George Paraskevopoulos

Time‐resolved reflectivity as a probe of the dynamics of laser ablation of organic polymers

The interaction of KrF excimer laser pulses with polyimide, poly(ethylene terephthalate), polystyrene, poly(methyl methacrylate), and polyethylene has been studied by time‐resolved reflectivity. The width, fluence, and peak intensity of a KrF (248 nm) laser pulse reflected from the polymer surface was determined over a large range of incident laser fluences, 1–3000 mJ/cm2. The reflected pulse was truncated once the incident fluence exceeded a critical value (threshold), characteristic for each polymer. Above the threshold the pulse decayed exponentially with time constants ranging from 2 to 6 ns. For polyethylene, such a critical value appears to lie above the highest fluence available in the present experiments. The threshold fluences for pulse truncation are usually somewhat lower than the ablation thresholds measured photoacoustically. The mechanism of the process is discussed in terms of time‐dependent absorption and scattering from particles emerging from the surface during ablation. Approximate calc...

Effect of optical pulse duration on the XeCl laser ablation of polymers and biological tissue

Photoacoustic spectroscopy was used to measure the pulse duration dependence of the XeCl laser ablation of polyimide, polyethylene terephthalate, and post‐mortem human aorta. It was observed that the ablation threshold exhibited only a weak dependence on pulse duration. Photoablation etch depth measurements of polyimide as a function of XeCl laser fluence indicated that over a practical etch depth range of 0.1 to 1 μm per laser pulse the etch depth was independent of the pulse duration.

Reaction of the Excited Oxygen Atoms (O1D2) with Isobutane

Reaction of O1D2 atoms, formed by photolysis of N2O at 2139 A, with isobutane has been studied in the gas phase at room temperature and at total pressures between 80 and 1250 torr. The major features of the reaction are similar to those observed previously in this Laboratory with propane. The excited atoms insert indiscriminately into the primary and tertiary CH bonds of isobutane. The “hot” butanols formed by insertion are stabilized at pressures much lower than those required for stabilization of “hot” propanols. The lifetimes of the “hot” butanols have been determined. The reaction mechanism is discussed.

The pressure dependence of the rate constant of the reaction of OH radicals with CO

Absolute rate constants for the reaction OH+CO+M have been determined in the gas phase, at 298 K, in the pressure range 20–700 Torr of He, N2, CF4, and SF6 and 0–20 Torr of H2O. Hydroxyl radicals were generated by flash photolysis of H2O vapor in the vacuum UV, and monitored by time‐resolved resonance absorption at 308.2 nm [OH (A 2Σ+→X 2Π)]. The second‐order rate constant was found to depend on the pressure and identity of the third body M. Approximate relative efficiencies of the gases as third bodies were estimated to be: H2O: SF6: CF4: N2: He≊ 1.0: 0.5: 0.3: 0.1: 0.02. In the presence of small amounts of O2 the OH decay became nonexponential and slower. Computer simulation of our system indicated that the effects of O2 are adequately explained by reactions of HO2 radicals formed in the presence of O2. A mechanism of the reaction which is consistent with the pressure effect is discussed. The values of the rate constant obtained with M=N2 are recommended for use in modeling atmospheric processes.

Excimer lasers in cardiovascular surgery: Ablation products and photoacoustic spectrum of the arterial wall

Photoacoustic spectra of normal artery wall and of atherosclerotic plaque are reported. Threshold fluences for ablative formation of gaseous products for each excimer laser line were calculated from the photoacoustic spectrum and the measured threshold for the KrF laser.

The effect of debris formation on the morphology of excimer laser ablated polymers

A study of the distribution of the debris formed by the XeCl laser ablation of polyimide and polyethylene terephthalate and the KrCl laser ablation of polyimide shows that it is the redeposition of debris rather than impurities which accounts for the cone structures commonly seen in the surface morphology.

Reaction of the Excited Oxygen Atoms O(1D2) with Neopentane

The reaction of O(1D2) atoms, formed by photolysis of N2O at 2139 A, with neopentane, has been studied at room temperature in the gas phase at total pressures from 5 to 1300 torr. The main features of the reaction are similar to those observed previously in this Laboratory with propane and isobutane. The reaction proceeds mainly along two paths: (a) insertion of O(1D2) into the CH bonds of neopentane to form vibrationally excited (“hot”) neopentanol accounts for about 66% of the total reaction, and (b) abstraction of hydrogen from neopentane to form neopentyl radicals and OH accounts for about 25% of the reaction. The “hot” neopentanol is stabilized at pressures much lower than those required for stabilization of the “hot” butanols. The mean lifetime of the “hot” neopentanol has been determined and is found to be 4.2 × 10−9 sec. The reaction mechanism is discussed.

The question of a pressure effect in the reaction OH + CO at room temperature☆

Abstract Absolute rate constants for the reaction of OH with CO were determined at 296 ± 2 K, with 50 torr of He and 0–350 torr of SF6. The rate constant was found to change from ≈1.0 × 1011 to ≈1.9 × 1011 cm3 mol−1 s−1 depending on the pressure and nature of the third body M, in agreement with our earlier results and with the three studies by Heicklen, Cox, Calvert, and their co-workers. However, it is not possible, at present, to attribute the effect with certainty to any particular cause.

Comparison of theoretical models of laser ablation of polyimide with experimental results

Abstract The results of detailed calculations of the etch rates for pulsed UV laser ablation of polyimide at 308 nm using several proposed theoretical models of the process are compared with experimental results. The theories which indicate that the etch depth per pulse is dependent on the fluence, and not the peak intensity of the laser pulse, are consistent with the experimental data for laser pulse widths of 7–500 ns. The implication of the results on various aspects of the models is discussed.

XeCl laser ablation of polyimide: Influence of ambient atmosphere on particulate and gaseous products

The gaseous and particulate products of the XeCl (308 nm) laser ablation of polyimide (Kapton H) are quantitatively determined and compared with the mass loss of the polymer in atmospheres of He, N2, air, or O2. In air and in pure O2, the observed mass balance is about 90%, but is lower for inert atmospheres. With increasing oxygen content in the atmosphere, the yield of CO2 increases at the expense of particulates and acetylene. The influence of laser fluence and nature of the ambient atmosphere on the product distribution is interpreted in terms of ejection of small reactive species which are involved in the competitive reactions of particulate formation and oxidation to CO2.