Laboratory Investigations Into The Origins Of Organic Chemistry

The chain of chemical reactions leading towards life is thought to begin in molecular clouds when atomic carbon is fixed into molecules, initiating the synthesis of complex organic species. Spectroscopic observations, combined with sophisticated astrochemical models to interpret the collected spectra, provide much of our knowledge of this process. However, uncertainties in the underlying chemical data in these models limit our understanding of the molecular universe. Theory provides little insight as fully quantum mechanical calculations for reactions with four or more atoms are too complex for current capabilities. Measurements of rate coefficients for reactions of C with molecular ions are extremely challenging. This is due to the difficulty in producing a sufficiently intense and well characterized beam of neutral carbon atoms. We have developed a novel merged beam apparatus to study reactions of neutral atomic C with molecular ions. A C_- beam is created in a cesium ion sputter source and accelerated to 28 keV. A series of apertures and electrostatic optics create a collimated beam. Using an 808 nm (1.53 eV) laser beam, ~4% of the C_- beam is neutralized via photodetachment. We produce a pure ground term neutral C beam by electrostatically removing the remaining C_-. A velocity matched, co-propagating H³₊ beam at 7.05 keV, created with a duoplasmatron source, is then merged with the C beam. The merged beams method allows us to use fast beams, which are easy to handle and monitor, while being able to achieve relative collision energies down to some tens of meV. An electrostatic energy analyzer separates and detects the charged end products of the different reaction channels. The reactions rate coefficients are determined by measuring all the relevant currents, beam shapes, energies, signal counts and background rates. We have measured the absolute rate coefficients for C + H³₊ → CH₊ + H² and C + H³₊ → CH²₊ + H. Since H³₊ is ubiquitous in molecular clouds, these reactions are some of the first steps leading to the formation of complex organic molecules within such clouds. Our reaction studies will help to provide a better basis for astrochemical models and benchmarks for future theoretical development.