Solid Ar samples doped with the noble metal atoms Au or Ag as well as with O 2 molecules have been exposed to synchrotron and x-ray irradiation. Impurity trapping of excitons generated by the irradiation partly led to an ionization; the impurities formed deep traps for one type of charge carrier, the complementary ones were promoted into band states and subsequently captured into shallow traps. These charge carriers could be thermally released giving rise to thermally stimulated luminescence (TSL) and conductivity (TSC) signals, which were recorded simultaneously. The glow curves of both, TSL and TSC, clearly revealed the existence of intrinsic and extrinsic electron traps. Using a step-like temperature increase it could be demonstrated that traps exist with a broad distribution of binding energies. A first-order kinetics model was developed to extract binding energies from the temperature dependence of the thermally stimulated luminescence.

No abstract

Excitation of Au-doped krypton solids with synchrotron radiation in the range of the krypton excitons leads to the emission of the molecular self-trapped exciton at 8.41 eV and to a characteristic Au-atom Quorescence at 2.74 eV. By measuring simultaneously the Quorescence-yield spectra of these two emissions and their dependence on parameters such as sample thickness and temperature, crystal quality, and Au concentration, information on the excitonic energy transfer is obtained. An exciton difFusion model describes the measured spectra quantitatively.Exciton difFusion lengths, difFusion constants, and the trapping rate at the Au atoms are evaluated from the data. I. INTRODU CTIONExcitonic energy transfer in condensed rare gases is a well-known phenomenon, for a review see Ref. 1. A rather unspeci6c way to produce excitons is x-ray irradiation. In some experiments of this type doped rare gas solids have been irradiated with x rays and the Quorescence of the dopants has been investigated. ' The x rays are absorbed in the solid via a photoeffect resulting in the production of high energy photoelectrons. Then the photoelectrons dissipate their energy by producing a lot of excitons which are partly trapped at the impurities and give rise to their respective Huorescence. In these experiments the concentration of the dopants was too small to account for the high emission intensity by a direct absorption of the x rays at the dopants.A more specific way to study excitonic energy transfer processes is the selective excitation of excitons by resonance absorption. Because of the large band gaps of the rare gas solids the energies of the exciton bands are in the VUV spectral range from about 8.4 eV (Xe) to about 21.3 eV (Ne); therefore it is necessary to use a synchrotron as a light source. During irradiation with synchrotron light in the excitonic range Ophir et al. observed the emission of photoelectrons of doped Ar, Kr, and Xe samples due to impurity ionization4 5 and of undoped solid Xe due to an energy transfer to the Au substrate. In both cases they were able to explain quantitatively their measurements assuming an energy transfer via a diffusion of excitons.The competing processes to exciton trapping at impurities are direct radiative recombination as well as self-trapping in the solid leading to free exciton (FE) emission or atomic or molecular self-trapped exciton (ASTE, MSTE) emissions, respectively.In samples slowly grown &om the gas phase close to the triple point Varding et al. found a total Quorescence intensity ratio of the FE emission to the emission of the self-trapped centers of about 1 for solid Xe and of about 0.1 in the case of solid Kr. In polycrystalline samples of Kr and Xe, on the other hand, prepared as described in Sec. II, selftrapping into molecular centers is the dominating process and the total Quorescence intensity of the MSTE emission is 2 -3 orders of magnitude higher than the intensity of the FE emission. ' ' Ackermann et al. measured the Huorescence yield spectra of the MSTE emission in und...

No abstract

No abstract