Chemistry Reference
In-Depth Information
substitution does not greatly affect the geometric structure, but does alter the
relative energies, and thus populations, of the different isomers allowing for the
observed PES to be deconvoluted into the contributions arising from the different
isomeric forms [
57
-
59
].
This wealth of information concerning the isomers of the anionic clusters is not
matched for the cationic species. The apparent lack of structural isomerism for most
cationic gold clusters may be an artefact of the methodology used to probe their
structures. Specifically, because the second ionisation energies of gold clusters are
prohibitively high for tabletop laser systems, a direct analogue to anion PES is not
possible, and instead the majority of the structural information comes from TIED
and ion drift measurements. The latter is not very sensitive to small structural
changes which do not alter the collision cross section significantly, whilst TIED
requires the presence of a considerable fraction of the isomer in order to resolve the
different structures. Furthermore, these techniques occur on a timescale much longer
than anion PES (which is almost always assumed to be a prompt photoemission
process), and so if the structural isomerism is connected with rapid inter-conversion
(fluxionality), it may be averaged over in the experiments.
3.2 Neutrals
Through the efforts of the above research relying on TIED, ion mobility measure-
ments and anion PES, a comprehensive understanding about the structures of the
charged gold clusters has been achieved. Comparatively little experimental work,
however, has been done on the neutral clusters. The origin of this discrepancy is
eminently clear: Experimentally the above techniques do not work, nor are they
easily adapted to work for neutral systems. All of them require the interaction of the
charge on the particles with external fields to allow for the clusters from the
nascent distribution to be resolved by size, either in time or space. Once separated
in this way, they can then be spectroscopically addressed on a species by species
basis.
Dissociation spectroscopy, as described above, can be performed on a non-mass-
selected molecular beam containing charged
and
neutral species. As the irradiation
of the cluster beam is done
prior
to mass analysis, it is possible to add an ionisation
step after the spectroscopic probe, allowing for the recording of mass-specific
spectra for the neutral species. Figures
5
and
6
show far-infrared spectra of
Au
7
,Au
19
and Au
20
measured with krypton as a messenger [
29
]. Au
n
Kr complexes
are formed at 100 K by seeding the He gas with a few per cent of Kr and they are
then interrogated using the IR-MPD technique as outlined in the methods section.
Ionisation is performed using 7.9 eV photons (F
2
laser). Structural assignments
