Wim Joosen

Raman scattering of KI:Ag+: Exploration of a nearly unstable defect-lattice configuration

Abstract The Raman spectra of Ag+ in KI exhibit between 6K and 25K two resonant modes and quasi-elastic scattering (QES), each with a different temperature behavior. The low-temperature Raman and infrared (ir) spectral features can be described within a harmonic force-constant model, which does not however account for the dramatic temperature dependence of the experimental data. They are compatible with a population transfer from a low-temperature on-center configuration to a second elastic configuration, in which the Ag+ ion is displaced from the regular lattice site. The QES, which dominates the Raman spectra above 20K, may reflect different types of rotational diffusion or the presence of a third elastic configuration.

Vibrational properties of the Ga0 and In0 centers of the Tl0(1) structure as studied by resonant Raman scattering

Abstract The Raman scattering spectra of M0(1) centers (M=Ga, In, and Tl) in KCl, which possess the laser-active type structure, have been resonantly excited in their optical transitions (OT3). The low frequency resonances at 59, 45, and 30cm-1, respectively, are attributed to the totally symmetric motion of the impurity atom along the fourfold axis in the C4v point group of the defects. The spectral distribution of the defect-induced first order phonon spectrum is remarkably similar for the three centers, showing that it essentially reflects the motion of the surrounding ions. The differences with an F-center-like phonon spectrum, in particular, the high relative intensity of the low frequency resonances indicate a dominant coupling of the electron with the motion of the heavy metal impurity and therefore illustrate the predominantly atomic character of these defects. The Raman spectrum of In0(1) in NaCl almost exclusively consists of the fundamental vibration of In0 and its overtones. A correspondence is indicated between this discrete character of the Raman spectrum and the electronic properties of the M0(1) defects in the NaCl host lattice.

Resonant Raman scattering of the laser active Tl°(1) defect in KCl

Abstract Resonant Raman measurements were performed on the A 1 vibrational mode of the Tl° atom in the Tl°(1) center in KCl:Tl + by excitation in the σ-polarized absorption band at 550 nm. The Tl°(1) center consists of a substitutional Tl° atom perturbed by a single nearest neighbor anion vacancy and it is known to be laser active with output in the infrared around 1.5 μm. A localized vibration at 30 cm -1 is proved to belong to the A 1 representation of a C 4v [001] symmetry by application of the recently developed Behavior Type method to the polarized Raman scattering data. Furthermore, there is evidence for a first order phonon spectrum induced by the Tl°(1) defect.

Behavior type method: Symmetry and structure of point defects in cubic crystals derived from polarized Raman scattering

Abstract A survey is given of the Behavior Type (BT) method (Zhou et al., Phys. Rev. B29 , 5509, 1984) for the analysis of the polarized Raman spectra of point defects in cubic crystals and its recent application to a variety of essentially different dynamical modes of color centers in the alkali halides. The symmetry group of a point defect and the irreducible representation(s) of its Raman transition(s) are reflected in the polarized Raman spectra through the form of the second-rank Raman tensor. In the BT method the polarized Raman intensities have been calculated for the symmetric Raman tensors of all possible point defect symmetry groups that occur in a cubic lattice. Both random and preferential distribution of the defects among their equivalent orientations have been treated. From a set of polarized Raman spectra of a specific transition, at least three independent intensity parameters can be determined and this allows one to eliminate the point groups and representations which are not consistent with the measurements, i.e. the method permits one to retrieve in an inductive way and to the largest possible extent the group-theoretical form of the Raman tensor. The BT method has been applied to the study of dynamical modes of (i) isolated and perturbed interstitial hydrogen atom centers, (ii) reorienting and static molecular impurities and (iii) substitutional impurity ions exhibiting off- or on-center positions. Resonant Raman scattering, requiring an extension of the BT method to non-symmetric Raman tensors, was performed on several F A centers and on defects with the Tl 0 (1) laser-active type structure. Some electronic Raman transitions of Sm 2+ in KCl were shown to possess an anti-symmetric tensor. In most of these cases, it is demonstrated by comparison that the BT method yields more precise conclusions about the symmetry and structure of the point defect than the traditional analysis of polarized Raman data. Further possible applications of the BT method are indicated.

Picosecond ground state recovery of the F-center in RbCl, KCl and KBr

Abstract Optical pump-probe measurements on the F-center in RbCl, KCl and KBr show a ps ground state recovery, when a substantial number of F-aggregates is present. The observed de-excitation process becomes faster for a larger concentration of aggregate centers and is related to energy transfer from the excited state of the F-center to aggregate centers. This energy transfer occurs towards F2- and N-centers in KCl. Our results are in accordance with measurements of the quantum efficiency of the F-center luminescence and with transient absorption measurements.

Photon echoes and site-selective raman scattering of samarium-vacancy dipoles

Abstract Electronic Raman scattering of Sm2+ in KCl is performed under resonant interconfigurational excitation around 500 nm. The position of the charge-compensating cation vacancy with respect to the substitutional Sm2+ ion is derived from both the Stark splitting of the J-levels and the polarization properties of the scattered light. The influence of the different site symmetries on the optical linewidth is investigated with photon echoes, stimulated from an accumulated grating, which is built up under intraconfigurational excitation between the 7 F 0 and the 5 D 1 terms of the 4f 65 s 25 p 6 ground state configuration. The echo decay time is about 10 ps and does not change sizably when tuning the excitation frequency over the inhomogeneous distribution. This shows that for zero-phonon line excitation, the Sm2+ sites are not very different. The temperature dependence of the echo is described by vertical relaxation to the bottleneck under multi-phonon emission.