182
11 Cluster Dissociation, Intracluster Reactivity and Effect of the Ligands
Therefore, it is difficult to discriminate between one-photon and multiphoton
processes, leaving a challenge to ensure correct conclusions when bracket cluster
dissociation energies using the photodissociation measurements. In order to eliminate such uncertainty, ion-molecule reactions (e.g., with ascertained bond activation energies) and collision-induced dissociation can be utilized simultaneously as a
reference to validate the energy values.
In comparison, photodissociation of diatomic metal cluster systems are relatively
simple dissociation process “M 2
+
→ M
+
+ M
+ ”. A typical study on this was reported
by Hettich and Freiser in 1987 [10]. With a focus on heteronuclear metal dimer ions
MFe
+ (M = Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Nb, and Ta) and utilizing Fourier transform mass spectrometry, they studied the photodissociation of these cluster systems,
which enabled a method of probing the fundamental bonding nature between two
bare transition-metal atoms. Both M
+ and Fe
+ were observed as photoproducts and
the one having a relatively lower ionization potential dominates the products respectively. The photodissociation spectra of MFe
+ obtained by monitoring the fragmentation of MFe
+ as a function of wavelength are displayed in Fig. 11.4. Among these
MFe
+ species, broad absorptions in the ultraviolet and visible spectral regions were
observed, revealing cross sections from 0.06 ´
Å
2 (for VFe
+ ) to 0.62 ´
Å
2 (for CrFe
+ ).
Bond energies obtained by noting the photoappearance onsets are in a range between
48 kcal/mol (for ScFe
+ ) and 75 kcal/mol (for VFe
+ ), which is in good agreement
with the values obtained by ion-molecule bracketing techniques. Moreover, ionization potentials for these MFe
+ can also be calculated comparing the ionic dimers
with their neutral metal dimer counterparts, as was found to be in the range of 5.4 eV
(for VFe) to 7.4 eV (for TaFe) [10].
11.1.3 Coulomb Explosion
Coulomb explosion is generally a process in which a molecule moving with high
velocity strikes a solid and the electrons that bond the molecule are torn off rapidly
in violent collisions with the electrons of the solid; as a result, the molecule is
suddenly transformed into a cluster of charged atomic constituents that then separate
under the influence of their mutual Coulomb repulsion. Coulomb explosions are most
studied using a particle accelerator which is normally employed in nuclear physics.
It could also be done under a narrow powerful laser beam, where a small amount of
solid explodes into plasma of ionized atomic particles; with their low masses, outer
valence electrons responsible for chemical bonding are easily stripped from atoms,
leaving them positively charged. Coulomb explosion has become a mechanism for
coupling electronic excitation energy from intense electromagnetic fields into atomic
motion. Given a mutually repulsive state between atoms whose chemical bonds are
broken, the material explodes into a small plasma cloud of energetic ions with higher
velocities than that seen in thermal emission.
11 Cluster Dissociation, Intracluster Reactivity and Effect of the Ligands
Therefore, it is difficult to discriminate between one-photon and multiphoton
processes, leaving a challenge to ensure correct conclusions when bracket cluster
dissociation energies using the photodissociation measurements. In order to eliminate such uncertainty, ion-molecule reactions (e.g., with ascertained bond activation energies) and collision-induced dissociation can be utilized simultaneously as a
reference to validate the energy values.
In comparison, photodissociation of diatomic metal cluster systems are relatively
simple dissociation process “M 2
+
→ M
+
+ M
+ ”. A typical study on this was reported
by Hettich and Freiser in 1987 [10]. With a focus on heteronuclear metal dimer ions
MFe
+ (M = Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Nb, and Ta) and utilizing Fourier transform mass spectrometry, they studied the photodissociation of these cluster systems,
which enabled a method of probing the fundamental bonding nature between two
bare transition-metal atoms. Both M
+ and Fe
+ were observed as photoproducts and
the one having a relatively lower ionization potential dominates the products respectively. The photodissociation spectra of MFe
+ obtained by monitoring the fragmentation of MFe
+ as a function of wavelength are displayed in Fig. 11.4. Among these
MFe
+ species, broad absorptions in the ultraviolet and visible spectral regions were
observed, revealing cross sections from 0.06 ´
Å
2 (for VFe
+ ) to 0.62 ´
Å
2 (for CrFe
+ ).
Bond energies obtained by noting the photoappearance onsets are in a range between
48 kcal/mol (for ScFe
+ ) and 75 kcal/mol (for VFe
+ ), which is in good agreement
with the values obtained by ion-molecule bracketing techniques. Moreover, ionization potentials for these MFe
+ can also be calculated comparing the ionic dimers
with their neutral metal dimer counterparts, as was found to be in the range of 5.4 eV
(for VFe) to 7.4 eV (for TaFe) [10].
11.1.3 Coulomb Explosion
Coulomb explosion is generally a process in which a molecule moving with high
velocity strikes a solid and the electrons that bond the molecule are torn off rapidly
in violent collisions with the electrons of the solid; as a result, the molecule is
suddenly transformed into a cluster of charged atomic constituents that then separate
under the influence of their mutual Coulomb repulsion. Coulomb explosions are most
studied using a particle accelerator which is normally employed in nuclear physics.
It could also be done under a narrow powerful laser beam, where a small amount of
solid explodes into plasma of ionized atomic particles; with their low masses, outer
valence electrons responsible for chemical bonding are easily stripped from atoms,
leaving them positively charged. Coulomb explosion has become a mechanism for
coupling electronic excitation energy from intense electromagnetic fields into atomic
motion. Given a mutually repulsive state between atoms whose chemical bonds are
broken, the material explodes into a small plasma cloud of energetic ions with higher
velocities than that seen in thermal emission.