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of cathode surface at the point where the He flux impinges on the electrode.
The mixture of helium-vaporized atoms quenches and cluster nucleation
takes place. The clusters are then carried out through the nozzle by the
supersonic expansion.
The cathode is continuously rotated by means of an external motor in
order to allow constant ablation conditions for all pulses and a homoge-
neous consumption of the rod. Higher deposition rates can be obtained by
substituting the simple cylindrical nozzle with a more complex one (called
focuser) as described in
Reference
28
.
Exploiting inertial aerodynamic
effects [28,29], the focuser reduces the angular semiaperture of the beam
from 12
to less than 1
concentrating the cluster on the center of the beam.
The PMCS (Pulse Microplasma Cluster Source) produces a pulsed
beam of carbon clusters seeded in helium with a mass distribution, with an
approximately shape peaked around 600 atoms/cluster and
extending up to several thousands atoms (Figure 2.2). The kinetic energy of
the clusters is of the order of 0.3 eV/atom. Characterization by high
resolution time-of-flight-mass spectrometry (TOF/MS) and in situ x-ray
photoemission spectroscopy indicates that no contaminants (oxygen,
nitrogen, hydrogen) are present in the beam and on the deposited films [30].
The PMCS source was mounted in the deposition apparatus CLARA
(cluster assembling roaming apparatus)
log-normal
[31],
schematically shown in
It consists of three differentially pumped vacuum chambers built to
UHV specifications and is equipped with a linear TOF/MS.
The first chamber hosts the cluster source, evacuated by two turbo-
molecular pumps (300 l/s). They are horizontally mounted facing each other,
in order to assure the correct operating conditions for the pulsed supersonic
FIGURE 2.2
Mass distribution of the cluster beam measured by a reflectron
TOF/MS. The masses are expressed as number of carbon atoms per clusters. As
shown in the enlarged windows clusters with both odd and even number of carbon
atoms can be observed.
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