184
11 Cluster Dissociation, Intracluster Reactivity and Effect of the Ligands
The valence electron cloud of metal clusters provides a finite fermion system with
remarkable properties as electronic shell effects and strong optical absorption in a
narrow frequency band [49–51]. The surface-plasmon resonance of metal clusters
depends sensitively on the geometry of the cluster and thus provides an ideal handle
for analyzing and for controlling cluster dynamics [52, 53]. The dynamical scenarios
become more involved when clusters stay in contact with a substrate, either embedded
inside or deposited on a surface. Although it is the aim of the present contribution
to explore, little has been done in the regime of highly non-linear dynamics on
metal clusters induced by intense laser fields [54–59]. Recently Fehrer et al. [60]
investigated the dynamical evolution of a Na 8 cluster embedded in Ar matrices of
various sizes, Na 8 Ar N (N = 30 to 1048). This system was excited by an intense short
laser pulse leading to high ionization stages, and the subsequent highly non-linear
motion of the cluster under Ar environment was analyzed in terms of trajectories,
shapes, and energy flow. The most prominent effects were found on the temporary
stabilization of high charge states for several ps, and sudden stopping of the Coulomb
explosion of embedded Na 8 clusters [60]. Döppner et al. [51] reported the charging
of free silver clusters in strong laser fields by dual-pulse excitation over a broad
cluster size range. Depending on the laser intensity and the cluster size, an optical
delay between 0.45-ps and 13.5-ps was found necessary to drive the plasmon mode
of the cluster into resonance with the laser field and allow for an effective charging of
the system. Note that the optimum time delay to reach maximum charging changes
with the cluster size and inversely with the laser intensity.
11.2 Intracluster Reactivity
Several investigations on metal clusters and metal cluster complex shed light on
the intracluster reactivity [34, 61–88]. For example, Fox et al. [34] reported the
intracluster reactivity on hydrated vanadium cations V
+ (H 2 O) n due to absorption
of black body radiation, as shown in Fig. 11.5. At a pressure of about 4 × 10
−10
mbar in the cell region, the fragmentation is induced by the black body background radiation. As shown in Fig. 11.5a, the initial cluster distribution which also
contained V(OH)
+
2 (H 2 O) m was observed after accumulating the ions for 2 s in the
cell; however, after a reaction delay of 2 s, the clusters shifted to lower masses, and
a distinct shift in favor of the V(OH)
+
2 (H 2 O) m clusters was observed. The V
+ (H 2 O) m
completely disappeared after 10 s, while the V(OH)
+
2 (H 2 O) m further evaporate water
until the final product V(OH)
+
2 (H 2 O) 3 was observed. This trend was also observed for
small hydrated vanadium cations (Fig. 11.5b), as well as mass-selected V
+ (H 2 O) 10
(Fig. 11.5c). In brief, besides the loss of water ligands, the V
+ (H 2 O) n clusters
were found to display two different intracluster redox reactions with size-dependent
branching ratios, resulting in V(OH)
+ (H 2 O) n or V(OH)
+
2 (H 2 O) n clusters together
with a concurrent release of atomic hydrogen. These behaviors reflect the properties
of the transition metals to form stable compounds in a variety of oxidation states
[34].
11 Cluster Dissociation, Intracluster Reactivity and Effect of the Ligands
The valence electron cloud of metal clusters provides a finite fermion system with
remarkable properties as electronic shell effects and strong optical absorption in a
narrow frequency band [49–51]. The surface-plasmon resonance of metal clusters
depends sensitively on the geometry of the cluster and thus provides an ideal handle
for analyzing and for controlling cluster dynamics [52, 53]. The dynamical scenarios
become more involved when clusters stay in contact with a substrate, either embedded
inside or deposited on a surface. Although it is the aim of the present contribution
to explore, little has been done in the regime of highly non-linear dynamics on
metal clusters induced by intense laser fields [54–59]. Recently Fehrer et al. [60]
investigated the dynamical evolution of a Na 8 cluster embedded in Ar matrices of
various sizes, Na 8 Ar N (N = 30 to 1048). This system was excited by an intense short
laser pulse leading to high ionization stages, and the subsequent highly non-linear
motion of the cluster under Ar environment was analyzed in terms of trajectories,
shapes, and energy flow. The most prominent effects were found on the temporary
stabilization of high charge states for several ps, and sudden stopping of the Coulomb
explosion of embedded Na 8 clusters [60]. Döppner et al. [51] reported the charging
of free silver clusters in strong laser fields by dual-pulse excitation over a broad
cluster size range. Depending on the laser intensity and the cluster size, an optical
delay between 0.45-ps and 13.5-ps was found necessary to drive the plasmon mode
of the cluster into resonance with the laser field and allow for an effective charging of
the system. Note that the optimum time delay to reach maximum charging changes
with the cluster size and inversely with the laser intensity.
11.2 Intracluster Reactivity
Several investigations on metal clusters and metal cluster complex shed light on
the intracluster reactivity [34, 61–88]. For example, Fox et al. [34] reported the
intracluster reactivity on hydrated vanadium cations V
+ (H 2 O) n due to absorption
of black body radiation, as shown in Fig. 11.5. At a pressure of about 4 × 10
−10
mbar in the cell region, the fragmentation is induced by the black body background radiation. As shown in Fig. 11.5a, the initial cluster distribution which also
contained V(OH)
+
2 (H 2 O) m was observed after accumulating the ions for 2 s in the
cell; however, after a reaction delay of 2 s, the clusters shifted to lower masses, and
a distinct shift in favor of the V(OH)
+
2 (H 2 O) m clusters was observed. The V
+ (H 2 O) m
completely disappeared after 10 s, while the V(OH)
+
2 (H 2 O) m further evaporate water
until the final product V(OH)
+
2 (H 2 O) 3 was observed. This trend was also observed for
small hydrated vanadium cations (Fig. 11.5b), as well as mass-selected V
+ (H 2 O) 10
(Fig. 11.5c). In brief, besides the loss of water ligands, the V
+ (H 2 O) n clusters
were found to display two different intracluster redox reactions with size-dependent
branching ratios, resulting in V(OH)
+ (H 2 O) n or V(OH)
+
2 (H 2 O) n clusters together
with a concurrent release of atomic hydrogen. These behaviors reflect the properties
of the transition metals to form stable compounds in a variety of oxidation states
[34].