16
2 Instrumentation for Cluster Science
the powers necessarily enough to evaporate metals with a Q-switched laser; (ii) as
the buffer gas used to carry the burgeoning clusters out of the source must be rapidly
pumped out of the vacuum instrument, it is much easier to manage when the laser
and thus the gas is pulsed; (iii) the formation of plasma around the target area may
briefly shield the material as tailing light is absorbed by the hot and opaque blowoff
material [68], but it takes several milliseconds (Fig. 2.3b) for the plasma-shielded
material to cool down when the plasma fully dissociates, so it is not necessary to use
too long laser pulse. Actually, if the laser faster than 200 Hz (including continuouswave lasers), stronger than 10
8 W/cm
2 and with a pulse duration longer than 10
−4 s,
the amount of material evaporated will be reduced and hence direct influence to the
intensity of the clusters produced [100].
2.1.4 Sputtering Sources
(a) Arc-discharge sputtering
The arc-discharge metal cluster source is to employ a DC discharge at a metal cathode
in a helium/argon flow [101–103]. Cluster ions are produced from sputtering of
the cathodic metal target by ionized Ar
+ and followed by further clustering in the
discharge plasma. Typically, Fig. 2.4a displays one of such cluster source used by
Ho et al. [103]. The use of carrier gas flowing in around the cathode feedthrough
helped to prevent the plating of metals onto the glass insulator. The cathode was
negatively biased (e.g., 3–5 kV) with respect to the grounded flow tube, where the
parameters of gas composition, flow rate and DC-voltage were adjustable to optimize
the cluster anion yields. In order to minimize the arcing from the cathode to ground,
they simply employed electrical ballast consisting of a 100 k resistor and a 4H
inductor connected in series with the DC power supply, resulting in a discharge with
Fig. 2.4 a Schematic diagram of the flowing afterglow system with the metal cathode discharge
ion source; b Mass spectra of Au −
n clusters produced in the metal cathode discharge ion source.
Adapted from Ref. [103]. Copyright 1994 by the American Physical Society
2 Instrumentation for Cluster Science
the powers necessarily enough to evaporate metals with a Q-switched laser; (ii) as
the buffer gas used to carry the burgeoning clusters out of the source must be rapidly
pumped out of the vacuum instrument, it is much easier to manage when the laser
and thus the gas is pulsed; (iii) the formation of plasma around the target area may
briefly shield the material as tailing light is absorbed by the hot and opaque blowoff
material [68], but it takes several milliseconds (Fig. 2.3b) for the plasma-shielded
material to cool down when the plasma fully dissociates, so it is not necessary to use
too long laser pulse. Actually, if the laser faster than 200 Hz (including continuouswave lasers), stronger than 10
8 W/cm
2 and with a pulse duration longer than 10
−4 s,
the amount of material evaporated will be reduced and hence direct influence to the
intensity of the clusters produced [100].
2.1.4 Sputtering Sources
(a) Arc-discharge sputtering
The arc-discharge metal cluster source is to employ a DC discharge at a metal cathode
in a helium/argon flow [101–103]. Cluster ions are produced from sputtering of
the cathodic metal target by ionized Ar
+ and followed by further clustering in the
discharge plasma. Typically, Fig. 2.4a displays one of such cluster source used by
Ho et al. [103]. The use of carrier gas flowing in around the cathode feedthrough
helped to prevent the plating of metals onto the glass insulator. The cathode was
negatively biased (e.g., 3–5 kV) with respect to the grounded flow tube, where the
parameters of gas composition, flow rate and DC-voltage were adjustable to optimize
the cluster anion yields. In order to minimize the arcing from the cathode to ground,
they simply employed electrical ballast consisting of a 100 k resistor and a 4H
inductor connected in series with the DC power supply, resulting in a discharge with
Fig. 2.4 a Schematic diagram of the flowing afterglow system with the metal cathode discharge
ion source; b Mass spectra of Au −
n clusters produced in the metal cathode discharge ion source.
Adapted from Ref. [103]. Copyright 1994 by the American Physical Society