No abstract

The thermal stability of fullerenes and dispersed in Ar was studied behind shock waves by following the time-dependent absorption and emission of the proposed decomposition product . The gas - particle mixtures filled into the shock tube were heated to temperatures at pressures of . Because of the relatively high vapour pressure of the fullerenes, they evaporate in a few microseconds so that both and are in the gas phase during most of the observation time. concentration profiles were measured by a ring dye laser spectrometer at . Simultaneously, the time behaviour of spectrally resolved light emitted from the shock-heated aerosol was recorded by an intensified CCD camera. A simplified reaction mechanism with both formation and consumption reactions was proposed and the experimental results were discussed in terms of kinetic parameters. Rate coefficients and of the initial pyrolysis reactions (R1) and (R2) were determined. Two different models for the further decomposition of fullerene fragments were discussed and compared with computer simulations.

The reaction of C2 radicals with nitrogen was studied behind reflected shock waves by time dependent absorption and emission measurements. Mixtures of fullerene C60 highly diluted in argon were shock heated and used as C2 sources. Perturbation of the reaction system by addition of N2 results in changes of C2 absorption and causes strong CN emission. The C2 concentration was quantitatively monitored by ring dye laser absorption spectroscopy at ν = 19 355.60 cm-1. Simultaneously, the time behavior of spectrally resolved light emitted from the shock-heated mixtures was recorded by an intensified CCD camera in the wavelength range 315 nm ≤ λ ≤ 570 nm. By comparing the integrated C2 and CN emission signals for the Δv = 0 progressions, the CN concentration can be determined on the basis of the known C2 concentration and the relative line strengths. The experiments were performed in the temperature range 2896 K ≤ T ≤ 3420 K at pressures 1.68 bar ≤ p ≤ 2.03 bar. Evaluation of C2 and CN concentration measurements lead to the following rate coefficient for the reaction C2 + N2 ⇌ CN + CN:  k 3 1.5 × 1013 exp(−21 000 K/T) cm3 mol-1 s-1. The uncertainty in k 3 is expected to be ±50%.

The reaction of O atoms with fullerene C60 was studied behind reflected shock waves by time dependent atomic and molecular resonance absorption spectroscopy. Mixtures of C60/N2O highly diluted in argon were shock heated, and the absorption of O atoms at λ = 130.5 nm and of CO[X∑+(ν‘ = 0)→A1Π(ν‘‘ = 1)] at 151.0 nm was monitored. Simultaneously, the time behavior of spectrally resolved light emitted by the shock-heated mixtures was recorded by an intensified CCD camera in the wavelenghth range 315 nm ≤ λ ≤ 570 nm. The experiments were performed in the temperature range 1870K ≤ T ≤ 2625 K at pressures around 1.30 bar. A kinetic model for the reaction system was developed. The evaluation of O atom concentration measurements resulted in the following rate coefficient for the reaction C60 + O→CO + products:  k 2 = 3.0 × 1015 exp(−11 450 K/T) cm3 mol-1 s-1. The uncertainty in k 2 is expected to be ±50%.

The reactions of H atoms and OH radicals with fullerene C60 were studied behind reflected shock waves by time-dependent absorption measurements. Shock-heated mixtures of C2H5I highly diluted in argon were used as a source for H atoms, and their absorption at λ = 121.6 nm was monitored using the highly sensitive ARAS technique. In a second series of experiments OH radicals were generated in H2/N2O/Ar reaction systems, and the absorption of OH was monitored at ν = 32 403.41 cm-1 using ring dye laser absorption spectroscopy (RDLAS). In each case, the respective mixtures were perturbed by C60. Simultaneously with the absorption measurements, emission spectroscopy with an intensified CCD camera was used to acquire additional information about the reaction systems. The H atom experiments were performed at temperatures between 2100 and 2300 K, while OH absorption was measured in the temperature range 2020 K ≤ T ≤ 2540 K. Postshock pressures were around 1.30 bar for all experiments. From the H-absorption measurements, an upper limit for the reaction C60 + H to products (R9) (k 9 < 1.0 × 1012 cm3 mol-1 s-1) was determined. For the H2/N2O/Ar + C60 reaction system, different kinetic models were discussed to verify the measured OH absorption. The reaction C60 + OH to products (R7) (k 7 = 1.0 × 1015 exp(−6030 K/T) cm3 mol-1 s-1) with the given rate coefficient represents the experimental findings quite well.