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Highly charged molecular ions, formed by either photoabsorption or charged particle impact, undergo dissociation. Dissociation leads to a sharing of the total molecular charge and conversion of the internal energy into translational energy of the fragment. Molecular ions possess a different electronic configuration as compared to the neutral molecule. Since a given molecular state is a function of the electronic and nuclear forces, the nuclear configuration (or the geometry) of the molecular ion would, in general, be different from that of the neutral molecule. The potential energy curve of an electronically excited molecular state may be entirely repulsive, and dissociation may occur. To obtain the properties of such electronic states of the precursor ion and to understand the energetics of the break up, it is necessary to determine complete kinematics of the dissociation products. We investigated dynamics of the dissociative ionization of CCl$_4$ and SF$_6$ using multiple ion coincidence imaging of fragment ions. Dissociative ionization was carried out by impact of high energy electrons on neutral molecules. Both chosen molecules have higher order of symmetry in their ground states. CCl$_4$ has tetrahedral geometry and SF$_6$ has octahedral geometry. Two observations concerning the dissociation fragments are common to these molecules. The first is the absence of parent ions (SF$_6^{q+}$ and CCl$_4^{q+}$) in the mass spectrum. The second is the preferential formation of molecular ions which have even number of electrons vi-{\`a}-vis molecular ions with odd number of electrons. In the mass spectrum of CCl$_4$, ions of the type CCl$_{2n+1}^{2+}$ are observed whereas CCl$_{2n}^{2+}$ are not, where $n=$ 0--1. In the case of SF$_6$, the probability of formation SF$_{2n+1}^{+}$ ions was observed to be higher than that of SF$_{2n}^{+}$ ($n=$ 0--2). Similarly, SF$_{2m}^{2+}$ ions were found to be more probable than SF$_{2m+1}^{2+}$, where $m=$ 0--2. This leads us to the conclusion that a high probability of formation of a certain fragment ion is strongly correlated to the number of electrons. For the case of CCl$_4$ ions possessing odd number of electrons have high probability of formation and for the case of SF$_6$ the ions possessing even number of electrons have high probability of formation. The number of electrons appears to be of further significance, as is seen from the following. The decay of dicationic SF$_6$ preferentially follows those decay channels which lead to fragments with even number of electrons. SF$_6^{2+}$ dissociating into F$^+$ and SF$_{2n-1}^+$ shows higher abundance than SF$_6^{2+}$ dissociating into F$^+$ and SF$_{2n-2}^+$, where ($n=$ 1--3). This effect is not seen in the dissociation of dicationic CCl$_4$. The dissociative ionization of a polyatomic molecule which has a high order of symmetry shows the Jahn-Teller effect [1]. In accordance with this effect a molecule or a molecular ion attains a minimum energy configuration by nuclear displacements. If this does not occur the molecule or the molecular ion dissociates via certain paths, as is seen in the above cases. This effect is prominent in those molecules which have $d$ hybridized molecular orbitals. This explains the alternation in the intensity of dissociation channels of SF$_6$. The S atom in the valence configuration 3s$^1$3p$^3$3d$^2$ together with six F atoms yields optimal bonding. CCl$_4$ dissociative ionization does not exhibit this pattern, since the $d$ orbital is absent in electronic configuration of CCl$_4$. The valence configuration of C is 2s$^1$3p$^3$, which yields optimal bonding with four Cl atoms at the apices of the tetrahedron. Reference: H A Jahn and E Teller , {\it Proceedings of the Royal Society of London Series A-Mathematical and Physical Sciences}, vol 161, 220 (1937). |
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