The correlation between reaction mechanism and the isotopic effects in charge transfer in systems implying deuterated methanes

The charge transfer reactions data are very useful in understanding phenomena relating to high-atmosphere, astro-physics, fusion plasma and mass spectrometry. A charge transfer process between a primary (reactant) ion AB+ and a neutral molecule CX4 (X being H or D) is of the type: AB+ + CX4 → AB + CX4+. The recombination energy (RE) of AB+ should be close to the appearance energy (AE) of the secondary ion CX4+ i.e. that Δ ε = AE - RE should be a small quantity. In the general case of asymmetrical charge transfer (Δ ε ≠ 0) the energy needed can be provided either by transfer of energy from the relative kinetic energy of the ion molecule system or from the vibrational energy of the primary ion. The measurements were carried out by using a perpendicular-type tandem mass spectrometer and a variable angle tandem mass spectrometer. The charge transfer processes studied involved H2+, CH4+, CH3+ and N+ as primary (reactant) ions and CH4, CH3D, CH2D2 and CD4 as neutral molecules. The secondary spectra were normalized and the relative intensities of CX3+ ions were used to calculate the intermolecular and intramolecular isotope effects, πi and Γi. In all the systems, it was observed a variation of the relative cross sections versus the kinetic energy of the primary ions. The variation was relatively small (as for the systems H2+ - CH4 and CH4+ - CX4) or it was quite important (as for the systems CH3+ - CX4 and N+ - CX4). However, in all the cases, the intermolecular isotope effects (πi and Γi) show a more important variation of the values versus the kinetic energy. For this reason, the isotope effects could be used as a more efficient parameter for the charge transfer reaction path. The reaction mechanism was derived from the experimental results. A modification of this mechanism is very probable for primary ion energies under 15 eV. This modification was also supported by the study of the momentum transfer measurements in the formation of CX3+ secondary ions. Conclusions were also obtained concerning the conversion of kinetic energy into internal energy in charge transfer reactions, for CX3+ ions containing more D atoms. (author)