Cross section measurements for molecular reactions of fundamental and astrophysical relevance

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The project aims at deepening our understanding of dense molecular clouds in the interstellar medium, which give birth to stars and planetary systems. For this purpose, it is essential to improve our understanding of the underlying gas-phase astrochemistry. Simulations of the gas-phase chemical evolution of molecular clouds rely directly on the knowledge of thermal rate coefficients for the individual molecular reactions. These reactions typically involve a charged and a neutral partner, since the low temperatures of molecular clouds are mostly not sufficient to overcome the potential barrier of neutral-neutral reactions. Theoretical models for astrophysically relevant molecular reactions may easily reach uncertainties up to a factor of two or more, and hence accurate experimental determination of key reaction rate coefficients can significantly enhance the reliability of molecular cloud simulations.My objective is to experimentally study one of the key reactions with deuterium, whose current uncertainty significantly hinders our ability to reliably model and analyze the star formation process in dense molecular clouds. The experiment will be performed at the merged-beams apparatus of the group of Dr. Daniel Savin at Columbia University, New York City. This apparatus is the only facility world-wide, which is currently capable of measuring the envisaged reaction. Here, molecular reactions between positively charged molecules and neutral atoms can be studied as a function of the relative collision velocity. Using the technique of merging two fast beams with small relative velocities, the low temperatures relevant for the reactions in molecular clouds can be reproduced. By measuring the intensities of both beams and counting the reaction products, the merged-beams rate coefficient as a function of the relative collision velocity is determined accurately on an absolute scale. Specifically, I will measure the merged-beams rate coefficient for the reaction D + H3+ -> H2D+ + H for relative collision energies from 3.5 meV to 20 eV. The convolution with a Maxwell-Boltzmann distribution will yield the thermal rate coefficient from the data. Implications of the results on astrophysical models will be analyzed.