Séminaire LATMOS à Guyancourt (amphi G. Mégie) le vendredi 6 janvier à 14h: Mark A. Smith (Department of Chemistry, 216 Science and Research Bldg. 1, University of Houston, Houston TX 77204 USA): "Ion-Molecule Reaction Processes in a Molecular Beam-Ring Electrode Trap"


Ion-Molecule Chemistry in the new Coaxial Molecular Beam-Radiofrequency Ring Electrode Trap (CoMB-RET)


We describe a new coaxial molecular beam radiofrequency ring electrode trap (CoMB-RET) for the study of ion-molecule reaction rates over a broad temperature range.  This instrument is capable of delivering molecular beams from equilibrated effusive nozzles at temperatures ranging from 20-500K into a trapped ion cloud equilibrated via buffer gas cooling with an independently temperature tunable rf trap at temperatures between 20 and 500K. Extensive differential pumping allows for very long trap times even at high temperature and the chopped molecular beam allows for selective ion-molecule exposure times between 10 ms and 1 s or longer.  In this manner, it is possible to study the separate internal energy effects of both neutral and ionic reactants on reaction rate coefficients.

In this contribution, we will discuss the properties and capabilities of this instrument and present initial reaction studies.  Following a discussion of the calibration reaction;

N2+ +  H2O  ->  H3O+ +  N2 (1)


we will discuss two studies investigating oxygenation of organics, or oxygen fixation, in the system of H3O+ with acetylene.


Simple oxygenation of organics via H3O+:  Radiative association with C2H2 vs. reaction of C2H2 dimers


Production of oxygenated organics in the interstellar medium (ISM) possibly represents the first step towards the abiotic generation of molecules critical to life. Understanding their production mechanisms is important if we are to begin to understand the plethora of complex organics now known to exist in the ISM as well as on other planetary bodies beyond Earth.  Acetaldehyde, CH3CHO, is ubiquitous in interstellar environments and has been detected in a variety of environments since it was first detected in 1973.  These environments include hot cores, dark clouds, and regions of star formation.


Radiative association plays a significant role in gas phase molecular ion synthesis in dense interstellar clouds and planetary atmospheres. The radiative association reaction between H3O+ and C2H2 monomer is considered as one of the most important steps for acetaldehyde production in the interstellar medium:


H3O+ +  C2H2 ->  C2H5O+ +  hv                                               (2)


This is followed by recombination of an electron to produce CH3CHO. Protonated water is one of the most stable forms of oxygen containing molecular ions and understanding its reactive potential with small hydrocarbons present in natural environments is critical.


The independent neutral molecule and ion temperature dependence of the rate coefficient for reactions between H3O+ and (C2H2)2 producing C2H5O+is determined using the coaxial molecular beam radiofrequency ring electrode ion trap (CoMB-RET). The rate coefficient for radiative association reaction between H3O+ and C2H2 monomer forming C2H5O+ is also studied in this system. The temperature of the H3O+ was varied from 25 K to 170 K while the neutral molecule C2H2/(C2H2)2 beam temperature was maintained at 160 K, 170 K, 175 K, 180 K and 200 K independently. For acetylene monomer study, the beam temperature was maintained above 300K to ensure the percent of acetylene dimer is less than 0.01%. The result demonstrates that acetylene dimer can be easily formed in the cold effusive molecular beam and its rate coefficient with H3O+ ions is close to the capture rate.   The rate coefficient decreases as the collision pair translational center-of-mass, or effective, temperature increases. The binding energy of acetylene dimer deduced from the experimental data is 436 ± 19 cm-1.   The rate  coefficient of the  H3O+ - C2H2 radiative  association  reaction is  below 1x10-13 molecule·cm3 when the effective temperature is 173K, consistent with the determination of Herbst[8].