Showing papers on "Excimer published in 1992"

Nanosecond and femtosecond excimer laser ablation of fused silica

Abstract: Ablation of fused silica using standard excimer lasers (20–30 ns pulse duration at 193, 248, and 308 nm) and a short pulse laser system (500 fs at 248 nm) is reported. Ablation rates range from several hundred nm/pulse (193 nm or fs-laser) up to about 6 μm/pulse (308 nm). The performance of the ablation is found to depend not only on wavelength and pulse duration but also on the existing or laser induced surface quality (e.g., roughness) of the material. Special ablation phenomena are observed. At 193 nm and moderate fluence (3 J/cm2) ablation takes place at the rear side of a plate without affecting the front side, whereas at higher fluence normal ablation at the front side occurs. At 248 nm (standard excimer) the existence of two consecutive ablation phases is observed: smooth ablation at low rate is followed by explosive ablation at high rate. Using fs-pulses smooth shaped holes are formed during the first pulses, whereas high pulse numbers cause the development of a ripple structure in the ablation craters.

A fluorescent calix[4]arene as an intramolecular excimer-forming Na+ sensor in nonaqueous solution

Abstract: A new fluorescent calix[4]arene 2 has been synthesized as an intramolecular excimer-forming Na+ sensor which shows a change in fluorescent characteristics specifically upon complexation with Na+

Time-resolved fluorescence studies of the chain dynamics of naphthalene-labeled polystyrene-block-poly(methacrylic acid) micelles in aqueous media

Abstract: The fluorescence data of poly(styrene-block-methacrylic acid) micelles in 1,4-dioxane/water medium were compared to that of micelles formed from the polystyrene-block-poly(tert-butyl methacrylate) copolymers in 1,4-dioxane/methanol solvent mixtures. It was found that intramolecular excimer formation is sterically hindered in the micellar cdores as they become more compact. Both steady-state and time-resolved excimer fluorescence are significantly influenced by the gradual collapse of the micellar cores and the increase in segment density within the cores with an increasing content of water

Steady-state and time-resolved fluorescence investigations of pyrene excimer formation in supercritical CO2

Abstract: Detailed studies on the formation of pyrene excimer in supercritical CO{sub 2} are reported The photophysics of pyrene are investigated as a function of temperature and fluid density Over the broad density range studied, there is no evidence for ground-state (solute-solute) interaction Comparison is made between excimer formation in supercritical CO{sub 2} and ground-state dimerization of pyrene in {gamma}-cyclodextrin ({gamma}-CD) Time-resolved fluorescence spectroscopy is used to recover the individual rate terms that describe the total emission process The recovered density-dependent bimolecular rates for pyrene excimer formation in supercritical CO{sub 2} follow a simple diffusion-controlled model This result parallels reports on pyrene excimer formation in normal liquid solvents Finally, the relative decrease in pyrene excimer formation, with increased fluid density, is easily explained from time-resolved experiments 73 refs, 15 figs, 3 tabs

Hyaluronic acid in the rabbit cornea after excimer laser superficial keratectomy

Abstract: Hyaluronic acid (HA) is not normally found in the corneal stroma. Rabbit corneas were examined for the presence of stromal HA after excimer laser treatment. One eye in each of 28 rabbits received a 60 microns deep superficial keratectomy with the excimer laser. After 1, 8, 21, and 60 days, the corneas were analyzed by quantitative and histochemical methods specific for HA. A statistically significant increase in the HA concentration compared to the baseline amount in the untreated fellow eye was seen at 8, 21, and 60 days. HA was visualized histochemically in the anterior stroma of the excimer-treated eyes at all times tested. The presence of HA after excimer surgery may influence the hydration, thickness, and transparency of the cornea. The reactive production of HA in the stroma may represent a nonspecific corneal tissue response to injury.

One-year results of excimer laser photorefractive keratectomy for myopia.

Abstract: BACKGROUND Excimer laser photorefractive keratectomy for the correction of myopia is presently under investigation in the United States by the Food and Drug Administration (FDA). The Phase II-B FDA study is being conducted on 75 normally sighted myopic eyes utilizing three currently available excimer lasers. This report presents the 1-year results on 12 myopic eyes treated with the VISX excimer laser system at the Ellis Eye Center at Cedars-Sinai Medical Center in Los Angeles under the Phase II-B FDA protocol. METHODS Twelve eyes of 12 patients with myopia between -1.75 and -5.00 diopters underwent 193 nm argon/fluoride excimer laser photorefractive keratectomy. The epithelium was mechanically removed, and fixation was accomplished with a suction ring which provided nitrogen flow across the corneal surface. The computer controlled corneal ablations were 5.00 mm in diameter and were accomplished with an iris diaphragm closing from large to small. RESULTS The preoperative spherical equivalent myopia was -3.50 D (SD = 1.02) and the postoperative myopia was -0.25 (SD = 0.48). Eleven of the 12 patients achieved an uncorrected visual acuity of 20/30 or better and were corrected to within +/- 0.50 D of emmetropia. All corneas demonstrated a mild reticular subepithelial haze which was barely visible at 1 year. There were no vision-threatening complications and none of the eyes experienced a loss of best corrected visual acuity. CONCLUSIONS In this small trial, the excimer laser appears to be capable of accurately changing the refractive power of the cornea for the correction of myopia with minimal side effects. Only when larger numbers of patients undergo the procedure will we be able to determine the safety and efficacy of photorefractive keratectomy as a refractive surgical procedure.

Multichromophoric cyclodextrins. 1. Synthesis of O-naphthoyl-.beta.-cyclodextrins and investigation of excimer formation and energy hopping

Abstract: The synthesis and photophysical properties of /3-cyclodextrins bearing seven 2-naphthoyloxy chromophores in specific positions, either on the primary face or the secondary face, or 14 2-naphthoyloxy chromophores, seven on each face, are reported. These multichromophoric cyclodextrins are good models for the study of excitation energy migration among chromophores in well-defined positions. The investigation was performed in dichloromethane and in a mixture of ethanol and methanol that can form a glass at low temperature. The absorption spectra show that the interactions between chromophores in the ground state are weak, whereas the fluorescence spectra reveal the existence of excimers at room temperature but not at low temperature in a rigid glass. Further evidence of excimer formation is provided by the fluorescence decays. Since excimers act as energy traps, the energy hopping process was studied in a rigid glass at low temperature by steady-state and time-resolved fluorescence depolarization techniques. The steady-state anisotropy is found to be one seventh of the theoretical limiting anisotropy 0.4, which means that excitation energy hops between chromophores with essentially randomly oriented transition moments at a rate much higher than the chromophore intrinsic decay rate. Energy hopping is indeed very fast as shown by the fluorescence anisotropy decay which is at least as fast as the apparatus time resolution (a few tens of picoseconds).

E-excimer and Y-type luminescence of perylene dimers in a Langmuir-Blodgett film at 1.5 K

Abstract: In this paper, E-excimer and Y-type luminescence of perylene dimers in a Langmuir-Blodgett film appear to represent excimer formation in sandwich dimers in the absence of crystalline order. Y-emission has a very weak temperature dependence. E-emission can be generated through the partially forbidden transition to the lowest excited dimer state. 24 refs., 6 figs.

Test of a model for reversible excimer kinetics : pyrene in cyclohexanol

Abstract: A model for reversible monomer–excimer kinetics that considers the time dependence of the excimer formation rate coefficient is proposed and tested for pyrene in cyclohexanol from 25 up to 85 °C. Simultaneous analysis of the monomer and excimer experimental decay curves allows the determination of all the relevant parameters for this reaction. The diffusion coefficient follows an Arrhenius plot with activation energy of 36±1 KJ mol−1, the encounter radius varies between 7.3 and 8.9 A, and the intrinsic rate constant for excimer formation varies between 2.4×109 and 1.1×1010 M−1 s−1. The intrinsic reciprocal lifetime of the excimer follows an Arrhenius plot with activation energy of 12±1 KJ mol−1, and the rate constant for excimer dissociation, determined for high temperatures (T>55 °C) when reversibility is important, has different values depending whether geminate pair effects are considered or not in the analysis. The binding energy of pyrene excimer obtained considering pair effects (ΔH=32±2 KJ mol−1) agrees with the published values for nonviscous solvents.

High-pressure glow discharge lamp

Abstract: A high-pressure glow discharge lamp (1) having a planar discharge vessel (2) which is sealed in a vacuumtight manner, which surrounds a discharge space (3) filled with a gas mixture which forms excimers, and whose parallel walls (4, 5) are formed from a dielectric material. The surfaces (6, 7) of the walls (4, 5) remote from the discharge space (3) are provided with planar electrodes (8, 9). At least one (5) of these walls with its associated electrode (8) is at least partly transparent to the generated radiation. The gas mixture comprises at least one of the rare gases Xe, Kr and Ar for forming an excimer and at least one of the halogens I₂, Br₂, Cl₂ and F₂. The partial pressure of the substance forming the excimer is at least 10 and at most 600 mbar in the case of Xe and/or Kr and at least 10 and at most 1000 mbar in the case of Ar. The partial pressure of the halogen is between 0,05 and 5% of the partial pressure of the substance forming the excimer. The atomic mass of the substance forming the excimer is greater than the atomic mass of the halogen. The lamp has a high radiant efficacy and can be constructed as a large-area, homogeneously emitting radiant source.

Influence of solute-fluid clustering on the photophysics of pyrene emission in supercritical C2H4 and CF3H

Abstract: In this paper, we continue our investigations of the photophysics of pyrene emission in supercritical fluids using steady-state and time-resolved fluorescence spectroscopy. Specifically, we study pyrene photophysics in sub- and supercritical C{sub 2}H{sub 4} and CF{sub 3}H. Steady-state fluorescence is utilized to determine how the local fluid density affects the ground and excited states of pyrene and to probe any potential ground-state dimerization (preassociation). Time-resolved fluorescence provides details of the mechanism of pyrene excimer formulation and the kinetics of solute-solute interactions. The temperature and density dependence of the recovered bimolecular rate constant for pyrene in supercritical C{sub 2}H{sub 4} follows that which had been reported in supercritical CO{sub 2} and liquid solvents, i.e., the excimer formation is diffusion controlled. In supercritical CF{sub 3}H this rate constant is much slower than predicted based on diffusion control arguments. These results indicate that enhanced solute-solvent interactions (clustering) strongly affect the pyrene excimer reaction in CF{sub 3}H. 53 refs., 10 figs., 4 tabs.

Expression of tenascin and fibronectin in the rabbit cornea after excimer laser surgery.

Abstract: In order to investigate the effects of excimer laser surgery on corneal wound healing, 25 rabbits underwent anterior keratectomy at a depth of 100 or 300 μm with a Meditec MEL 50 excimer laser. After various intervals the animals were killed and the cornea excised and investigated immunohistochemically for the expression of extracellular matrix (ECM) proteins, fibronectin and tenascin. Fibronectin was shown to occur earlier than tenascin, and the two also had different distribution patterns. Wound depth showed no clear effect on the localization and time of ECM protein expression. This study indicates that corneal wounds caused by excimer laser radiation and those caused by mechanical surgery differ as to healing mechanisms.

Evidence for intramolecular hydrophobic association in aqueous solution for pyrene-end-capped poly(ethylene oxide)

Abstract: Evidence is presented that in dilute aqueous solutions of a Py-PEO-Py polymer (Py=pyrene, PEO=poly(ethylene oxide)) (M n =8000, M w /M n =1.08), 7% of the polymeric molecules exist in an end-associated conformation. When excited by UV light, dilute (less than 10 -6 M) solutions of the polymer exhibit typical pyrene monomer and excimer fluorescence. The excimer fluorescence is shown to arise from intramolecular association only. part of the excimer emission arises from diffusional approach and end-to-end cyclization during the lifetime of the excited pyrene monomer. The observation is that a significant portion of the excimer emission arises from direct UV excitation of polymers that exist in an intramolecularly end-associated conformation

Photodissociation spectroscopy of excimers in naphthalene clusters

Abstract: The naphtalene excimers, produced upon selective excitation of the van der Waals trimer and tetramer, are studied by photodissociation spectroscopy with fluorescence detection These excimers absorb efficiently in the near-IR and visible regions, with a maximum at 700 nm, which results in a strong depletion of the excimer fluorescence The absorption band is assigned to a transition between the excimer states, both originating from configuration interaction of the 1 L a molecular excitation states and the charge resonance states

Photoacoustic injury and bone healing following 193nm excimer laser ablation

Abstract: The argon-fluoride excimer laser was investigated as a cutting-ablating tool for bone surgery. A total of 52 rats were divided into two experimental groups and two control groups. In one experimental group cortical bone defects were made; in another experimental group defects penetrating into the medullary space were performed. In the two control groups similar defects were achieved using water-cooled carbide burs. The rats were sacrificed on each of the 3, 7, 10, 20, 30, and 40 postoperative day. The cortical bone, the medullary space, and the extrabony tissue were examined by means of light microscopy. In both experimental groups, bone damage, represented by osteocyte destruction, extended to 1,050–1,450 μm ahead from the irradiated site, and bone healing was very much impaired. In the control groups no histological changes could be identified and bone healing appeared to be within normal limits. We believe this extensive bone damage, following 193 nm irradiation, to be a result of photoacoustic waves propagating in the bone following each pulse. In view of our results we feel that excimer lasers presently in use are not suitable for bone surgery. This problem of photoacoustic damage can be overcome in one of two ways: by designing a CW excimer laser or by reducing the pulse width to the picosecond regime. © 1992 Wiley-Liss, Inc.

The application of a Raman-shifted tunable KrF excimer laser for laser-induced fluorescence combustion diagnostics

Abstract: Four wavelength extensions have been investigated by stimulated Raman scattering in hydrogen or deuterium gas of the 248 nm fundamental output of a narrow-band tunable KrF excimer laser. They have been used to acquire laser-induced fluorescence spectra of NO and OH in flames at atmospheric pressure. NO is detected in relatively high rotational states within the γ-band electronic system at 225 nm. OH was excited at 291.5 nm in the (1–0), at 313 nm in the (1–1) and at 268.5 nm in the predissociative (3–1) band of the 2Σ–2Π electronic band system, respectively. Prospects of 2D imaging for concentration and temperature measurements in flames using these wavelength extensions are discussed.

Fluorescence and energy migration in the random copolymers of (1-naphthylmethyl)methacrylamide and sodium 2-(acrylamido)-2-methylpropanesulfonate

Abstract: Fluorescence spectra of the copolymers in organic solution showed predominant monomer fluorescence of the naphtyl (Np) residues even when the Np content in the copolymers is as high as 58 mol%. In aqueous solutions the copolymers showed a strong preference for excimer emission. The excimer intensity increased with increasing Np content. The results imply that little or no self-quenching site exists and rapid singlet energy migration occurs solely to preformed excimer sites in the copolymers with a higher Np content in aqueous solutions.

Gas chromatographic and mass spectroscopic analysis of excimer and erbium: yttrium aluminum garnet laser-ablated human cornea.

Abstract: The photoablative products from human cadaver corneas treated with lasers were investigated using gas chromatography and mass spectroscopy. The photoablations were done separately with an excimer laser (193 nm) and with an erbium: yttrium aluminum garnet (Er:YAG) laser (2.94 microns). More than 20 different types of molecules with a molecular weight of 40-400 mass units could be identified, most of which were found to be alkanes. The comparison of the two types of laser ablations showed larger fragments and fewer types of molecules present after excimer laser treatment than after Er:YAG photoablation.

Excimer formation dynamics in pure and mixed naphthalene clusters

Abstract: The excited‐state dynamics of the pure naphthalene clusters and of the mixed clusters involving two naphthalenes solvated with durene have been investigated as a function of the excess vibrational energy of the initially excited clusters All of the clusters studied exhibit excimer fluorescence, depending strongly on the excess vibrational energy and on the solvation number The isomerization rate has been measured by detecting a buildup in the excimer fluorescence and used as a probe of the cluster geometry The trimer requires an excess energy of 870 cm−1 to promote the excimer formation with a rise time of 32 ns, suggesting a structurally less favorable process The dimer and tetramer undergo rapid isomerization at much lower energies The solvent addition is found to impede excimer formation, due to geometry restriction The singly solvated cluster exhibits an excimer evolution time as slow as 60 ns

Effect of the ArF excimer laser on human enamel.

Abstract: Human enamel surface was irradiated with ArF excimer laser and examined under light microscopy and scanning electron microscopy (SEM). Enamel surface was irradiated at three different areas with different energy fluences. It is demonstrated that the ArF excimer laser causes ablation of the calcified hard enamel tissue. Ablation curves were measured. There was no significant difference found in the etch depth between the three different areas of enamel surface. The morphology of the irradiated areas seen under the SEM was found to be dependent on energy fluence. It changed with increase in energy fluence from being etched to forming a smooth, fused, glaze-like surface and then at very high energy fluences producing a rough surface. The influence of the laser irradiation was confined to the irradiated area only, with no visible heat damage to the surroundings. These results suggest that excimer laser could be applied in a controlled and defined manner for tooth enamel treatments in dentistry.