Juergen Troe

Ultraviolet laser ablation of polymers: spot size, pulse duration, and plume attenuation effects explained

A versatile model for ultraviolet (UV) laser ablation of polymers is presented, which is very successfully applied to the calculation of a variety of different properties of this process, including the influence of plume attenuation dynamics. The polymer is described as a system of chromophores with two possible electronic states. The model is based on the combination of photothermal decomposition and photodissociative bond breaking in the electronically excited state. Laser induced chemical modifications are incorporated via different absorption coefficients for the initial and for the modified polymer after absorption of UV light. Dynamic attenuation of the expanding ablation plume and heat conduction are taken into account. The results of the theoretical calculations are compared with the results of three different series of experiments performed with polyimide (PI) and polymethylmethacrylate at the excimer laser wavelength 248 nm and with PI also at 308 nm: (1) Measurement of the ablation rate as a fu...

Electron capture by polarizable dipolar targets: Numerical and analytically approximated capture probabilities.

Numerically accurate and analytically approximated partial wave probabilities for the capture of a charged particle by a stationary polarizable dipole are presented over wide ranges of collision energies. The results facilitate the analysis of electron-molecule attachment in terms of capture rates, contributions from electron-phonon coupling, and kinetic properties when metastable anions are formed.

Pressure and temperature dependence of dissociative and non-dissociative electron attachment to CF3: Experiments and kinetic modeling

The kinetics of electron attachment to CF(3) as a function of temperature (300-600 K) and pressure (0.75-2.5 Torr) were studied by variable electron and neutral density attachment mass spectrometry exploiting dissociative electron attachment to CF(3)Br as a radical source. Attachment occurs through competing dissociative (CF(3) + e(-) → CF(2) + F(-)) and non-dissociative channels (CF(3) + e(-) → CF(3)(-)). The rate constant of the dissociative channel increases strongly with temperature, while that of the non-dissociative channel decreases. The rate constant of the non-dissociative channel increases strongly with pressure, while that of the dissociative channel shows little dependence. The total rate constant of electron attachment increases with temperature and with pressure. The system is analyzed by kinetic modeling in terms of statistical theory in order to understand its properties and to extrapolate to conditions beyond those accessible in the experiment.

Activation of Methane by FeO+: Determining Reaction Pathways through Temperature-Dependent Kinetics and Statistical Modeling

The temperature dependences of the rate constants and product branching ratios for the reactions of FeO(+) with CH4 and CD4 have been measured from 123 to 700 K. The 300 K rate constants are 9.5 × 10(-11) and 5.1 × 10(-11) cm(3) s(-1) for the CH4 and CD4 reactions, respectively. At low temperatures, the Fe(+) + CH3OH/CD3OD product channel dominates, while at higher temperatures, FeOH(+)/FeOD(+) + CH3/CD3 becomes the majority channel. The data were found to connect well with previous experiments at higher translational energies. The kinetics were simulated using a statistical adiabatic channel model (vibrations are adiabatic during approach of the reactants), which reproduced the experimental data of both reactions well over the extended temperature and energy ranges. Stationary point energies along the reaction pathway determined by ab initio calculations seemed to be only approximate and were allowed to vary in the statistical model. The model shows a crossing from the ground-state sextet surface to the excited quartet surface with large efficiency, indicating that both states are involved. The reaction bottleneck for the reaction is found to be the quartet barrier, for CH4 modeled as -22 kJ mol(-1) relative to the sextet reactants. Contrary to previous rationalizations, neither less favorable spin-crossing at increased energies nor the opening of additional reaction channels is needed to explain the temperature dependence of the product branching fractions. It is found that a proper treatment of state-specific rotations is crucial. The modeled energy for the FeOH(+) + CH3 channel (-1 kJ mol(-1)) agrees with the experimental thermochemical value, while the modeled energy of the Fe(+) + CH3OH channel (-10 kJ mol(-1)) corresponds to the quartet iron product, provided that spin-switching near the products is inefficient. Alternative possibilities for spin switching during the reaction are considered. The modeling provides unique insight into the reaction mechanisms as well as energetic benchmarks for the reaction surface.

Near-UV laser ablation of doped polymers

Near UV excimer ablation of transparent polymers can be improved or even enabled by doping using organic dyes. Polymethylmethacrylate (PMMA) and polystyrene (PS) have been investigated using both photostable and photoreactive dopants. The best results with regard to etch quality and effectivity have been achieved with 1,3- diphenyltriazene, which supports the ablation process by photoeliminating nitrogen. Etch rates of 50 jim/pulse can be reached at 308 nm and 351 nra. The etch rate does not depend entirely on the low level absorption coefficient of the doped polymer.

Plasma and plume effects on UV laser ablation of polymers

Laser ablation of polyimide (PI) and polymethyl-methacrylate (PMMA) at 248 nm with pulse lengths τ ranging from 200 fs to 200 ns was investigated. The measured ablation rates increase towards long and very short pulses and show minima for pulse lengths of about 5 ps (PMMA) or 50 ps (PI). Measurements of the reflected and transmitted portion of the ablating laser pulse give additional information about the factors influencing the ablation rates. The minimum of the ablation rate coinciding with a maximum of the reflectance of the ablating pulse can be explained by a dynamic plasma reflection model: A fast build up of a dense plasma is followed by high obscuration for a brief transition time and self-regulating opacity for the rest of the pulse. This model of plasma mediated ablation leads to a τ1/4-dependence of the ablation rate at fixed fluence, which fits very well to the measured data in the range above 50 ps. In the long pulse regime (20 ns to 200 ns), the attenuation of the ablating laser pulse by the expanding ablation plume was investigated by using a small sensing hole in the sample within the ablation spot. In combination with ablation rate measurements with varying laser spot size, these results lead to the conclusion, that the three dimensional plume expansion determines the fraction of the laser pulse actually reaching the sample. Concerning plasma and plume effects at high fluence irradiation, three effects influence the ablation behavior on different time scales: The time constant for plasma formation is important on the fs-ps time scale, self regulating plasma on the ps-ns time scale, three dimensional plume expansion on the ns-µs time scale.

Direct measurement of photoisomerization lifetimes for laser-excited methylcycloheptatriene molecules

Methylcycloheptatriene is irradiated by 15 ns flashes from a frequency-quadrupled Nd–YAG laser at 265 nm. The excited molecules undergo rapid internal conversion to vibrationally excited electronic ground state molecules. The lifetime for photoisomerization of these state-selected species under collision-free conditions is measured via light absorption of the hot species and/or the hot products. The nature of these spectra is discussed. The lifetimes are compared with calculations based on thermal isomerization experiments.

Mutual Capture of Dipolar Molecules at Low and Very Low Energies. II. Numerical Study

The low-energy rate coefficients of capture of two identical dipolar polarizable rigid rotors in their lowest nonresonant (j(1) = 0 and j(2) = 0) and resonant (j(1) = 0, 1 and j(2) = 1, 0) states are calculated accurately within the close-coupling (CC) approach. The convergence of the quantum rate coefficients to their quantum-classical counterparts is studied. A comparison of the present accurate numerical with approximate analytical results (Nikitin, E. E.; Troe, J. J. Phys. Chem. A 2010, 114, 9762) indicates a good performance of the previous approach which was based on the interpolation between s-wave fly wheel quantal and all-wave classical adiabatic channel limits. The results obtained apply as well to the formation of transient molecular species in the encounter of two atoms at very low collision energy interacting via resonance dipole-dipole interaction.

Kinetic and Spectroscopic Studies of the Reaction of CF2 with H2 in Shock Waves

The reaction of CF2 with H2 was studied in shock waves by monitoring UV absorption signals. CF2 was prepared by thermal dissociation of C2F4 (or of c-C3F6). The rate constant of the reaction CF2 + H2 → CHF + HF near 2000 K was found to be close to 1011 cm3 mol-1 s-1, consistent with earlier information on the reverse reaction CHF + HF → CF2 + H2 and a modeled equilibrium constant. The kinetic studies were accompanied by spectroscopic investigations. Absorption cross sections of C2F4 between 190 and 220 nm were measured near 1000 K and compared with room-temperature values from the literature. Likewise, absorption cross sections of CF2 near 2000 K were measured between 210 and 300 nm and compared with room-temperature data. Additional superimposed absorption signals were recorded during the reaction and identified by their time dependence and by quantum-chemical calculations employing time-dependent density functional theory. A previously unknown absorption spectrum of CHF radicals near 200 nm was identified and its wavelength dependence determined. Further strong absorptions between 190 and 300 nm were attributed to CH2F and CF radicals. Absorptions at longer wavelengths, reaching up to 510 nm, were postulated to arise from C2 radicals formed at later stages of the reaction.

Modeling of velocity and surface temperature of the moving interface during laser ablation of polyimide and poly(methyl methacrylate)

A recently developed model, which is very successful in explaining pulse duration and spot size dependences of nanosecond laser ablation rates of polyimide (PI) and poly(methyl methacrylate) (PMMA) [H. Schmidt, J. Ihlemann, B. Wolff-Rottke, K. Luther, J. Troe, J. Appl. Phys., 83 (1998) 5458.], is applied to calculate the temporal behavior of position and temperature of the moving interface between the condensed phase of the polymer sample and the ablation plume, i.e., the receding sample surface, during the ablation process. The model describes the polymer as a system of chromophores with two possible electronic states. It is based on the combination of photothermal decomposition and photodissociative bond breaking in the electronically excited state. Laser-induced chemical modifications are incorporated via different absorption coefficients for the initial and for the UV-modified polymer. Dynamic attenuation of the incoming radiation by the expanding ablation plume and heat conduction are taken into account. The model predicts maximum surface temperatures during ablation of about 3000 K in the case of PI and about 700 K in the case of PMMA. Typical maximum velocities of the moving interface range from 20 m/s (PI) to 100 m/s (PMMA) for a pulse duration of 20 ns at a fluence of 4 J/cm2. The simulations show that the attenuation of the laser pulse by the plume of ejected material, which reaches a factor of up to 5 shortly after the peak of the pulse, starts to decrease towards the end of the laser pulse. These calculations support the significance of three dimensional plume expansion for the increase of ablation rates with growing pulse duration and decreasing laser spot size.