178
11 Cluster Dissociation, Intracluster Reactivity and Effect of the Ligands
Integral cross sections for CID of Al
+
2–7 by xenon were measured for all product
ions at collision energies ranging from 0.0 to 10.0 eV. As results, a similar trend
existed for Al
+
2–6 on the collision energy dependence of the total CID cross sections.
However, Al 7
+ displays a higher threshold and a slower rise with increasing collision
energy, which indicates its better stability as a 20-electron cluster species. Another
common trend is that, for all reagent clusters, the two lowest energy channels both
involve the loss of a single atom hence produce Al
+ or Al
+
n-1 as ionic products,
expressed as [21],
Al
+
n + Xe → Al n−1 + Al
+
+ Xe
(11.2)
Al
+
n + Xe → Al
+
n−1 + Al + Xe
(11.3)
The threshold CID method has also been applied to study the fragmentation
patterns and to measure the dissociation energies of small anionic copper clusters
(Cu
−
n , n = 2–8) and their monocarbonyls (Cu n CO
− , n = 3–7). Similar to the above
case for Al
−
n , the main reaction channels for the bare clusters Cu
−
n (n = 2–8) were
also found to be the loss of an atom and loss of a dimer,
Cu
−
n + Xe → Cu
−
n−1 + Xe + Cu
(11.4)
Cu
−
n + Xe → Cu
−
n−2 + Xe + Cu 2
(11.5)
Furthermore, it is interesting to mention that all the clusters Cu
−
n (n = 2–8) were
found to take a reaction channel as Eq. 11.4, but among them only Cu
−
3–5 and Cu
−
7
support another reaction channel as Eq. 11.5. As Cu
−
7 is an 8-electron stable cluster
which is consistent with closed shells in the jellium model, and Cu
−
7 has the highest
dissociation energy, so it is reasonable for Cu
−
8 to take a dominant reaction pathway
as Eq. 11.4 leading to the species Cu
−
7 while give rare opportunity for it to undertake a
reaction channel as Eq. 11.5. Moreover, the dissociation energies for the loss of a Cu
atom from bare copper cluster anions also show even-odd alternation, indicating the
different stability of the odd and even electron copper cluster systems. Copper cluster
monocarbonyls (Cu n CO
− ) were also found to undertake the similar CID reactions,
but the main reaction channel is loss of CO. Similar to Cu
−
7 , the species Cu 5 CO
−
also has eight valence electrons and displays the highest carbonyl desorption energy
[37].
While CID is inherently a low-resolution method for the measurements of dissociation energies, it also provides a tool to measure good quality physical data on
systems which are not complicated by spectroscopic methods, such as approximate
ionization potentials. From the product branching ratios and cross section magnitudes
based on the CID investigations, qualitative structural information of the correlated
clusters could also be derived [21]. It is still notable that, high energy collisions of
metal clusters could be also associated with fragmentation induced by thermal effect
(known as thermal dissociation) [2, 4–6, 38] and even shock waves [39].
11 Cluster Dissociation, Intracluster Reactivity and Effect of the Ligands
Integral cross sections for CID of Al
+
2–7 by xenon were measured for all product
ions at collision energies ranging from 0.0 to 10.0 eV. As results, a similar trend
existed for Al
+
2–6 on the collision energy dependence of the total CID cross sections.
However, Al 7
+ displays a higher threshold and a slower rise with increasing collision
energy, which indicates its better stability as a 20-electron cluster species. Another
common trend is that, for all reagent clusters, the two lowest energy channels both
involve the loss of a single atom hence produce Al
+ or Al
+
n-1 as ionic products,
expressed as [21],
Al
+
n + Xe → Al n−1 + Al
+
+ Xe
(11.2)
Al
+
n + Xe → Al
+
n−1 + Al + Xe
(11.3)
The threshold CID method has also been applied to study the fragmentation
patterns and to measure the dissociation energies of small anionic copper clusters
(Cu
−
n , n = 2–8) and their monocarbonyls (Cu n CO
− , n = 3–7). Similar to the above
case for Al
−
n , the main reaction channels for the bare clusters Cu
−
n (n = 2–8) were
also found to be the loss of an atom and loss of a dimer,
Cu
−
n + Xe → Cu
−
n−1 + Xe + Cu
(11.4)
Cu
−
n + Xe → Cu
−
n−2 + Xe + Cu 2
(11.5)
Furthermore, it is interesting to mention that all the clusters Cu
−
n (n = 2–8) were
found to take a reaction channel as Eq. 11.4, but among them only Cu
−
3–5 and Cu
−
7
support another reaction channel as Eq. 11.5. As Cu
−
7 is an 8-electron stable cluster
which is consistent with closed shells in the jellium model, and Cu
−
7 has the highest
dissociation energy, so it is reasonable for Cu
−
8 to take a dominant reaction pathway
as Eq. 11.4 leading to the species Cu
−
7 while give rare opportunity for it to undertake a
reaction channel as Eq. 11.5. Moreover, the dissociation energies for the loss of a Cu
atom from bare copper cluster anions also show even-odd alternation, indicating the
different stability of the odd and even electron copper cluster systems. Copper cluster
monocarbonyls (Cu n CO
− ) were also found to undertake the similar CID reactions,
but the main reaction channel is loss of CO. Similar to Cu
−
7 , the species Cu 5 CO
−
also has eight valence electrons and displays the highest carbonyl desorption energy
[37].
While CID is inherently a low-resolution method for the measurements of dissociation energies, it also provides a tool to measure good quality physical data on
systems which are not complicated by spectroscopic methods, such as approximate
ionization potentials. From the product branching ratios and cross section magnitudes
based on the CID investigations, qualitative structural information of the correlated
clusters could also be derived [21]. It is still notable that, high energy collisions of
metal clusters could be also associated with fragmentation induced by thermal effect
(known as thermal dissociation) [2, 4–6, 38] and even shock waves [39].