Principles

The basic setup	1. The Basic Setup Reactive scattering experiments help to elucidate the intimate details of a chemical reaction. This would not be possible in bulk experiments such as in test tubes. Our experiments are carried out in a crossed molecular beams apparatus, where two pulsed beams of reactants collide under well defined conditions. The resulting products are analyzed in a rotatable detector.    The adjacent figure shows a schematic top view of the setup. The molecular beams machine consists of two source chambers (each containing a pulsed valve which releases a pulsed beam of molecules or atoms), a stainless steel scattering chamber and a rotatable detector. A chopper wheel selects the velocity of the primary beam, whereas the copper cold shield hold at 4.5 K reduces the background molecules 'seen' by the detector. In phase I of the experiments, reactively scattered species shall be detected with a Quadrupole Mass Spectrometer which is currently being upgraded for 'soft electron impact ionization'.
Crossed Beams Experiment	2. The Timing The timing of a typical experiment is shown in the adjacent animation; you can click it to get a bigger GIF . Normally we are working with carbon, dicarbon, tricarbon, boron, cyano, or ethinyl beams which are produced via laser ablation. Also, phenyl radicals have been prepared via flash pyrolysis. In case of carbon, an ultraviolet laser pulse hits a rotating carbon rod to release carbon species which are subsequently seeded in the helium or neon pulse of the primary valve. This beam passes a skimmer, a chopper wheel (not shown in the animation for simplicity) and the cold shield to reach the interaction region, where it collides with the secondary pulsed beam. This secondary pulsed beam can be for example acetylene. As seen in the animation, a correct timing of the pulsed valves and the laser is necessary for the experiment to work. At the collision of the two beams, some of the reactants will react and the pro- ducts will be scattered in the chamber. Those products flying in the direction of the detector will be analyzed at different angles in the time-of-flight mode, i.e. recording the intensity of an ion of a selected mass-to-charge ratio versus the time. The detector is shown here in a fast moving motion which is only to demonstrate its variable viewing angle.
Schematics of data accumulation and transformation	3. The Data Accumulation The detector is not only able to resolve the incoming products according to mass, but also in time, i.e. it can measure the time the products need from the interaction point to the detector, taking so called time-of-flight-spectra (TOFs). These TOFs taken at different laboratory angles are depicted schematically in the adjacent jpg . Together with the laboratory angular distribution we can use these data to transform it into the center-of-mass (CM) system to get a unique picture of the reaction dynamics. later