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Articles you may be interested inRotational spectrum of SO3 and theoretical evidence for the formation of sixfold rotational energy-level clusters in its vibrational ground state Terahertz spectroscopy and solid-state density functional theory calculation of anthracene: Effect of dispersion force on the vibrational modes Ab initio potential energy and dipole moment surfaces, infrared spectra, and vibrational predissociation dynamics of the 35 Cl − H 2 / D 2 complexesThe energy relaxation times of the fundamental stretching modes in the electronic ground state of OH Ϫ , OD Ϫ , SH Ϫ , and TeH Ϫ in a variety of alkali halides are measured by incoherent laser saturation and found to vary from 0.3 to 3 ns at 1.7 K. These vibrational lifetimes are between 4 and 8 orders of magnitude smaller than those of other heteronuclear diatomics diluted in crystals, including the ionic systems of CN Ϫ in salts and the neutral deuterides, DCl and ND, and oxides, CO and NO, in rare-gas matrices. Unlike these other systems, the chalcogen-hydride-doped alkali halides have a librational mode at frequencies well above the top of the host phonon band. This makes the librational decay channel a lower order process than relaxation into phonons. An energy gap law can be fit to the data, in which the relaxation times vary exponentially with the number of accepting reorientational modes. This model can explain the fact that OH Ϫ and OD Ϫ in KCl have nearly the same lifetimes, since the vibrational and librational frequencies both have the same isotope shift. Furthermore, previous persistent spectral hole burning measurements of SH Ϫ in mixed crystals are consistent with a picture in which the defects reorient during vibrational de-excitation. It is found that the reorientational decay rates are much faster than the equal-decay-order relaxation of CN Ϫ into translational modes in the silver and sodium halides. This could be explained by a factor of ϳ3 enhancement in the vibrational coupling constant to reorientations as compared to translations, but the relative strengths of the appropriate sidebands do not appear to support such an enhancement. The relaxation times of the diatomic hydrides are also found to be much smaller in ionic than in van der Waals hosts, even for equal order reorientational relaxation, suggesting that Coulombic forces significantly increase the V -R coupling strength.

Theoretical lattice-dynamics analysis and new optical experiments on KI:Ag + reveal a nearly unstable low-temperature defect-host configuration, whose population decreases dramatically with temperature, but whose dynamics are consistent with a temperature-independent harmonic model involving collective motion. This behavior is in marked contrast to the temperature-dependent evolution of the dynamics of "soft-mode" systems. PACS numbers: 78.30.-j, 63.20.Pw, 63.20.RyThe silver ion in KI is the most thermally unstable lattice-defect combination known. At 1.2 K the Ag + ion takes the same equilibrium position as the K + ion it replaces but by 20 K it is in an off-center position. 1,2 A fundamental problem is to understand within the framework of lattice dynamics how the system moves between these two lattice-defect arrangements with temperature. 3 Missing from the characterization of the dynamics have been complementary temperature-dependent optical and Raman data and a realistic Lifshitz-type calculation of the KI:Ag + lattice dynamics.To experimentally track the temperature dependence of different aspects of the coupled defect-host system, we first present uv measurements which follow the Ag + ion itself and then Raman scattering results which monitor the disappearance of the E g mode and the appearance of a new A\ -symmetry resonant mode as the temperature is raised. Next, the spectral properties of ir-and Ramanactive features for the on-center configuration are calculated within the harmonic approximation, and the interplay between theoretical and experimental results is described for the first time. The harmonic approximation is shown to give a good account of the low-temperature ir and Raman data, for a model in which the defect and surrounding host ions remain strongly coupled yet are quite close to being unstable against T\ u displacements. Our results imply that the observed temperature dependence of the on-center spectral features simply reflects the population in this state. The system is in either the ground-state configuration or not; a continuous evolution of the on-center defect dynamics with temperature does not occur.To monitor the position of the Ag + ion with temperature we make use of the known optical behavior of this ion in alkali-halide crystals, which can be understood in terms of the 4d 10 ->4d 9 5s parity-forbidden transitions of the defect. 4 " 6 These transitions are made allowed by vibronic couplings to the odd-parity vibrational modes, or if the defect is off center, by the static odd-parity lattice displacement; the larger the odd-parity contribution the larger the line strength. Figure 1(a) shows the measured uv absorption spectrum of KI:Ag + for two different temperatures. Note that over this temperature range the center frequencies remain essentially fixed. A linear concentration depen-10 2 10 1 10°

Severe scattering losses from KDP crystals have been correlated with the exposure of porous sol AR coated crystals to ambient humidity. The scattering is attributed to formation of etch pits which develop under the coating on the KDP surface along crystallographic axes. This high angle scattering can in turn produce laser damage of downstream optics either through modulation of the beam or by optic contamination from ablation of adjacent metal structures.We have developed a simple tool to characterize the evolution of scatter from sol-coated KDP surfaces. We have measured the rate of etch pit formation as a function of relative humidity and surface treatment using both microscopy and scattering. We will discuss various surface treatments which can be utilized to retard or eliminate the environmental degradation of KDP crystals.