Klaus Luther

Trajectory simulations of collisional energy transfer in highly excited benzene and hexafluorobenzene

Quasiclassical trajectory calculations of the energy transfer of highly vibrationally excited benzene and hexafluorobenzene (HFB) molecules colliding with helium, argon and xenon have been performed. Deactivation is found to be more efficient for HFB in accord with experiment. This effect is due to the greater number of low frequency vibrational modes in HFB. A correlation between the energy transfer parameters and the properties of the intramolecular potential is found. For benzene and HFB, average energies transferred per collision in the given energy range increase with energy. Besides weak collisions, more efficient ‘‘supercollisions’’ are also observed for all substrate–bath gas pairs. The histograms for vibrational energy transfer can be fitted by biexponential transition probabilities. Rotational energy transfer reveals similar trends for benzene and HFB. Cooling of rotationally hot ensembles is very efficient for both molecules. During the deactivation, the initially thermal rotational distribution heats up more strongly for argon or xenon as a collider, than for helium, leading to a quasi‐steady‐state in rotational energy after only a few collisions.

Shock wave study of the unimolecular dissociation of H2O2 in its falloff range and of its secondary reactions

The thermal decomposition of H2O2 was studied behind reflected shock waves using absorption spectroscopy at 215 nm with light from a laser source and at 215, 230, and 290 nm with light from a UV lamp. Due to the improved detection sensitivity for H2O2 and HO2, rate constants for the reactions H2O2 (+Ar) → 2HO (+Ar) (1), HO2 + HO2 → H2O2 + O2 (3) and HO + HO2 → H2O + O2 (4) could be derived with much better precision than possible in earlier work. Rate constant minima for reactions (3) and (4) near 800 and 1100 K, respectively, were reconfirmed. Falloff curves of reaction (1) near to the low pressure limit were measured in experiments at about 1, 4, and 15 bar. Limiting low pressure rate constants for reaction (1) of about k1,0 = [Ar]1016.36±0.23exp[−21 962(±608) K/T] cm3 mol−1 s−1 were obtained over the temperature range 950–1250 K. The deviations from the low pressure limit at relatively low pressures may provide experimental evidence for non-exponential lifetime distributions of dissociative H2O2 such as recently observed in classical trajectory calculations on the ab initio potential of H2O2.

Experimental and modelling study of the recombination reaction H + O2 (+M) → HO2 (+M) between 300 and 900 K, 1.5 and 950 bar, and in the bath gases M = He, Ar, and N2

The recombination reaction H + O(2) (+M) --> HO(2) (+M) was studied by laser flash photolysis in a high pressure flow cell, over the temperature range 300-900 K, the pressure range 1.5-950 bar and in the bath gases M = He and N(2). Earlier experiments by Hahn et al. (Phys. Chem. Chem. Phys. 2004, 6, 1997) in the bath gas M = Ar were also extended. The data were analyzed in terms of unimolecular rate theory employing new calculations of relevant molecular parameters. Improved energy transfer parameters for the bath gases M = He, Ar, N(2), and H(2)O could thus be obtained and complete falloff curves were constructed. In the case of water, the high pressure rates well connect with pulse radiolysis results obtained in supercritical water by Janik et al. (J. Phys. Chem. A 2007, 111, 79).

Experimental and modeling study of the reaction C2F4 (+ M) ⇔ CF2 + CF2 (+ M).

The thermal dissociation reaction C2F4(+ M) → 2CF2(+ M) was studied in shock waves monitoring CF2 radicals by their UV absorption. The absorption coefficients as functions of wavelength and temperature were redetermined and are represented in analytical form. Dissociation rate constants as functions of bath gas concentration [M] and temperature, from previous and the present work, are presented analytically employing falloff expressions from unimolecular rate theory. Equilibrium constants are determined between 1200 and 1500 K. The data are shown to be consistent, with a C-C bond energy of 67.5 (±0.5) kcal mol(-1). High-pressure limiting rate constants for dissociation and recombination are found to be unusually small. This phenomenon can be attributed to an unusually pronounced anisotropy of the potential energy surface, such as demonstrated by quantum-chemical calculations of the potential energy surface.

A quasiclassical trajectory study of energy transfer in benzene–benzene collisions

Collisional energy transfer from large, highly vibrationally excited molecules has been studied by quasiclassical trajectory calculations and results for large polyatomic colliders are reported for the first time. Highly excited benzene molecules in a thermal bath gas of benzene were investigated using a sum of atom–atom Lennard‐Jones interactions for the intermolecular potential. Excellent agreement with experiment has been found for the first moment of energy transfer 〈ΔE〉 and its energy dependence. 〈ΔE〉 is increasing with a slight leveling off at high energies. The results suggest that vibrational energy loss of the excited molecule is dominated by V–V transfer.

Photochemically assisted laser ablation of doped polymethyl-methacrylate

Abstract Polymethyl-methacrylate doped with organic dye molecules can be photoablated with 308 or 351 nm nanosecond excimer laser pulses. For comparison, pure polymethyl-methacrylate cannot be photoablated at these wavelengths. The definition of the ablation pattern is comparable to the results achieved by the use of 248 nm pulses on pure polymethyl-methacrylate. Etch rates of about 50 μm per pulse can be reached, which is ten times the maximum etch rate of 248 nm pulses. An example of an effective dopant molecule is 1,3-diphenyltriazene. The ablation process is governed by gas production due to laser-induced photodissociation of the dopant. The ablation rate increases with growing laser fluence up to a saturation level, which is approximately proportional to the reciprocal dopant concentration (0.2 to 2 wt%).

The role of the radical-complex mechanism in the ozone recombination/dissociation reaction

The data bases for low-pressure rate coefficients of the dissociation of O3 and the reverse recombination of O with O2 in the bath gases M=He, Ar, N2, CO2 and SF6 are carefully analyzed. At very high temperatures, the rate constants have to correspond solely to the energy transfer (ET) mechanism. On condition that this holds for Ar and N2 near 800 K, average energies transferred per collision of -DeltaE/hc=18 and 25 cm-1 are derived, respectively. Assuming an only weak temperature dependence of DeltaE as known in similar systems, rate coefficients for the ET-mechanism are extrapolated to lower temperatures and compared with the experiments. The difference between measured and extrapolated rate coefficients is attributed to the radical complex (RC) mechanism. The derived rate coefficients for the RC-mechanism are rationalized in terms of equilibrium constants for equilibria of van der Waals complexes of O (or O2) with the bath gases and with rate coefficients for oxygen abstraction from these complexes. The latter are of similar magnitude as rate coefficients for oxygen isotope exchange which provides support for the present interpretation of the reaction in terms of a superposition of RC- and ET-mechanisms. We obtained rate coefficients for the ET-mechanism of k/[Ar]=2.3x10(-34) (T/300)(-1.5) and k/[N2]=3.5x10(-34) (T/300)(-1.5) cm6 molecule-2 s-1 and rate coefficients for the RC-mechanism of k/[Ar]=1.7x10(-34) (T/300)(-3.2) and k/[N2]=2.5x10(-34) (T/300)(-3.3) cm6 molecule-2 s-1. The data bases for M=He, CO2 and SF6 are less complete and only approximate separations of RC- and ET-mechanism were possible. The consequences of the present analysis for an analysis of isotope effects in ozone recombination are emphasized.

Pressure dependence of the reaction H+O2(+Ar)→HO2(+Ar) in the range 1–900 bar and 300–700 K

The reaction H+O2 (+Ar)→HO2 (+Ar) was studied in a high pressure flow cell in the bath gas argon at pressures between 1 and 900 bar and temperatures between 300 and 700 K. H atoms were generated by laser flash photolysis of NH3 at 193.3 nm, HO2 radicals were monitored by light absorption at 230 nm. The results are consistent with experimental low pressure rate constants k0=[Ar] 2.5×10−32(T/300 K)−1.3 cm6 molecule−2 s−1 and theoretical high pressure rate constants k∞=9.5×10−11(T/300 K)+0.44 cm3 molecule−1 s−1 from the literature. The intermediate falloff curve was found to be best represented by k/k∞=[x/(1+x)]Fcent1/(1+(a+logx)2/(N±ΔN)2) with x=k0/k∞, a≈0.3, N≈1.05, ΔN≈0.1 (+ΔN for (a+logx) 0), and Fcent(Ar)≈0.5 independent of the temperature. A comparison with literature data between 300 and 1200 K does not confirm major deviations from third order kinetics in earlier medium pressure experiments.

Collisional energy transfer of highly vibrationally excited toluene and pyrazine: Transition probabilities and relaxation pathways from KCSI experiments and trajectory calculations

New experimental results for the collisional energy transfer of highly vibrationally excited toluene and pyrazine employing the method of “kinetically controlled selective ionization (KCSI)” are presented. By means of a master equation approach we determine complete and detailed collisional transition probabilities P(E′,E) for energies up to 50000 cm−1. The same monoexponential representation P(E′,E)∝exp[ − ((E − E′)/α1(E))Y] (for E′⩽E) with a parametric exponent Y in the argument and linearly energy dependent α1(E) = C0 + C1E successfully used in our earlier investigation [T. Lenzer, K. Luther, K. Reihs and A. C. Symonds, J. Chem. Phys., 2000, 112, 4090] can reproduce the toluene and pyrazine results for the whole range of bath gases studied. The parameters Y, C0 and C1 of P(E′,E) show a smooth increase with the size of the collider. An approximately linear energy dependence of the first moment of energy transfer 〈ΔE〉 is observed for all bath gases. Literature data from infrared fluorescence (IRF) experiments in general show significantly smaller − 〈ΔE〉 values outside the uncertainty limits of the KCSI results. It is shown that this can mainly be traced back to the critical dependence of the IRF data on small uncertainties in the calibration curve. Some of the trends with respect to the energy transfer efficiencies of different colliders observed in the KCSI experiments are easily rationalized on the basis of accompanying trajectory calculations on the deactivation of highly vibrationally excited pyrazine by n-propane and CO2. The negligible influence of the V–V relaxation channel in the pyrazine + CO2 system observed in earlier IR diode laser studies is confirmed.

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.