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Concerted optical and mass spectroscopic investigations on a molecular level of the heterogeneous chemical reactions important in the depletion of polar stratospheric ozone


The goal of the investigations is the spectroscopic characterization of chlorine and bromine containing molecules involved in important heterogeneously catalyzed reactions leading to the depletion of polar stratospheric ozone, and to obtain reliable information about the mechanisms, rate constants, and branching ratios of these reactions on a molecular level.

The contractors will measure the rate constants for the heterogeneous chlorine activation by polar stratospheric cloud aerosols to obtain quantitative insights into the reaction dynamics and information about the structure and properties of the reactive intermediates involved.

The techniques used to study the reactions and to monitor the reactants and products are Fourier Transform Ion Cyclotron Resonance mass spectroscopy (FTIR (far-infrared, visible and ultraviolet). Two state-of-the-art instruments are available in Garching, a Spectrospin CMS 47 X FT-ICR and a Bunker IFS 120 HR FT-IR. In Dublin there is a blank far-infrared spectrometer.

The FT-ICR technique is a mature method that is used to obtain the mass spectra, at very high resolution of ions stored in the ICR trapping cell. Many masses can be determined at one time. The unambiguous identification of the stoichiometry of these ions is routinely possible. The ability to perform collision-induced dissociation experiments allows the identification of stable substructures (daughter ions). The rate constants of gas-phase chemical reactions under single collision conditions are obtained by time-resolved product detection and consecutive modelling to pseudo first order kinetics. The FTIR method lacks the high sensitivity of the mass spectroscopy, but does give direct information about the structural properties of the reactive intermediates and products, which can not be obtained by mass spectroscopy alone.

Two types of experiments will be done to mimic the stratospheric reactions. Large clusters will be formed in a miniature flow reactor using a pulsed expansion of a mixture containing water and a suitable reactant which will coat the walls of the reactor. Subsequently, a carrier gas with suitable reactive species will flow through the reactor and the reactions taking place on the surface will be monitored. This will occur, a) by sampling the effluent gases through an orifice into the FT-ICR and measuring their mass spectrum, and b) by trapping the product in solid matrices, and measuring their optical spectra. The advantage is that in step a) the products or intermediates are identified by their masses. This then facilitates in step b) performed under identical experimental conditions in assigning the observed optical spectra.

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