Cluster-ToF mass-spectrometer
Cluster-ToF II at MPI Stuttgart
The cluster ToF is illustrated by the two sketches on this page. It is a
high-resolution time-of-flight mass
spectrometer (ToF) with a resolution of up to m/Delta(m) = 40000. This makes it one of the
best machines of it's kind in the world.
Figure 1: The standard setup of the cluster ToF
Production of clusters:
The cluster source shown can produce all kinds of clusters from "simple" metal
clusters to sophisticated
multi-layered heterogenous clusters. The source is a rare gas aggregation cell containing
either one or
two ovens to evaporate the various materials to be studied. Alternatively, elements can be
evaporated by targeting a laser through a window onto refractory materials inside the oven
chamber. A steady helium gas flow cools the vapour(s) inside the oven chamber causing the
atoms to condense to clusters. By means of a liquid-nitrogen-cooled nozzle and a system of
skimmers, a neutral cluster beam is formed and directed towards the ionization region.
Ionization and acceleration of clusters:
The ionization spot lies within a region of strong electric field which "turns"
the clusters by 90 degrees and accelerates them down the drift tube. The clusters first
pass a quadrupole focussing section which compensates for the remaining lateral velocity
components, and then are bounced back by a reflector field ( => reflectron-TOF) towards
the ion detector. The reflection compensates for differences in the starting coordinates
(and therefore kinetic energy) of the clusters due to the finite size of the ionization
laser spot thereby enhancing the achieved mass resolution. The clusters are detected using
a multi-channel-plate as secondary electron multiplier. The arrival times are read out by
a TDC (time-to-digital-converter).
Figure 2: This is the setup with a selection ToF.
A more refined way of preselecting a certain cluster mass range for
measurement is made possible by the "Mini-TOF",
which is illustrated in Fig. 2. It is a small reflectron-TOF that can be installed on the
axis of the neutral cluster beam. The clusters are ionized just after having entered the
acceleration section of the Mini-TOF and return from the Mini-TOF reflector into the main
acceleration section already mass separated. The acceleration field of the main TOF is
switched on when the clusters of interest are located within the small volume that is
projected onto the detector. This small portion of the initial cluster size distribution
can then be studied in more detail with the main ToF.
Applications:
|| home || products
|| contact
||