Mario E. Fajardo

Cooperative photoabsorption induced charge transfer reaction dynamics in rare gas solids. I. Photodynamics of localized xenon chloride exciplexes

Cooperative charge transfer absorptions are demonstrated in HCl, Cl2, and Cl doped rare gas solids. The four‐body, two‐photon cross section for Xe+HCl+2hν→Xe+(HCl)− at 308 nm is >10−42 cm4 s. The same mechanism explains the efficient Cl atom photogeneration in Cl2 doped solids. The weak field cross sections for the three‐body one‐photon and four‐body one‐photon transitions, Xe+Cl+ hν→XeCl(B) and Xe+Xe+Cl+hν→Xe2Cl(4 2Γ), are comparable near the B←X resonance, the cross sections are estimated as 3 and 1(±0.5)×10−16 cm2. The XeCl C→A emission, B←X absorption, and C state lifetime [75 (±5) and 60 (±5) ns in Ar and Kr], can be accounted by bulk dielectric solvation of the molecular dipole. The Xe2Cl(4 2Γ) emission [573 nm in Ar, Kr, and Xe], and lifetime [250 (±10), 210 (±10) and 225(±10) ns in Ar, Kr, Xe] cannot be treated by the same model. The XeCl (B)→XeCl(C) and XeCl(C)+Xe→Xe2Cl(4 2Γ) reactions are fast, >2×1010 s−1. The XeCl(C) and Xe2Cl(4 2Γ) emissions can be modeled with one‐ and two‐dimensional potent...

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)

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...

Energy storage and thermoluminescence in halogen doped solid xenon. III. Photodynamics of charge separation, self‐trapping, and ion–hole recombination

The optically accessed excitonic charge transfer states of solid xenon doped with atomic halogens relax by one of two channels: self‐trapping of the exciton to form the triatomic molecular exciplex, or self‐trapping of the hole which leads to charge separation by the creation of a pair of oppositely charged small polarons. The latter channel leads to long term storage of optical energy. Charged pair storage densities of 1017 cm−3 are routinely realized, and retention times as long as 35 h have been directly verified. The trapped ion–hole pair may recombine either through tunneling or thermal activation of the self‐trapped hole. Tunneling leads to temperature independent phosphorescence which decays with a hyperbolic time dependence. Temporally and spectrally resolved phosphorescence and thermoluminescence are used for the characterization of the trapped state energetics and their recombination dynamics. A first order kinetic treatment of the recombination kinetics yields 800(±200) cm−1 as the lattice rela...

Potential energy surfaces and transition moments of a Cl atom in a Xe solid matrix

Xe solid containing a Cl atom is considered by the semiempirical diatomics‐in‐ionic‐systems method which takes into account the charge delocalization in an ionic state and the coupling between the neutral and ionic states. The calculation shows that the Cl atom has motional freedom in substitutional trapping sites which leads to the broadening of absorption bands. The vertically accessed ionic states are treated as Xe+12 Cl− molecules with a nonuniform positive charge distribution. The most stable ionic complex has the Xe+2 Cl− molecular configuration. Cl atoms at interstitial sites are treated as Xe6Cl molecules with Xe+6 Cl− excited states. The results of the calculation are in general agreement with recent experiments.

Stimulated radiative dissociation and gain measurements of Xe2Cl in solid xenon

The molecular charge transfer states of Cl-doped solid xenon form an ideal four-level laser system. UV excitation on the molecular XeCl (B+X) pair potentials leads to the formation of the diatomic exciplex which relaxes with a nearly unity quantum yield to the triatomic Xe,’ Cl- (4 ‘F ) state. The radiative dissociation of the triatomic exciplex can be stimulated to provide monoenergetic atoms % 1 eV above ground. While these systems are characterized by large gain coeffkients, scattering losses predominate in samples prepared by standard matrix isolation techniques. Scattering losses are due to the inability of the lattice to accommodate the excess kinetic energy released in the bound to repulsive transition of the triatomic exciplex.

Rydberg series of charge transfer excitations in halogen-doped rare gas crystals

Abstract Charge transfer excitations of halogen-doped rare gas solids are treated as Rydberg progressions of hole states. The analysis is in good agreement with experiments performed in Cl-, Br- and I-doped xenon.

Cooperative photoproduction of Xe2+Cl− in liquid Cl2/Xe solutions: Stimulated emission and gain measurements

Abstract Cooperative photogeneration of Xe 2 + Cl − can be accomplished by near UV laser excitation of Cl 2 /Xe solutions with a high quantum efficiency. Gain measurements on the Xe 2 + Cl − (4 2 Γ) transition are reported. The liquid phase rare gas halides should be regarded as a family of ideal “dye” lasers.