Showing papers on "Excimer published in 1987"

Femtosecond UV excimer laser ablation

Abstract: Experiments on the ablation of polymethylmethacrylate (PMMA) with 300 fs uv excimer laser pulses at 248 nm are reported for the first time. With these ultrashort pulses, ablation can be done at fluences up to five times lower than the threshold fluence for 16 ns ablation of PMMA, and the surface morphology is improved, also for several other materials. A model for ablation is proposed, assuming a non-constant absorption coefficient αeff depending on the degree of incubation of the irradiated material and the intensity of the incoming excimer laser pulse. The agreement between our model and our experimental observations is excellent for 16 ns excimer laser pulses, also predicting perfectly the shape of a pulse transmitted through a thin PMMA sample under high fluence irradiation. Qualitative agreement for 300 fs excimer laser pulses is obtained so far.

High-Speed Photography of Excimer Laser Ablation of the Cornea

Abstract: • We have used laser-based highspeed photography to investigate excimer laser ablation of the cornea. Photographs of the ablation plume were obtained 500 ns to 150 μs after incidence of a 193- or 248-nm excimer laser pulse on the surface of the cornea. Ejection of material from the cornea begins on a time scale of nanoseconds and continues for 5 to 15 μs following the excimer pulse. At 193 nm the ablation plume resembles a burst of smoke, and individual particles are too small to be optically resolved with our apparatus. At 248 nm the plume resembles a spray of larger, discrete droplets. Material is ejected from the cornea at supersonic velocity but decelerates rapidly; the velocity for the first 500 ns following the excimer pulse averages 400 m/s at 193 nm. Plume size and velocity increase with increasing fluence.

Picosecond fluorescence spectroscopy on excimer formation and excitation energy transfer of pyrene in Langmuir-Blodgett monolayer films

Abstract: Molecular association of pyrene chromophores has been studied with Langmuir-Blodgett (LB) monolayer films consisting of stearic acid and small amounts of 16-( 1-pyreny1)hexadecanoic acid. Picosecond time-resolved fluorescence spectra have shown four fluorescence bands originating from (1 ) a ground-state dimer (band D with vibrational peaks at 381, 401, and 422 nm), (2) a monomer (band M with vibrational peaks at 377, 397, and 421 mm), ( 3 ) an excimer (a structureless band El with a peak at 420 nm), and (4) another type of excimer (a structureless band E2 with a peak at 470 nm). E, was assigned as a fluorescence from a two-center, sandwich-type excimer which corresponds to the well-known pyrene excimer in solution, while E, was assigned as a fluorescence from a one-center type excimer. In the initial time region (0-200 ps), the D band is dominant, and its decay is associated with a rise of the E2 band. The excimer band El forms much faster than band E,. The fluorescence decay curves of the isolated monomer (band M) are interpreted in terms of two-dimensional energy transfer and trapping, leading to the conclusion that the LB monolayer film exhibits an island structure, Le., a nonuniform distribution of guest molecules.

Cytotoxicity and Mutagenicity of Low Intensity, 248 and 193 nm Excimer Laser Radiation in Mammalian Cells

Abstract: The cytotoxicity of 193 and 248 nm excimer laser radiation was compared to that produced by a germicidal lamp (predominantly 254 nm) using Chinese hamster ovary cells (CHO), and a human diploid fibroblast line, AG-1522A Excimer laser radiation at 248 nm (35 X 10(2) w/m2) and germicidal radiation (53 X 10(-5) w/m2) caused toxicity in both cell lines, with the AG-1522A cells (D37 = 7-8 J/m2) being slightly more sensitive than the CHO cells (D37 = 11 J/m2) Incident 193 nm radiation was less cytotoxic than 248 nm to AG-1522A and CHO cells with D37 values of 18 and 85 J/m2, respectively The mutagenic potential of UV excimer radiation at 193 and 248 nm was evaluated using the hypoxanthine guanine phosphoribosyl transfer assay system with CHO cells Excimer laser radiation at 248 nm induced mutation in proportion to dose (17 X 10(-5) resistant colonies per survivor per J/m2 incident radiation) up to 14 J/m2, similar to results reported for 254 nm light However, excimer laser radiation at 193 nm did not cause mutation greater than the dark control The decreased cytotoxicity and mutagenicity of 193 nm radiation may be due to the shielding of the nucleus by cytoplasmic and membrane components or to the formation of different DNA photoproducts These differences between 193 and 248 nm radiation may be important in choosing an excimer wavelength for ablation in biological systems

Quantitative imaging of temperature fields in air using planar laser-induced fluorescence of O(2).

Abstract: Planar laser-induced fluorescence of O2 has been used to acquire quantitative, instantaneous two-dimensional images of temperature in heated air flows. O2 is excited by using a broadband ArF excimer laser at 193 nm, and the resultant fluorescence signal is converted to temperature by using a theoretical calculation of the dependence of the fluorescence on temperature. This calculation has been confirmed experimentally, and validating data are presented.

Direct observation of excimer-laser photoablation products from polymers by picosecond-uv-laser mass spectroscopy

Abstract: Excimer laser (248 nm, 15 ns) photoablation products from the polymers PMMA and polystyrene are directly observed using picosecond uv laser (290 nm, 1 ps) ionization time-of-flight mass spectroscopy. Species heavier than the monomer masses are detected, which cannot be attributed to simple direct fragments of the polymer chains, but rather show the importance of fast chemistry and/or photochemistry during the ablation process.

160-fs XeCl excimer amplifier system

Abstract: An amplifier system based on XeCl gain modules, which, generates bandwidth-limited, 160-fsec, 308-nm, 12-mJ pulses, is described. The UV seed pulses are derived from a colliding-pulse mode-locked laser system.

Time-resolved and temperature-dependent fluorescence spectra of anthracene and pyrene in crystalline and liquid states

Abstract: Excimer states of anthracene have been found not only in the cyrstalline phase at high temperatures but also in the liquid phase by observing stationary and time-resolved fluorescence spectra. Fluorescence decay times in the liquid phase are very short, 0.2-0.3 ns, without any emission wavelength dependence even at the excimer band. On the other hand, there seems to appear a separate component around the excimer band of pyrene liquid, of which the origin is tentatively assigned to a monomer emission band. The change of profiles in Raman spectra of anthracene at the transition from crystal to liquid has been observed and discussed in relation to the disordered and/or ordered structures.

High-Repetition-Rate Excimer-Based UV Laser Excitation Source Avoids Saturation in Resonance Raman Measurements of Tyrosinate and Pyrene

Abstract: UV resonance Raman excitation using low-duty-cycle pulsed lasers such as the Nd:YAG can result in photophysical processes that interfere with Raman spectral studies of ground-state species due to the high incident laser energy fluxes. Depletion of the ground state occurs due to optical absorption and due to the population of intermediate levels which have lifetimes comparable to or longer than the excitation pulse width. In addition, formation of photochemical intermediates can occur. For example, excitation in resonance with the tyrosinate La electronic transition (∼240 nm) results in formation of tyrosyl radicals which deplete the concentration of ground-state tyrosinate molecules; as a result, decreased resonance Raman intensities are observed for vibrational modes of ground-state tyrosinate. For pyrene, excitation in resonance with the S4 electronic transition results in population of the long-lived S1 state via rapid internal conversion. This long-lived state bottlenecks relaxation back to the ground state, thus causing saturation of the ground-state pyrene Raman intensities. Given similar incident average laser powers and focusing conditions, higher-duty-cycle lasers result in decreased saturation. A comparison between a 20-Hz Nd:YAG and a 200-Hz excimer laser-based UV Raman excitation source demonstrates superiority of the excimer in avoiding both Raman saturation and interferences from photochemical transients. For the identical energy flux per pulse, the accompanying tenfold increase in average energy flux for the excimer, over the YAG, results in a dramatic improvement in the spectral signal-to-noise ratios. We report the first measurement of the absolute resonance Raman cross section of pyrene within the S4 transition. The Raman cross section of 48 barns/str measured for the 1632-cm−1 vibration with 240-nm excitation is the largest observed to date.

Excimer fluorescence determination of solid-liquid interfacial pyrene-labeled poly(acrylic acid) conformations

Abstract: The conformations of a pyrene-labeled poly(acrylic acid) adsorbed at the water-solid interface were studied by using fluorescence of the pyrene excimer as an indication of the presence or absence of polymer strand coiling. It is found that adsorption from aqueous solution onto positively charged alumina is rapid and that the conformation of the polymer at the surface reflects the state of the polymer in solution prior to adsorption. In the case of high pH adsorption, the extended polymer (low excimer contribution) bound to the surface is not affected by lowering the pH of the solution. In contrast, when the polymer is initially adsorbed in coiled form (low pH), raising the pH of the solution partially leads to extension of the polymer on the surface. Binding to positively charged silica leads to a more extended conformation of the polymer which is relatively unaffected by the pH of the solution; that is, adsorption at high pH followed by a decrease in pH does not change the relative amount of excimer present.

Excimer formation in pyrene-labeled hydroxypropyl cellulose in water: picosecond fluorescence studies

Abstract: Excimer formation of pyrene chromophores in an aqueous solution of hydroxypropyl cellulose has been studied with a picosecond time-resolved fluorescence spctrophotometer. The time-resolved spectra show two types of broad structureless fluorescence bands at (1) 420 nm in a short time region (0-7 ns) and (2) 470 nm in the time region after 50 ns. The latter is the well-known sandwich-type pyrene excimer, and the other is assumed to be a one-center-type excimer similar to those observed in several other molecular assemblies such as LB films, vapor-deposited films, and single crystals. This is the first report on the detection of such excimers in a dilute polymer solution.

Measurement of 248-nm, subpicosecond pulse durations by two-photon fluorescence of xenon excimers.

Abstract: A technique for measuring the duration of single ultrashort pulses at KrF* wavelengths has been developed that employs fluorescence from xenon excimers excited by two-photon absorption of atom pairs. Pulses of ~350-fsec duration have been measured at 248 nm.

Gas chromatographic-light microscopic correlative analysis of excimer laser photoablation of cardiovascular tissues: evidence for a thermal mechanism.

Abstract: The present series of experiments used gas chromatography to identify vapor-phase photoproducts liberated during excimer laser irradiation of cardiovascular tissues in air and blood. In air, laser beams produced from ArF (193 nm) and XeF (351 nm) excimer laser gas mixtures were delivered to samples of myocardium and atherosclerotic coronary arterial segments through the wall of a quartz cell, using 8-40 mJ/pulse. In blood, 351 nm were delivered via an optical fiber, using 14 mJ/pulse. When the experiments were performed using an air-tissue interface, the dominant photoproducts identified in order of elution from the gas chromatographic column were methane, acetylene, ethylene, ethane, propyne, allene, propylene, propane, and butene. When a fiberoptic was used to accomplish 351-nm excimer laser tissue ablation in a blood field, a similar gas chromatographic spectral distribution was observed. These vapor-phase photoproducts are indistinguishable from those observed following continuous wave laser irradiation or flame torching of cardiovascular tissues. Thus, despite the fact that excimer laser ablation of cardiovascular tissues is characterized by the absence of signs of thermal injury, the results of these experiments suggest that the predominant mechanism of excimer ablation is, like continuous-wave laser irradiation, a thermal process.

Heavy‐ion excitation of rare‐gas excimers

Abstract: Beams of high‐energy heavy ions (Ar and U) from the UNILAC accelerator have been used to excite rare gases at pressures near 1 bar. The dominant spectroscopic feature observed in Ar, Kr, and Xe gases was molecular excimer emission at the second excimer continuum at 130, 150, and 170 nm, respectively. The excimer radiation was studied as a function of time (with respect to the excitation pulse), ion‐beam current, pressure, and excitation density. The efficiency of excimer production from heavy‐ion‐beam energy was found to be several percent. Details of spectral shape, especially the ratio of first‐to‐second continuum emission, were found to depend on pressure and exciting beam type.

Transient effects in pyrene monomer-excimer kinetics

Abstract: A new method of analysis of time-dependent rate phenomena in an excimer formation process is presented. The method uses a convolution relation between the monomer and excimer decay curves that includes the time-dependent rate coefficient. From the analysis of the decay curves the mutual diffusion coefficient ( D ) and the effective radius of interaction (R’) can be calculated. The method was applied to the intermolecular excimer formation of methyl 4-(l-pyrene)butyrate (1) in toluene between -1 6 and 22 O C . With the help of the theory of partially diffusion controlled reactions a critical distance for reactions was calculated to be 7.6 & 0.7 A. The temperature dependence of the diffusion coefficient of 1 gives an activation energy of 2.5 kcal mol-’.

Optical fiber transmission of high power excimer laser radiation

Abstract: An experimental investigation of optical fiber transmission of high power excimer laser radiation is presented. Different types of commercially available UV fiber have been tested, measuring energy handling capabilities and transmission losses of short samples at the XeCl (308-nm) and KrF (249-nm) wavelengths by using a standard excimer laser. A power density dependent damage process has been observed over 1 GW/cm(2). Fiber losses due to different radii of curvature are also reported. Experimental results have been examined to evaluate the effectiveness of excimer laser transmission through optical fibers for such medical uses as laser angioplasty, including also a comparison between the use of KrF or XeCl emission lines for this purpose. Finally, optimum excimer laser characteristics to increase the energy coupling in fibers are discussed.

Transient effects in monomer–excimer kinetics

Abstract: A theoretical model for diffusion‐controlled excimer formation in the presence of excimer dissociation is presented. We assume that the rate coefficient for excimer formation k1(t) has the form k1(t)=a+bt−1/2. The equations obtained for monomer and excimer fluorescence decays are given in terms of Laplace transforms. Numerical analysis of monomer and excimer fluorescence decays are given in order to show the influence of dissociation, diffusion, and solution concentration on fluorescence decays. It has been shown that dissociation leads to exponentialization of decay curves. Two different methods of calculation quantum yields of monomer and excimer fluorescence are given in the case of time‐dependent excimer formation and dissociation.

The paradox of thermal ablation without thermal injury

Abstract: Previous investigations have demonstrated in vitro that the excimer laser may be used to accomplish cardiovascular tissue ablation without causing thermal injury to boundary sites. Initial investigations suggested that results achieved with the excimer laser were related to photochemical breaking of molecular bonds, rather than thermal degradation. More recent investigations, however, have suggested that the mechanism of excimer tissue ablation may not be unique. Results indistiguishable from those accomplished with the excimer laser have been reproduced using both visible and infrared wavelengths. Experiments utilizing gas chromatography have indicated that the vapour-phase photoproducts liberated during excimer laser tissue ablation are indistinguishable from those observed following continuous wave laser irradiation or flame torching of cardiovascular tissue. While photoemission spectroscopic analysis has identified free radicals released in gas phase during excimer laser ablation, electron paramagnetic resonance spectroscopy has similarly identified free radicals released in solution during continuous wave laser ablation; while these two techniques may detect different types of radicals with different kinetic behaviour, these results indicate, that the finding of free radical photoproducts per se during laser ablation does not necessarily exclude a non-thermal mechanism. Finally, plasma-mediated photodisruption represents a third alternative to explain pulsed laser ablation; experiments are required, however, to define whether plasma generated during pulsed laser irradiation is central to the ablation process, or represents an epiphenomenon.

Pressure effects on the fluorescence decay of pyrene in micellar solutions

Abstract: The monomer fluorescence decay of pyrene was measured in sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium chloride (HDTCl) micellar solutions as a function of pressure from 0.1 to 203 MPa at 25 /sup 0/C by a single-photon-counting technique. The decay curves were analyzed successfully by a conventional fluorescence decay analysis for micellar solution over the pressure ranges examined. From the analysis, it was found that the rate constant (k/sub E/) for intramicellar excimer formation decreased significantly with increasing pressure in both micellar solutions, while the cmc and the aggregation number (N) for SDS and HDTCl micelles were found to be almost independent of pressure. The activation volumes for excimer formation were 19 and 16 cm/sup 3//mol for SDS and HDTCl micellar solutions, respectively. An attempt to obtain an activation volume for a nonionic detergent was unsuccessful because of the failure of the decay profile to fit the theoretical model. However, activation volumes for microviscosity were obtained by studying the pressure dependence of intramolecular excimer formation of a dinaphthylpropane probe. The diffusion process in micelles and microviscosities are discussed by comparing the activation volume for excimer formation with those for microviscosities determined by the pressure effect on intrmolecular excimer formation, and bymore » comparing the results with those reported in the literature for a dipyrenylpropane probe.« less