3.1. Principle of the field-free ion imaging
The principle of ion imaging is shown in Fig. 3. Let the DCT process take place at time t0 in a small reaction volume located at zr on the molecular beam axis. The velocity v ion of a product ion is given by the flow velocity vbeam of the particles in the beam and the additional velocity vDCT gained in the reaction process:
vion = vbeam + vDCT
The tip of the velocity vectors of the particles, and, consequently, the dissociation fragments themselves form on sphere (the so-called Newton sphere). The centre of this sphere propagates along the beam axis, while the radius of the sphere expands. In contrast to the existing ion-imaging designs which use electric fields to extract the ions from the reaction zone, we keep the reaction and expansion zone free of any electric fields. As soon as the ions have passed through the entrance mesh of the ion optics, they are focused and accelerated onto position sensitive detector (PSD, see also Fig. 4). The obtained two-dimensional images bear information about the velocity (i.e., the kinetic energy) distribution of the ions released in the reaction. Provided the velocity distribution of the ions is axially symmetric, the three-dimensional information can be numerically reconstructed yielding the relative cross sections of the DCT process as a function of the released kinetic energy.