194
12 Charge Transfer and the Harpoon Mechanism
Here X generally refers to halogen, while M aims at alkali metals. Researches
on this interesting topic retrospect to 1986 when Dudley R. Herschbach, Yuan T.
Lee, and John C. Polanyi received the Nobel Prize in Chemistry for elucidating the
collision dynamics of elementary chemical reactions [40]. Their research has been
of great importance for the development of a new field of research in chemistry (i.e.,
reaction dynamics) and has provided a much more detailed understanding of how
chemical reactions occur, including the discovery of harpoon mechanism. In a very
recent study, Castleman considered the validity of the harpoon mechanism in gasphase cluster reactivity of coinage metal clusters [41]. Interesting reactions of copper
and silver cluster anions toward chlorine were observed and the harpoon mechanism
is identified for “[Cu 8 ]
– /[Ag 8 ]
–
+ Cl 2 ”. This finding revealed the harpoon mechanism
in cluster reactivity which remains a subject of increasing interest and activity as a
bridge in probing atoms and macroscopic matter.
12.2 Charge-Transfer Reactions of Clusters
Charge transfer reactions between ionic and neutral species are basic processes in
physics and chemistry. In a simple case of ion–atom collisions, the development of
technology has largely improved the experimental approaches to control beams of
atoms and atomic ions with much ease and over an increasing range of impact energies
[42]. On the other hand, the theoretical demonstration for the elementary processes
has reached a high level of accuracy and enables predictions in the cases where charge
transfer is difficult to be investigated by experimental method. Further progress has
also been made for molecule/cluster targets with the advances in the controllable
production of molecular beams and clusters [43–45]. In particular, charge transfer
involving metal clusters has attracted reasonable research interest in the late decades
[46–51].
As an electron jump is often the first step of a chemical reaction, the direct observation of an electron transfer from an atom/cluster to a molecule/particle is of primary
interest in surface science and catalysis. A fundamental question is for which cluster
size the colliding partner interacts either with the whole cluster or only a part of it;
and from which cluster size does the interaction resemble an atom-surface interaction? Insights into the evolution of charge transfer properties between clusters and
molecules/atoms as a function of particle size is very important to address these questions. By collisional neutralization of mass selected cluster ions [52–55], researchers
have been able to estimate cluster ionization potentials through experimental data
[56], which concerns the availability of electron transfer to form the correlative
neutral clusters [50].
Early work by Bréchignac et al. [46] reported a study of charge exchange between
mass-selected sodium cluster ions Na
+
n and Cs atoms,
Na
+
n + Cs → Na n + Cs
+
, (1 ≤ n ≤ 21)
(12.5)
12 Charge Transfer and the Harpoon Mechanism
Here X generally refers to halogen, while M aims at alkali metals. Researches
on this interesting topic retrospect to 1986 when Dudley R. Herschbach, Yuan T.
Lee, and John C. Polanyi received the Nobel Prize in Chemistry for elucidating the
collision dynamics of elementary chemical reactions [40]. Their research has been
of great importance for the development of a new field of research in chemistry (i.e.,
reaction dynamics) and has provided a much more detailed understanding of how
chemical reactions occur, including the discovery of harpoon mechanism. In a very
recent study, Castleman considered the validity of the harpoon mechanism in gasphase cluster reactivity of coinage metal clusters [41]. Interesting reactions of copper
and silver cluster anions toward chlorine were observed and the harpoon mechanism
is identified for “[Cu 8 ]
– /[Ag 8 ]
–
+ Cl 2 ”. This finding revealed the harpoon mechanism
in cluster reactivity which remains a subject of increasing interest and activity as a
bridge in probing atoms and macroscopic matter.
12.2 Charge-Transfer Reactions of Clusters
Charge transfer reactions between ionic and neutral species are basic processes in
physics and chemistry. In a simple case of ion–atom collisions, the development of
technology has largely improved the experimental approaches to control beams of
atoms and atomic ions with much ease and over an increasing range of impact energies
[42]. On the other hand, the theoretical demonstration for the elementary processes
has reached a high level of accuracy and enables predictions in the cases where charge
transfer is difficult to be investigated by experimental method. Further progress has
also been made for molecule/cluster targets with the advances in the controllable
production of molecular beams and clusters [43–45]. In particular, charge transfer
involving metal clusters has attracted reasonable research interest in the late decades
[46–51].
As an electron jump is often the first step of a chemical reaction, the direct observation of an electron transfer from an atom/cluster to a molecule/particle is of primary
interest in surface science and catalysis. A fundamental question is for which cluster
size the colliding partner interacts either with the whole cluster or only a part of it;
and from which cluster size does the interaction resemble an atom-surface interaction? Insights into the evolution of charge transfer properties between clusters and
molecules/atoms as a function of particle size is very important to address these questions. By collisional neutralization of mass selected cluster ions [52–55], researchers
have been able to estimate cluster ionization potentials through experimental data
[56], which concerns the availability of electron transfer to form the correlative
neutral clusters [50].
Early work by Bréchignac et al. [46] reported a study of charge exchange between
mass-selected sodium cluster ions Na
+
n and Cs atoms,
Na
+
n + Cs → Na n + Cs
+
, (1 ≤ n ≤ 21)
(12.5)