Showing papers on "Excimer published in 1988"

UV excimer radiation from dielectric-barrier discharges

Abstract: Dielectric-barrier (silent) discharges are ideally suited for efficient excitation of high-intensity UV radiation from excimers. The molecular continuum of xenon at 172 nm could be obtained with an efficiency of close to 10%. Model calculations for excimer formation and UV efficiencies in such discharges are presented. The possibility of obtaining many other wavelengths (e.g. KrF* at 248 nm and XeCl* at 308 nm) and the variety of conceivable geometries (plane or cylindrical) makes this new UV source an attractive choice for many photophysical and photochemical applications.

Two‐photon processes in defect formation by excimer lasers in synthetic silica glass

Abstract: Defect formation in dehydrated silica glasses was investigated using various excimer lasers with different energies. The ArF laser (6.4 eV) generates the E’ center more effectively than the KrF laser (5.0 eV), while the XeCl laser (4.0 eV) generated no centers. Defect generation was found to be proportional to the square of the laser photon density, indicating that it occurs dominantly due to a two‐photon process which makes band‐to‐band excitation possible. The E’ center probably originated from oxygen‐deficient centers. Contributions to the E’ center formation from a process involving direct absorption at the sites of intrinsic defects in SiO2 glass were discussed on the basis of the excitation energy dependence and a comparison with the effect of a low‐pressure mercury lamp.

Side effects in excimer corneal surgery. Corneal thermal gradients.

Abstract: Thermal loading is a significant side effect during excimer laser surgery of the cornea The maximal temperature rise, which is dependent on fluence and repetition rate of the radiation, amounts to 20° C in excised corneas and 75° C in the in situ cornea The temperature decreases exponentially with increasing distance from the incision with a half-value distance of 610 ± 100 μm Exposure parameters are recommended to avoid temperature rises greater than 11° C

Dependence of the damage and transmission properties of fused silica fibers on the excimer laser wavelength.

Abstract: The damage and transmission properties of selected commercially available fused silica fibers have been measured as a function of excimer laser wavelength. Two-photon absorption and color center formation in fused silica currently limit the use of these fibers at the excimer wavelengths of 193 nm (ArF) and 248 nm (KrF).

Two-photon absorption at 248 nm in ultraviolet window materials

Abstract: Two-photon-absorption coefficients are measured for fused silica, LiF, and the alkali earth halides BaF2, SrF2, CaF2, and MgF2, using 07-psec, 248-nm pulses with intensities in the range (05–30) × 1010 W/cm2 The consequences of these measurements in the design of high-brightness excimer lasers are discussed

Side effects in excimer corneal surgery. DNA damage as a result of 193 nm excimer laser radiation.

Abstract: UV radiation is known to cause actinic damage to the DNA. Excimer laser light, possibly used for keratorefractive surgery, should not produce this damage, as the penetration depth is far less than the diameter of a cell. However, photoreactivation experiments with yeast cells show a significant amount of DNA repair after excimer irradiation. The zone of influence of a small slitlike exposure has a diameter of 2 cm. Consequently, the limbus, the critical location of epithelial neoplasia, always lies within the sphere of actinic damage. Radiation damage is induced by secondary radiation rather than by direct interaction.

Attaining the wavelength range 189–197 nm by frequency mixing inβ-BaB2O4

Abstract: Wavelengths in the range from 188.9 to 197 nm have been obtained by type-I sum frequency generation (SFG) in β-BaB2O4. The fundamental beams were supplied by pulsed dye lasers one of which tuned between 780 and 950 nm and the other frequency-doubled at 497 nm. The possibility of shifting the excimer wavelength 248.5 nm to the excimer wavelength 193 nm has been demonstrated, replacing the frequency-doubled dye laser by KrF excimer lasers of different beam properties.

Novel method for measuring excimer laser ablation thresholds of polymers

Abstract: Conical features produced on excimer laser ablated polymers are shown to provide a sensitive technique for determining ablation thresholds. This has been applied to polyimide, polyethylene terephthalate, polyethylene, and nylon 66 ablated using the 157 nm vacuum ultraviolet F2 laser.

Mobility of pyrene on chemically modified silica surfaces

Abstract: Pyrene was adsorbed on three different chemically modified silicas, RP-2, RP-8, and RP-18, commonly used in liquid chromatography separations. The mobility of pyrene on the surfaces and its dependence of temperature was studied by measuring the time dependence of pyrene excimer formation with a fluorescence method. The rate constant for excimer formation is 1.2 x 10/sup 8/ s/sup -1/ M/sup -1/ at room temperature on RP-18. The apparent activation energy for diffusion E/sub d/, of pyrene on RP-18 is approx. 20 kJ/mol in the temperature range 290-325 K. For lower temperatures E/sub d/ increases rapidly, which indicates a phase transition in the temperature range 273-290 K. The values for k/sub DM/ and E/sub d/ at room temperature are consistent and show that the surface of RP-18 has liquid-like properties. The authors found that pyrene excimer formation is a dynamic process on RP-2 and RP-8 as well. In these cases the evaluation of the different rate constants is more complex, mainly due to differences in the microenvironment for different pyrene molecules.

Half‐collision studies of the Hg–NH3 excimer

Abstract: The Hg–NH3 complex has been studied by forming the complex in a supersonic jet and probing the bound‐to‐bound transitions to the two excited electronic states correlated to Hg(6 3P1)+NH3. Laser‐induced fluorescence and action spectroscopy have been combined with isotopic studies to map out the characteristics of these states. Both excited states are found to be bound by more than 5000 cm−1, over 20 times greater than the ground state binding energy. Extensive vibrational structure is found and interpreted in terms of a stretching progression of the Hg–NH3 bond and bending of the NH3 moiety with respect to the mercury atom. The two states show striking differences in their behavior with respect to predissociation to Hg(6 3P0). The B state is not observed in fluorescence, but predissociates efficiently to Hg(3P0)+NH3, while the A state shows predominant fluorescence with only a minor amount of Hg(6 3P0) formation. Rotational band contour analysis has been used to assign the B state as the 3E and the A s...

The effect of the nuclear motion on the electronic decay of core hole excited states in molecules

Abstract: We have measured the vibrational fine structure in the electronic decay spectrum of the N 1s→1πg core electron excited state in molecular nitrogen. The Franck–Condon analysis of the vibrational substructure shows that within the 10−14 s lifetime of the core hole excited state the molecule adjusts to the excited state configuration, characterized by a change in bond length by 0.071 A. Interference effects due to the short lifetime of the intermediate state play only a minor role.

Non-conjugated bichromophoric systems. Part 4. Synthesis and photochemical study of bis-9-anthryls with a four-membered chain; influence of the replacement of methylene links by oxygen atoms or dimethylsilyl groups on the formation of intramolecular excimers and photocyclomers

Abstract: Bichromophoric systems (ICC, IOO, IO1, IO2, and ISic) incorporating various four-membered links were prepared in order to study the influence of the nature of the linkages upon the photophysical and photochemical properties; these systems are bisanthracenes A–Z–A where A = 9-anthryl and Z =(CH2)4(ICC), O(CH2)2O (IOO), O(CH2)3(IO1), CH2O(CH2)2(IO2), and SiMe2(CH2)2SiMe2(ISiC). By irradiation at high dilution, each of these compounds yields a single photocyclomer PXX linked through the 9,10 positions of one ring and the 9′,10′ positions of the other, except for ISic where the bonds were formed between the 9,10 and 1′,4′ positions. The reaction quantum yields were measured at 365 nm in methylcyclohexane. The fluorescence was examined in the same solvent under stationary conditions at different temperatures and by transient kinetic analysis (single-photon counting). Three of these compounds (ICC, IOO, IO1) did not display any intramolecular excimer fluorescence; a thermodynamic study indicates that the non-radiative decay of the S1 excited state leads essentially (φi.s.c. being very small) to the photoproduct PXX and the starting material IXXvia a minimum of the potential surface (pericyclic minimum) with respective partition coefficients α and (1 –α): α= 0.13 (ICC), 0.28 (IOO), 0.27 (IO1). This shows the influence of the replacement of a CH2 group by oxygen. The other two derivatives (IO2 and ISiC) exhibit locally excited and excimer fluorescence, the latter being suggestive of large overlap between the aromatic rings. The excimers were postulated to be on the reaction pathway; the first collapses readily to the photocyclomer PO2–(φR 0.28) whereas the second, due to steric strain, leads to PSiC with a very low yield (φR 8 × 10–4;kRDca. 105 s–1). It was observed that the introduction of an oxygen atom instead of a CH2 group enhances the cycloaddition efficiency but the second oxygen did not show any further improvement. The presence of the bulky SiMe2 groups helped the excimer state to be reached (rate of formation at room temperature kDMca. 109 s–1) but strongly hindered the formation of the photocyclomer (kRDca. 105 s–7). The last result clearly shows that photocyclomerization depends first on the intra-chain interactions controlling the rate of formation of an overlapping conformation and then on possibly more severe inter-linkage and chain-ring (with the so-called ‘peri’ hydrogens) repulsions, which can be a powerful brake in the last step of the build up of cycloadduct.

X-Ray production by irradiation of solid targets with sub-picosecond excimer laser pulses

Abstract: A table-top excimer laser system generating sub-ps pulses was used to irradiate solid targets at intensities of up to 4×1015 W/cm2. Soft X-ray spectra of various materials were measured. The X-ray conversion efficiencies were between 1–5%. Streak camera measurements showed instrument-limited X-ray pulse duration of a few ps.

Percutaneous, in vivo excimer laser angioplasty: results in two experimental animal models.

Abstract: Pulsed ultraviolet light from an excimer laser was successfully transmitted via conventional fused silica optical fibers and used to accomplish recanalization of stenotic or totally occluded arteries in an intact, flowing blood field of two atherosclerotic animal models. The fibers, 300-600 micron in diameter, were delivered percutaneously in wire-guided multilumen catheters and then used to transmit wavelengths of 308 or 351 nm from excimer lasers with pulse durations of 12 nsec or less. Lesions from 70-100% diameter narrowing, and 0.6 to 5.5 cm in length were successfully recanalized (less than 50% residual diameter narrowing) in eight animals, using 3-4 J/cm2/pulse, 10-50 Hz, and 48-370-sec cumulative exposure. Necropsy examination in six of the eight animals disclosed no signs of thermal injury. Perforations were observed in four of eight animals. Thus, while use of an excimer laser power source did not obviate vascular perforation as a complication of laser angioplasty, these preliminary results indicate that energies of pulsed ultraviolet light sufficient to ablate atherosclerotic plaque can be both transmitted via conventional fused silica optical fibers and used successfully within an intact, flowing blood field. It may therefore be possible to use pulsed ultraviolet light from an excimer laser to accomplish percutaneous ablation of atherosclerotic arterial obstructions in humans.

Emission from ionic cesium fluoride excimers excited by a laser-produced plasma.

Abstract: CsF vapor generated in a heat pipe was excited by a laser-produced plasma. Temporally integrated spectra and time-resolved fluorescence on the ionic excimer transition Cs(2+)F(-) ? Cs(+)F were observed at 185 nm. The pressure dependence of the fluorescence is influenced by self-absorption in the CsF vapor. The fluorescence is assigned to the Cs(2+)F(-) ((2)Sigma(1/2), B)? Cs(+)F ((2)Sigma(1/2),X) transition.

Photophysics at solid surfaces. Evidence of dimer formation and polarization of monomer and excimer fluorescences of pyrene in the adsorbed state on silica-gel surfaces

Abstract: The fluorescence, fluorescence excitation and fluorescence-polarization spectra of pyrene adsorbed on a silica gel surface activated at 473 K have been observed as a function of the amount of adsorbed pyrene, as well as the ratio of fluorescence quantum yields of excimer-like fluorescence to monomer-like fluorescence. Kinetic considerations reveal that a part of the adsorbed pyrene forms a ground-state dimer. The excited dimer rearranges its conformation during the fluorescence lifetime from which excimer-like fluorescence is emitted. The degree of polarization, P, has values other than zero. These findings indicate that the excimer formation can be accomplished by slight displacement which does not depolarize the emission on the silicagel surface during the formation of the excimer from the ground-state dimer. It is also shown that the polarization of excimer emission is along the a axis of the pyrene molecule, and the excimer is favourably formed by b-axis excitation rather than a-axis excitation.

Effects of molecular organization on photophysical behavior. Time-resolved fluorescence of a pyrene-labeled phosphatidylcholine in spread monolayers of dioleoylphosphatidylcholine

Abstract: The time-resolved behavior of 1-(6-pyreneylhexanoyl)-2-palmitoyl-sn-glycero-3-phosphocholine (1-PyPC) has been examined in spread of monolayers of dioleoylphosphatidylcholine (DOL) at the N/sub 2/-water interface as functions of concentration of 1-PyPC and of surface pressure within the layer. Both excited monomer emission at 380 nm and excimer emission at 480 nm were monitored after excitation by a 80-ps pulse at 354 nm. It was found that for first-order processes the rate constants are comparable to those reported for pyrene in cyclohexane. Rate constants for the diffusion-controlled excimer formation exhibit a small but definite dependence on surface pressure and may be used to obtain diffusion coefficients of the probe of (5-7) x 10/sup -7/ cm/sup 2//s.

Excimer Formation and Intramolecular Self-Complexation of Double-Armed γ-Cyclodextrin

Abstract: Circular dichroism and fluorescence studies of γ-cyclodextrin bearing two naphthyl moieties (1) suggest that 1 exists as two intramolecular self-complexation forms in water-rich solutions with one or two of the naphthyl moieties included in its cavity. Upon addition of (−)-borneol as a guest, 1 forms a 1 : 1 host–guest complex, exhibiting slightly diminished excimer and circular dichroism bands. Ethylene glycol converts the intramolecular complexes into a relaxed form, in which two naphthyl moieties dangle outside the cavity. (Remark: Graphics omitted.)

Formation of pyrene excimer in an association dimer of pyrene-appended γ-cyclodextrin

Abstract: Pyrene-appended γ-cyclodextrin (1) forms an association dimer, in which excimer formation between two pyrene moieties occurs.