The research domain of molecular dynamics is being developed from a few visionary experimental and theoretical investigations to a fully grown discipline. The rise of the discipline is strongly related to the discovery of a very powerful tool used in this study: molecular beam techniques. However, elementary chemical processes were already investigated decades before the development of the molecular beam method.
Already in 1932 Polanyi observed in sodium flame experiments that the cross section of the reaction between Na and Br2 or I2 is about 150 Å2. It means that the reaction takes place when the particles are at a distance of about 7Å. This is an order of magnitude larger than the range of the Van der Waals forces (1-2 Å).Polyani suggested the basic features of a model, the harpoon model, which could explain the large cross sections for the formation of sodium halides, which he deduced from his flame experiments. This model was later developed by Magee in a theoretical study, explaining the mechanism of the electron jump from the alkali atom towards the reactant molecule, which jump initiates the reaction. A very lucid and clear discussion on the harpooning mechanism has been given by Herschbach in connection with reactive scattering in molecular beams.
Briefly what this harpoon model comes to is that in the collision between an alkali atom and an electronegative molecule at some distance Rc an electron will jump over from the approaching alkali atom towards the molecule. The negative molecular ion in most cases is formed in a highly excited vibrational state, close to the dissociation limit or even in a dissociative state. The Coulomb force exerted by the alkali ion during the remainder of the trajactory completes the dissociation, and an alkali halide molecule is formed. The vibrational and rotational energy of the molecule will be high
Crucial in this harpoon model is the transfer of the alkali valence electron at the crossing point, which initiates the reaction. The electron jump is due to the configuration interaction between the purely ionic and the purely covalent state at the point where these states cross at some cross distance Rc . This initial step now can be investigated by applying the molecular beam method in the electronvolt energy range . In that case the transfer of the electron would not lead to MX formed in a chemical reaction, but to an ion-pair M+ + XY-. These experiments might indicate in how far a generalised Landau-Zener formula could be formulated
About a decade ago the investigation of reactive collisions by means of molecular beams became a well established method for measuring cross sections and investigating reaction dynamics. In the early experiments mainly alkali beams were used for the obvious reason of detection feasibility. As a consequence much attention has been paid to charge transfer reactions. Especially the akali atom-halogen-molecule reactions, including the methyl halides and hydride halides as reactants have been subject of detailed experimental investigations.
With the development of the cathode sputtering method and the charge exchange method , it became possible to investigate these chemi-ionisation scattering processes in the electronvolt range, allowing for the analysis of the 'pure' first step of the harpoon reaction, namely the ion pair formation.
The research project initiated at the Institute for Atomic and Molecular Physics in Amsterdam is concerned with this charge transfer process in atom-atom and atom-molecule collisions . We use atomic and molecular beam techniques in the electronvolt range to study} the scattering process, including total and differential cross sections, the electronic transfer probability and the theoretical description and interpretation.