196
12 Charge Transfer and the Harpoon Mechanism
ion arising from collision-induced dissociation (Na
+
1–8 ), and fast neutrals arising
principally from the charge-transfer process. In contrast, Fig. 12.1c shows a series of
reionization spectra for Na 9 , Na 3 , and Na 7, respectively. The spectra of Na 3 and Na 7
showed mostly the parent, evidently due to charge exchange. In contrast, for Na 9
the reionization spectrum displays Na 8
+ and Na 7
+ dominating the mass spectrum,
indicating that an evaporation of a monomer is associated with the charge transfer,
expressed as [46]
Na
+
n + Cs → Na n−p + Na p + Cs
+
, (1 ≤ n ≤ 21, p < n)
(12.6)
Higher masses of Na
+
n (e.g., Na 21 ) were also noted to exhibit evaporation of a
single atom in the re-ionization spectrum. Similar experiment on “K
+
n + Cs” was
also observed. Measurements on Na
+
n and K
+
n charge transfer cross-sections with
cesium atoms have been interpreted using the Rapp and Francis formalism [47].
Extensive investigations regarding the charge transfer and fragmentation in collisions of metal clusters have been undertaken in several other groups [50, 57–
95]. The charge transfer and fragmentation in collisions of alkali metals attracted
major interest, either theoretically through a microscopic framework called nonadiabatic quantum molecular dynamics, or experimentally through mass spectrometry.
Figure 12.2 displays such a study on Li
+
n clusters. Alike to the above, clusters were
produced as an evaporative ensemble containing some internal energy; thus, they
partially undergo uni-molecular dissociation during their propagation in the TOF.
Under such experimental conditions, the dissociation is dominated by evaporation
of a neutral monomer, hence a pathway as Eq. 12.7.
Li
2+
31 → Li
2+
30 + Li
(12.7)
Apparently, the dissociation ratio Li 30
2+ /Li 31
2+ depends on the two time windows
of the experiment: the residence time in the accelerating region (t 1 ) and the propagation time in the drift tube of the TOF (t 2 ), as shown in Fig. 12.2A. Products of the
dissociation process propagate in the first drift tube with the center-of-mass velocity
of the parent; and then they are spatially resolved into individual mass packets in the
second drift tube with dependence on the V R . When the collision cell is activated
(i.e., the Cs pressure is larger than zero), the structure displayed by the retarding field
images the combined effects of dissociation and charge transfer. By comparing the
spectra obtained cell-on and cell-off, the signals that are exclusively due to CT can
be identified.
Figure 12.2B presents three mass spectra for Li 31
2+ corresponding to the V R values
at 1250 and 2500 V, compared with the case of no collisions and no electrostatic
analysis (i.e., V R = 0 V) respectively. Charge transfer results in a neutral Li 31 cluster
which still gives chance to evaporate one and two Li atoms leading to smaller neutral
species Li 30 and Li 29 (seen as cationic Li 30
+ and Li 29
+ after the reionization). Based
on the experimental observations, the following channels are demonstrated: (i) CT
occurring for both the parent and its dissociation product,
12 Charge Transfer and the Harpoon Mechanism
ion arising from collision-induced dissociation (Na
+
1–8 ), and fast neutrals arising
principally from the charge-transfer process. In contrast, Fig. 12.1c shows a series of
reionization spectra for Na 9 , Na 3 , and Na 7, respectively. The spectra of Na 3 and Na 7
showed mostly the parent, evidently due to charge exchange. In contrast, for Na 9
the reionization spectrum displays Na 8
+ and Na 7
+ dominating the mass spectrum,
indicating that an evaporation of a monomer is associated with the charge transfer,
expressed as [46]
Na
+
n + Cs → Na n−p + Na p + Cs
+
, (1 ≤ n ≤ 21, p < n)
(12.6)
Higher masses of Na
+
n (e.g., Na 21 ) were also noted to exhibit evaporation of a
single atom in the re-ionization spectrum. Similar experiment on “K
+
n + Cs” was
also observed. Measurements on Na
+
n and K
+
n charge transfer cross-sections with
cesium atoms have been interpreted using the Rapp and Francis formalism [47].
Extensive investigations regarding the charge transfer and fragmentation in collisions of metal clusters have been undertaken in several other groups [50, 57–
95]. The charge transfer and fragmentation in collisions of alkali metals attracted
major interest, either theoretically through a microscopic framework called nonadiabatic quantum molecular dynamics, or experimentally through mass spectrometry.
Figure 12.2 displays such a study on Li
+
n clusters. Alike to the above, clusters were
produced as an evaporative ensemble containing some internal energy; thus, they
partially undergo uni-molecular dissociation during their propagation in the TOF.
Under such experimental conditions, the dissociation is dominated by evaporation
of a neutral monomer, hence a pathway as Eq. 12.7.
Li
2+
31 → Li
2+
30 + Li
(12.7)
Apparently, the dissociation ratio Li 30
2+ /Li 31
2+ depends on the two time windows
of the experiment: the residence time in the accelerating region (t 1 ) and the propagation time in the drift tube of the TOF (t 2 ), as shown in Fig. 12.2A. Products of the
dissociation process propagate in the first drift tube with the center-of-mass velocity
of the parent; and then they are spatially resolved into individual mass packets in the
second drift tube with dependence on the V R . When the collision cell is activated
(i.e., the Cs pressure is larger than zero), the structure displayed by the retarding field
images the combined effects of dissociation and charge transfer. By comparing the
spectra obtained cell-on and cell-off, the signals that are exclusively due to CT can
be identified.
Figure 12.2B presents three mass spectra for Li 31
2+ corresponding to the V R values
at 1250 and 2500 V, compared with the case of no collisions and no electrostatic
analysis (i.e., V R = 0 V) respectively. Charge transfer results in a neutral Li 31 cluster
which still gives chance to evaporate one and two Li atoms leading to smaller neutral
species Li 30 and Li 29 (seen as cationic Li 30
+ and Li 29
+ after the reionization). Based
on the experimental observations, the following channels are demonstrated: (i) CT
occurring for both the parent and its dissociation product,