12
2 Instrumentation for Cluster Science
sputtering sources including arc-discharge sputtering and magnetron sputtering. The
working principles of these sources are described in detail below.
2.1.1 Thermal Heated Oven Sources
Metal cluster sources have been used as an integral part of molecular beam experiments for many years. Among them, thermal heated oven sources (also known as
Knudsen ovens) were widely used in the early investigations [4–14]. Afterwards
Larsen et al. [4] extended this kind of sources and applied to the study of alkali
metal atom/cluster systems by using a supersonic nozzle and “seeding” technique.
Their design of the oven, as displayed in Fig. 2.1, allowed the beam gas of metal
atoms/clusters mixing with a light diluent gas, and hence the beam species accelerated to a velocity equal to that of the diluent gas. The seeded beams were used
to measure cross sections for electronic excitation in collisions of fast alkali atoms,
utilizing thermal energy beams of the alkali atoms and mercury atoms, as well as
various diatomic and polyatomic molecules [4].
Similar sources were also built with thermionic emission from heated metal salt
cathodes, and decomposition of volatile organometallics [15]. For example, Draves
et al. [16] developed a thermionic and molecular-beam source and firstly applied it
to the studies upon Cs(CH 3 OH)
+
n cluster systems. The thermionic metal-ion source
was combined with an electrostatic lens system, a quadrupole mass filter, and an
electron multiplier for signal detection. The apparatus consistsed of two chambers,
a source chamber and a detector chamber which were pumped by diffusion pumps,
respectively. During the operation, the metal-ion source was placed away from the
nozzle at a linear distance of ~5 mm. An electric current of ~5 A from a floating
power supply (±100 V) was passed through the filament. The metal-ion emission
can be enhanced by biasing the filament relative to ground [16].
2.1.2 Electron Impact and Electrospray Ionization (ESI)
Electron impact (EI) and electrospray ionization (ESI) sources [17–19], as another
kind of free-jet themes, are known of great value applied in mass spectrometry [17–
19]. Utilizing the ESI technique, protonated water clusters and series of alkali metal
halide clusters were successfully prepared and investigated [20–22]. Up to now,
both for practical purposes and to discern the mechanisms of ESI itself, various salt
cluster ions have been studied by the ESI mass spectrometry (ESI-MS) [18, 23–27].
Figure 2.2 depicts a sketch drawing of a typical ESI source [28], where the nebuliser
(N 2 gas) makes the ionised liquid to spray through a taylor-cone nozzle, resulting
to aerosol plume and forming gas-phase cluster ions. There are two mechanisms
for the ESI process: the ion evaporation model proposed by Iribarne and Thomson
[29, 30], and the charge residue model originated by Dole et al. [17] and developed
2 Instrumentation for Cluster Science
sputtering sources including arc-discharge sputtering and magnetron sputtering. The
working principles of these sources are described in detail below.
2.1.1 Thermal Heated Oven Sources
Metal cluster sources have been used as an integral part of molecular beam experiments for many years. Among them, thermal heated oven sources (also known as
Knudsen ovens) were widely used in the early investigations [4–14]. Afterwards
Larsen et al. [4] extended this kind of sources and applied to the study of alkali
metal atom/cluster systems by using a supersonic nozzle and “seeding” technique.
Their design of the oven, as displayed in Fig. 2.1, allowed the beam gas of metal
atoms/clusters mixing with a light diluent gas, and hence the beam species accelerated to a velocity equal to that of the diluent gas. The seeded beams were used
to measure cross sections for electronic excitation in collisions of fast alkali atoms,
utilizing thermal energy beams of the alkali atoms and mercury atoms, as well as
various diatomic and polyatomic molecules [4].
Similar sources were also built with thermionic emission from heated metal salt
cathodes, and decomposition of volatile organometallics [15]. For example, Draves
et al. [16] developed a thermionic and molecular-beam source and firstly applied it
to the studies upon Cs(CH 3 OH)
+
n cluster systems. The thermionic metal-ion source
was combined with an electrostatic lens system, a quadrupole mass filter, and an
electron multiplier for signal detection. The apparatus consistsed of two chambers,
a source chamber and a detector chamber which were pumped by diffusion pumps,
respectively. During the operation, the metal-ion source was placed away from the
nozzle at a linear distance of ~5 mm. An electric current of ~5 A from a floating
power supply (±100 V) was passed through the filament. The metal-ion emission
can be enhanced by biasing the filament relative to ground [16].
2.1.2 Electron Impact and Electrospray Ionization (ESI)
Electron impact (EI) and electrospray ionization (ESI) sources [17–19], as another
kind of free-jet themes, are known of great value applied in mass spectrometry [17–
19]. Utilizing the ESI technique, protonated water clusters and series of alkali metal
halide clusters were successfully prepared and investigated [20–22]. Up to now,
both for practical purposes and to discern the mechanisms of ESI itself, various salt
cluster ions have been studied by the ESI mass spectrometry (ESI-MS) [18, 23–27].
Figure 2.2 depicts a sketch drawing of a typical ESI source [28], where the nebuliser
(N 2 gas) makes the ionised liquid to spray through a taylor-cone nozzle, resulting
to aerosol plume and forming gas-phase cluster ions. There are two mechanisms
for the ESI process: the ion evaporation model proposed by Iribarne and Thomson
[29, 30], and the charge residue model originated by Dole et al. [17] and developed