Improving Models of Molecular Clouds and Planetary Atmospheres: Dissociative Recombination Measurements for Molecular Ions of Astronomical Interest

Columbia University, New York NY

Investigators

Abstract

The PI and his team will measure dissociative recombination branching ratios and rate coefficients for important chemical molecular ion reactions in cold interstellar clouds. This work on dissociated recombination reaction rates and cross-sections is needed to understand the molecular inventory and cooling of the interstellar medium, and to put astrochemical models onto a firm base. Molecules are key components of diffuse, translucent and dense molecular clouds, hot cores, photon dominated regions, protostellar disks, protoplanetary disks, planetary and satellite ionospheres, cometary comae, and circumstellar envelopes around dying stars Models and interpretations of the chemical composition, charge balance, emission and/or absorption spectra, and thermal structure in the various astronomical environments depend on reliable knowledge of the underlying molecular collisions which control these properties. Among the dissociative recombination reactions the primary neutralizing reactions for molecules in cosmic plasmas are particularly important. Dissociative recombination involving ion-molecule reactions is often the terminating step for particular synthesis pathways in chemical networks. One needs to know rates and branching ratios for final products in order to understand reaction pathways, and to understand whether a compound can be produced in the gas phase or if grain surface chemistry must be invoked. If the end products of dissociative recombination are energetic, they can collisionally heat the plasma; if they are in excited states, they can cool the gas through radiative relaxation. The experiments are done at the unique heavy-ion Test Storage Ring (TSR) facility at the Max-Planck Institute in Heidelberg, Germany. The ions to be studied are HF+, H2F+, CF+, 16O16O+, 18O16O+, HSiO+, CH2O+, and CH3O+. They are selected based on their importance for ground-based spectroscopic observations combined with astrochemical modeling studies. The selected molecules can be stored long enough in TSR to cool to their lowest electronic and vibrational levels, except for 16O16O+ which will electronically relax but lacks a dipole moment and will not vibrationally radiatively relax. The PI and his team will generate total dissociative recombination rate coefficients versus temperature for the lowest vibrational level of each molecule as well as branching ratios for the various possible outgoing final channels. Such and related data will also find applications in other areas of astronomy such as planetary sciences, e.g., for 16O16O+, they may be able to generate these data as a function of vibrational level, which is needed for martian atmosphere studies. This project has an ongoing international collaboration and strong commitment to education and involves offers a research experience to high school teachers in Staten Island in collaboration with the Columbia Summer Research Program for Science Teachers.