How do carbon dioxide molecules prevent an electron from escaping a Br2− ion in a cluster?

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cluster Figure 1: The four stable geometric structures of the seven-atom cluster of argon, in order of increasing energy: (A) A pentagonal bipyramid. (B) A regular octahedron with one face capped by the seventh atom. (C) A regular tetrahedron with three of its faces capped by other atoms. (D) A trigonal bipyramid with two of its faces capped by other atoms; although this has the highest energy of the four structures, it is very close in energy to the tricapped tetrahedron.

When a negatively charged bromine molecule ( ${Br}_{2}^{-}$ ) is encased by a sufficient number of inert carbon dioxide ( ${CO}_{2}$ ) molecules—typically 10 to 15—the surrounding cluster acts as a cage that prevents an excess electron from escaping after being detached by light.1

Instead of escaping, the electron loses its energy to the surrounding ${CO}_{2}$ molecules. This energy transfer causes some of the ${CO}_{2}$ molecules to boil off, while the electron eventually recombines with the neutral bromine molecule to re-form the original ${Br}_{2}^{-}$ ion. This process illustrates a competition between a particle's attempt to escape a cluster and the energy-dissipating interactions provided by the surrounding cage of inert molecules.1