Dissociative Charge Transfer of Interstellar Dimethyl ether and Methyl Formate in collisions with He+. Experimental and theoretical study
Abstract
Ion-molecule reactions play an important role in the chemistry of planetary ionospheres and interstellar molecular clouds. Helium ions are found in the interplanetary space and have also been detected in Titan's magnetosphere and in the Earth's polar cusp region.[1] For this reason, studies on collisions of molecules with He+ are of fundamental importance for modeling planetary and cometary atmospheres, but especially the interstellar medium.[2] In the experiment that we report, dissociative charge transfer reactions are carried out with the smallest organic O-containing molecules: dimethyl ether (CH3OCH3), DME, and methyl formate (HCOOCH3), MF. The former molecule (DME) is particularly important in hot cores[3], while the latter (MF) is another important specie observed by radioastronomy in hot cores and it is very abundant in star forming regions.[3] These molecules play both a key role in understanding the origin of life, because they lead to the synthesis of bio-polymers.[4] For this reason, they immediately evoke great interest and several models were developed to explain why and how these molecules are formed and destroyed in space.[6] Furthermore, recent observations show the presence of these molecules in regions where the dust temperature is less than 30K: pre-stellar cores [5] and cold envelopes of low-mass protostars.[6] A new model proposes that gasphase reactions, triggered by the nonthermal desorption of methanol from the outer shell of the cold core, where the temperature is circa 10K, give the major contribution to DME and MF formation in cold regions.[7] However, to expand this model it is necessary to consider also the decomposition reactions. Therefore, experimental results about the reactions of He+ with dimethyl ether and methyl formate are of crucial importance, because they represent the most important fragmentation channel of these molecules. We have investigated these reactions by using the home-built Guided-Ion Beam Mass Spectrometer (GIB-MS) apparatus. Absolute cross sections and product branching ratios have been measured as a function of the collision energy in the hyperthermal energy range (i.e. from about 0,5 to 7 eV). In addition, charge transfer processes and the fragmentation dynamics have been studied with theoretical methods for the system DME – He+ .