Cluster Catalysis


Experimental Setup and Apparatus

In the apparatus which is described in the following sub-nano- and nanoclusters are synthesised in a Magnetron-Sputtering source. Through an expansion into a high vacuum chamber, a molecular beam is formed which is then skimmed and focussed by ion optical components. In a quadrupole mass filter, mass separation of the charged particles is achieved. Afterwards, the beam is deflected about 90° to separate the ions from neutral clusters.

Picture 1: Schematic representation of the beam path.
Picture 1: Schematic representation of the beam path.

The Magnetron-Sputter source consists of a double pipe which is cooled with liquid nitrogen. Inside this pipe is a water-cooled sputter head with the cylindrical target material from which clusters are made. The target acts as a cathode and a nearby stainless steel ring as an anode. Between these electrodes, a DC voltage is applied which ionises inflowing Argon gas and accelerates ions towards to target surface. The impact of argon ions leads to the formation of atoms, electrons and fragments from the solid target material. The free electrons are constrained to magnetron trajectories through the NdFeB permanent magnets behind the target material. This leads to an increased rate of ionisation and, therefore, lowers the pressure to several mbars which is needed to sustain the sputtering process. Through collisions with He, the metal atoms condense to clusters. When this mixture expands into the high vacuum chamber a molecular beam is formed. Afterwards, a skimmer is used as a pressure stage to separate the source chamber from the mass selection chamber and to segregate the noble gas atoms which are in the outer region of the molecular beam from the metal clusters near the centre. Right after the skimmer the beam passes einzel lens 2 and the quadrupole entrance lens 3 which focus the beam. Between the main rods 5, the mass filtering occurs which can be described by Mathieu’s equations. The quadrupole exit lens und the following three einzel lenses 7, 8 and 9 focus the cluster ions into the quadrupole ion deflector (QID), which deflects the beam about 90° in order to separate charged particles from neutral ones. On every side of the QID, perforated plates are used to further focus the incoming and outcoming beam. After passing the ion deflector the particle beam reaches the grounded sample surface.