled gas. Since excimer light is obtained from one end face 7, a reflection film 9 of aluminum is provided on the other outer end face 8. Further, a mask means for shaping the excimer light in dot shape or straight line shape or one part of ring is provided at the end face 7 for taking out excimer light or at its front. Then. the excimer light is applied to a reduction projection exposure device with the reflection system as a main body. thus exposing photo resist through a photo mask.

Antioxidants reduce corneal light scattering after excimer keratectomy in rabbits

Abstract: Background and Objective: Excimer laser-corneal tissue interaction liberates highly reactive free radicals. Our aim was to determine if intraoperative application of antioxidants reduces the postoperative corneal light scattering by minimizing free radical-mediated cellular injury. Study design/materials and methods: Phototherapeutic keratectomy (PTK) was performed in 20 rabbit eyes. Following 40-μm epithelial ablation, rabbits were divided into two groups and received 1-minute intraoperative application of either 50% dimethyl sulfoxide and 1% superoxide dismutase or vehicle (balanced salt solution) only. This was followed by a 6-mm diameter 100-μm PTK. Corneal light scattering was measured for 18 weeks using scatterometry. A light scattering index (SI) ranging from 0–10 was calculated;SI = 1 represents normal scattering. Light microscopic examination was performed in selected corneas. Results: The mean baseline SI was similar for both groups (P-= 0.95). Corneal haze followed a pattern of gradually increasing to peak at 2–3 weeks, and regressing partially thereafter. The decrease in mean SI following antioxidant application (dimethyl sulfoxide and superoxide dismutase) as compared to control group approached significance at 3 weeks (0.1 > P > 0.05) and was highly significant at 9 weeks (P < 0.05). Conclusion: Intraoperative application of antioxidants decreases light scattering following excimer keratectomy in rabbit corneas. UV-induced free radicals may play a role in the pathogenesis of corneal light scattering following excimer laser keratectomy.

Pumping rate of electric-discharge excimer lasers

Abstract: An experimental investigation was made of the conditions of excitation of electric discharges ensuring attainment of the maximum efficiency of KrF, ArF, and XeCl excimer lasers. The influence of the degree of ultraviolet preionisation on the lasing characteristics was different for KrF and XeCl lasers. The efficiencies achieved for KrF and ArF lasers were 3.9% and 2.1%, respectively.

Intramolecular Pyrene Excimer Probing the Sol-Gel Process

Abstract: The sol-gel-xerogel process in a system consisting of tetraethyl orthosilicate (TEOS)/water/ethanol with acid catalysis was monitored, using as probes polystyrene chains labeled either at one or both ends with a 1-pyrene derivative, in very low concentrations ( <10 -6 M). The probe labeled at one end enabled us to conclude that at these concentrations pyrene emits only in the monomeric form. For the chain labeled at both ends, besides the monomeric emission, a new band centered at 470 nm was observed, which was attributed to intramolecular excimer emission. The excimer forms either by a dynamic process, in which an excited pyrene at one end of the chain encounters a ground state monomer at the other end, or by a static process, in which a ground state dimer resulting from the cyclization of the chain is excited and rearranges to the excimer. The ratios of excimer to monomer fluorescence intensities at two excitation wavelengths (345 and 360 mm) are different even at the beginning of the hydrolysis reactions, showing that some pyrene dimers are preformed in the initial solution. From the evolution of this ratio at 345 nm with time (which mainly monitors the dynamic excimer formation process), it has been possible to detect an increase in the local medium viscosity with the progress of reaction. The same ratio at 360 nm (which mainly monitors the excimer formation via the dimer) increases over time, initially slowly and afterwards very steeply, and finally attains a plateau, allowing the determination of the gel point and probably of the critical point of drying of the gel. A type I nitrogen adsorption isotherm was determined for the xerogel. Being characteristic of a microporous solid, it indicates that the average radius of the pores is around 10 A, which is lower than the radius of gyration of the polymer chain, supporting that in the xerogel the polymers are constrained in one pore, enhancing the intramolecular pyrene aggregation.

The room-temperature phosphorescence of 2-chloronaphthalene induced by inclusion complexation with 6-iodo-6-deoxy-β-cyclodextrin and the excimer fluorescence of a β-cyclodextrin-2-chloronaphthalene inclusion complex in aqueous solution

Abstract: In aqueous solution, 6-iodo-6-deoxy-β-cyclodextrin (β-CDI) forms a 1:1 inclusion complex with 2-chloronaphthalene (2CN), with a formation constant of 1400 ± 300 mol −1 dm 3 . The fluorescence of 2CN is quenched by the formation of the inclusion complex with β-CDI. In addition, in deaerated aqueous solution, the room-temperature phosphorescence of 2CN is observed from the inclusion complex. Although parent β-cyclodextrin (β-CD) also induces the room-temperature phosphorescence of 2CN on formation of an inclusion complex with this compound, β-CDI as a heavy atom perturber is more than 1.2 times as effective in enhancing the room-temperature phosphorescence than β-CD. β-CD forms an inclusion complex with 2CN, from which the excimer fluorescence of 2CN is observed. It is probable that the excimer fluorescence is due to a 2:2 β-CD-2CN inclusion complex.

Na-like autoionization states of copper ions in plasma, heated by excimer laser

Abstract: A 100 ns excimer laser-produced copper plasma was used to investigate the Na-like Cu XIX n = 4-2 satellite emission in a higher density and lower temperature regime than previously studied. Theoretical model calculations were used to identify a number of radiative transitions from autoionizing Na-like copper levels. These transitions were measured with an accuracy of 0.001 A. A comparison of these spectral measurements to spectra obtained from lower density Nd-glass laser-produced copper plsmas demonstrated the changing role of different excitation mechanisms for different plasma conditions.

Molecular Recognition in Organic Solvents. The Importance of Excimer Fluorescence Spectroscopy

Abstract: The present work clearly demonstrates the usefulness of fluorescence spectroscopy as a sensitive tool for the study of molecular recognition in solution, whenever other established experimental techniques are not applicable. Excimer fluorescence spectroscopy was used, in combination with NMR, to study the hydrogenbonding interactions between a carboxylic group and a nucleic acid base. Two different organic solvents were employed: chloroform, which can participate in H-bonding, albeit weakly, and cyclohexane, which is totally devoid of this ability. The complexes formed between the reactants were determined from the quantitative analysis of the fluorescence and NMR spectra. It was found that in these solutions the predominant H-bonding interactions iead to the formation of the three dimers, acid-base, acid-acid and base-base in order of decreasing strength. The formation of trimeric units, although reported in the literature for similar systems, was not confirmed by our study.

Spectroscopic study of rare-gas excimer formation in a direct-current discharge with supersonic expansion

Abstract: Emission spectra of the rare‐gas excimers Ar2*, Kr2*, and Xe2* were excited in a dc jet discharge with supersonic expansion. Absorption and fluorescence emission measurements provided atomic population densities for levels of the 1s manifold. Changes in intensities of the atomic resonance lines and the VUV bands were examined as the plasma was irradiated with laser radiation tuned to specific atomic transitions between levels of the 1s and 2p manifolds. This technique of optical pumping has established that excimers in the A 3Σ+u state are the main contributors to the observed bands from this source. Rate equations were developed to explain the observed intensity changes. Comparisons of the calculated and observed changes indicated that population mixing amongst levels of the 1s manifold caused by electron collisions is an important process in regulating the population in the 1s5 atomic level that leads to the formation of rare‐gas excimers by collisions with ground level atoms.

Polystyrene cyclization under high hydrostatic pressure

Abstract: Hydrostatic pressure effects on the cyclization of polystyrene chains (M n = 2600, M w /M n = 1.07 ; M n = 4500, M w /M n = 1.08), both ends labeled with 1-pyrenyl groups, were studied by measuring fluorescence spectra and pyrene monomer and excimer decay curves. The experiments were performed at room temperature in toluene (good solvent), cyclopentane (Θ solvent), and methylcyclohexane (poor solvent), covering a pressure range of 1-3500 bar. The cyclization rate constant decreases monotonically with pressure, as a result of the increase of solvent viscosity. However, changes in the quality of solvent for the polymer induced by pressure also influence chain cyclization. Activation volumes for chain cyclization compare well with solvent viscosity activation volumes when solvent quality to the polymer is pressure independent. The rate of excimer dissociation and ring-opening increases with pressure, especially in Θ and poor solvents. This effect is ascribed to the lowering of the excimer binding energy, induced by changes on the excimer conformation.

Spectroscopic study of vibrational relaxation and cooling of rare‐gas excimers formed in a direct current discharge with supersonic expansion

Abstract: Emission spectra (in the region 100–200 nm) of the excimers Ar2*, Kr2*, and Xe2* produced in a dc jet discharge with supersonic expansion were used to determine vibrational temperatures of the excimers, and to analyze the process of excimer relaxation from high vibrational levels near dissociation of the A3Σ+u(1u) states to the lowest levels. The observed narrow bandwidths of the second continua yielded population distributions with at least 80% in the v=0 levels at thermal equilibrium, for all three gases, corresponding to vibrational temperatures of 280, 95, and 82 K for Ar2*, Kr2*, and Xe2*, respectively. Changes in relative intensities of the first and second continua with pressure were monitored by optical pumping of atoms in the 1s5 metastable level [precursor of the excimer A3Σ+u(1u) state], and the resulting difference spectra were analyzed in terms of vibrational relaxation to the v∼0 levels. A simple model based on collisional relaxation was developed to calculate the VUV band spectra. Compariso...

Photodissociative excitation processes of XeF2 in the vacuum ultraviolet region 105–180 nm

Abstract: Photodissociative excitation processes of XeF2 are studied by absorption and photofragment fluorescence spectroscopies in the vacuum UV region 105–180 nm. The quantum yields are measured along with the dispersed fluorescence spectra for emissions from XeF* (B, C, and D) excimers formed from XeF2 excited by linearly polarized, monochromatized synchrotron radiation. Fluorescence anisotropy, R, of XeF* fragment emission was also measured against excitation light wavelength in the region 105–165 nm. The quantum yields for XeF* excimer formation are found to be very high; especially in the valence band region shorter than 130 nm it reaches almost unity. From the fluorescence measurements we have observed new peaks assigned to 5πu→5dR Rydberg transitions. The intense 5πu3/2→6sR bands which appear around 143 nm exhibit no fluorescence excitation bands; even fluorescence dips are observed at 143.18 and 144.25 nm whereas very weak positive fluorescence bands are observed at 141.04 and 142.11 nm. The R values for t...