3D Velocity Mapping: Vollständige Charakterisierung der bimolekularen Reaktion O(1D) + N2O → NO + NO (English shop)

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In this work, the 3D imaging technique was employed for the first time to elucidate a bimolecular reaction. Through the use of a novel investigative method, it was possible to determine the complete velocity vector distribution of the NO molecules formed in the reaction of O(1D) with N2O selectively for individual quantum states. From the information thus obtained on the distribution of kinetic as well as internal energy, conclusions can be drawn about the underlying mechanism. The 3D velocity mapping technique developed and instrumentally realised within the framework of this work is distinguished by the known advantages of 3D imaging, on which it is based. Thus, the reaction products are detected mass-spectrometrically following resonance-enhanced multiphoton ionisation, which results in a state- and mass-selective detection with a detection probability close to unity. Since the velocity vectors are determined directly, the necessity of an error-prone reconstruction algorithm is eliminated. Furthermore, the internal energy of the reactants is defined by the use of the molecular beam technique, whereby the energy balance of the system becomes accessible. The application of the 3D imaging technique to questions of reaction dynamics could be achieved through further development of the method. Among the most important extensions are the novel signal analysis, which endows the system with multi-hit capability, and the integration of a pulsed molecular beam nozzle, which has made accessible the density range necessary for the formation of the reactant pair. Likewise, a second laser system for initiating the reaction as well as a means for the exact positioning of the corresponding excitation volume had to be integrated. A further optimisation indispensable for the characterisation of the reaction consists in the improvement of resolution achieved through the use of an ion optics assembly. In order to characterise the individual components, a large number of preliminary investigations were carried out: on the basis of photodissociation experiments with various carrier gases, the quality of the molecular beam as well as the homogeneity of the TOF acceleration field were characterised. Furthermore, these experiments were used to determine the optimal parameters for the operation of the overall system. Examples include the synchronisation of the pulsed molecular beam nozzle as well as the optimisation of the parameters used in the newly developed analysis software.

In addition, photodissociation experiments were carried out to characterise the ion lens. On the basis of the fragmentation of HCl it could be confirmed that the detection position of an ion in velocity mapping mode depends exclusively on its velocity vector, and that the position of the particle at the moment of ionisation therefore has no influence. As could be shown, the compensation of the finite excitation volume achieved in this way leads to a significant increase in resolution. The characterisation of the bimolecular reaction of O(1D) with N2O was successfully carried out by means of a large number of measurements. The rotational excitation of the NO products formed as a function of their vibrational state could be determined by recording the corresponding spectra. In this way it was also possible to confirm unambiguously the formation of products in the levels v = 0,3,4,5,7. Taking into account the corresponding Franck-Condon factors, it can be concluded from the relative intensities for the various vibrational transitions that v = 0 is populated considerably more strongly than the higher states. By means of a series of 3D measurements, the product velocity vectors in the laboratory coordinate system were determined for individual rovibronic states. The three-dimensional distributions derived from these show no anisotropy and, just like the velocity distributions extracted from them, differ only very slightly for the individual states. Comparative measurements with significantly higher pulse energies of the dissociation wavelength made it possible to determine a second velocity distribution of non-resonant product states. By combining the individual results, it was possible to establish a reaction mechanism for the reaction of O(1D) with N2O in the “constrained geometry” of the reactant pair. The forward/backward scattering of the NO products occurring in the proposed stripping mechanism leads to the same results as the transformation of the experimentally determined velocity vector distribution from the laboratory into the centre-of-mass coordinate system. Both the isotropic product distribution and the velocity distribution determined for the resonant vibrational states agree with the expectations for this reaction pathway. A simulation carried out according to the Monte Carlo method was able, through its very good agreement with the experimental data, to confirm the validity of the proposed model.

In summary, it can be stated that the investigative method of 3D velocity mapping developed within the framework of this work can be successfully applied to questions of reaction dynamics. The experiments for characterising the individual components showed that the performance of the method for photofragmentation exceeds that of 3D imaging in all respects, while bimolecular reactions have only become accessible through the new method. The successful characterisation of the reaction investigated is proof of the capability of the 3D velocity mapping technique to induce bimolecular reactions under defined conditions and to investigate the products formed both with regard to their rovibronic excitation and with regard to their angular and velocity distribution, and thus completely.