Charge transfer photodynamics in halogen doped xenon matrices. II. Photoinduced harpooning and the delocalized charge transfer states of solid xenon halides (F, Cl, Br, I)

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Abstract
The optically accessed charge transfer states of solid xenon doped with atomic halogens are excitonic in nature: an electron localized on the guest halogen atom and a delocalized hole centered on xenon atoms. These excitonic states are effectively self-trapped such that luminescence is observed exclusively from the localized molecular charge transfer states: the triatomic xenon halide exciplexes. The latter relax radiatively. The emission spectra of Xe+2 I−, Xe+2 Br−,Xe+2 Cl−, and Xe+2 F− are centered at 390, 480, 573, and 775 nm, and their radiative lifetimes are 130, 185, 225, and 190 ns, respectively. The charge transfer excitation spectra of the atomic solids are presented. In the case of F doped solids, the vertical transitions correspond to the diatomic XeF (B←X) and (D←X) absorptions: fluorine is bound to xenon in the ground state. The heavier halogens isolate atomically. Their excitation spectra are treated by a modified reflection approximation: reflection of the halogen–xenon radial distribution function from the hole transport potential. Ion–hole pairs separated by many lattice sites can be created by optical excitation, hence the spectra yield both the long range hole transport potential and the extended structure of the solid around the impurity site. The structure is fcc. Cl atoms generated by photodissociation of HCl or Cl2 are born at interstitial sites and convert to substitutional upon annealing of the solids. The atomic solids are prepared by two-photon induced harpoon reactions between xenon and molecular halogens: Xe+X2+2hν→[Xe+X−2 ]→Xe+X−+X. The inordinate efficiency of these reactions are attributed to the ionic potentials and the intimate participation of the polarizable cage atoms in ejecting the neutral halogen—a ‘‘negative’’ cage effect is postulated.