Chemistry Reference
In-Depth Information
These reviews focus on the charged gold clusters and surprisingly large differences
in, for example, their geometric shapes and the size for the 2D-3D transition have
been found between the charge states. Such a dramatic influence of charge state
raises intriguing questions concerning the intermediate case, that of the neutral
clusters, which shall be discussed in more detail here.
2 Experimental Methods
2.1 Mass Spectrometry
Conceptually one of the simplest experimental techniques applied to clusters, and
indeed the basis of most gas-phase studies, is mass spectrometry. Very different
techniques for mass analysis have been applied to separate and/or trap charged
clusters, for example, classical electric or magnetic sector-field instruments, time-
of-flight measurements, quadrupole mass filters, and Paul or Penning traps. Mass
spectrometry is indispensable in the analysis and control of cluster formation in
cluster sources where typically a supersaturated atomic vapour aggregates into a
distribution of cluster sizes. By now, this field of mass spectrometric characterisa-
tion of clusters is very well established and so we shall refrain from discussing
further technical details here. It should be noted that mass spectrometry requires the
presence of charged clusters; however, by employing suitable ionisation techniques
prior to mass analysis, information on the neutral clusters may also be obtained.
Despite its conceptual simplicity, a great deal of information can be extracted from
the mass analysis of a cluster distribution, concerning relative stabilities of different
clusters (cf. “magic” sizes) and the reactivity of the clusters.
In addition to this, kinetic reaction studies can also be performed, i.e. analysing
the products formed with changing reaction times or reactant partial pressure.
Knowledge of the kinetics allows for the individual steps in the reaction paths to
be modelled and ultimately can lead to the elucidation of complete reaction
mechanisms [
16
,
17
].
2.2 Trapped Ion Electron Diffraction
One approach to investigating cluster structures is trapped ion electron diffraction
(TIED) [
22
,
23
]. In TIED, an electron beam of several tens of keV is directed at a
sample of size-selected clusters held in an ion trap. The electrons are diffracted by
the atoms and the resulting diffraction pattern is recorded, typically with a CCD
camera. The radial variance in the electron intensity pattern is then compared with
simulated curves, and structural assignment is made based upon the best agreement
between theory and experiment. Experimentally the cluster sizes which can be
