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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 8682-8687 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Single crystals of KTiOPO4 (KTP) often contain trace amounts of isolated platinum impurities. When present in sufficient concentration, these ions increase the KTP crystal's susceptibility to form gray tracks during frequency doubling of high-power laser beams. Electron paramagnetic resonance (EPR) has been used to characterize three platinum centers in a crystal of KTiOPO4. Initially, the platinum ions are present in nonparamagnetic forms. Exposure to ionizing radiation (i.e., x rays or above-band-gap laser beams) at room temperature converts them into paramagnetic centers. Once formed, these centers are stable for weeks at room temperature; however, their EPR spectra can only be observed at temperatures near or below 30 K. An angular study provided principal values and principal directions for the g matrices and the platinum hyperfine matrices. The Pt(A) center has principal g values of 1.9397, 2.4463, and 2.5900 and is assigned to a Pt3+ ion substituting for a Ti4+ ion. In contrast, the Pt(B) center has principal g values of 1.6019, 1.9754, and 2.7788 and the Pt(C) center has values of 1.4656, 1.8597, and 2.9576. We suggest that these latter two centers are Pt+ ions substituting for K+ ions. © 2000 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 47-52 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electron paramagnetic resonance (EPR) has been used to characterize a hole trap and several electron traps in single crystals of potassium dihydrogen phosphate (KH2PO4 or KDP). The paramagnetic charge states of these centers are produced by ionizing radiation (e.g., x rays or a 266 nm beam from a pulsed Nd:YAG laser) and are stable for days and even weeks at room temperature. One center consists of a hole trapped on an oxygen ion adjacent to a silicon impurity located on a phosphorus site. This defect has a small, but easily observed, hyperfine interaction with the adjacent substitutional proton. The other centers are formed when an electron is trapped at an oxygen vacancy. These latter defects are best described as (PO3)2− molecular ions, where the primary phosphorus nucleus is responsible for a large hyperfine splitting (500–800 G in magnitude). Five EPR spectra representing variations of these oxygen vacancy centers are observed, with the differences being attributed to the relative position of a nearby cation vacancy, either a missing proton or potassium. An angular study of the EPR spectra, conducted at room temperature, provided principal values and principal directions for the g matrices and hyperfine matrices for the hole center and two of the electron centers. © 2001 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 75 (1999), S. 1503-1505 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The intrinsic "self-trapped" hole center in KD2PO4 crystals has been identified using electron paramagnetic resonance and electron-nuclear double resonance. These defects, labeled [D2PO4]o centers, can be formed at 77 K by irradiating with either 60 kV x rays or the fourth harmonic (266 nm) of a pulsed Nd:YAG laser. The hole is equally shared by two adjacent oxygen ions, and hyperfine interactions with one phosphorus and two equivalent deuterons are observed. The sample used in this investigation was approximately 80% deuterated, thus both [D2PO4]o and [HDPO4]o centers were detected, with the former being dominant. These intrinsic self-trapped hole centers are of interest because of their potential role in the transient optical absorption produced in KD2PO4 crystals at room temperature by intense 266 nm laser pulses. © 1999 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 81 (2002), S. 622-624 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electron paramagnetic resonance (EPR), photoluminescence, and infrared optical absorption have been used to investigate a ZnO crystal before and after a thermal anneal for 1 h in air at 900 °C. The sample was an undoped high quality crystal grown by the chemical vapor transport method. In addition to shallow donor impurities, the crystal contained trace amounts of copper ions. Prior to the thermal anneal, these ions were all in the Cu+ (3d10) state and the observed luminescence at 5 K, produced by 364 nm light, consisted of a broad structureless band peaking at 500 nm. After the high-temperature anneal, the Cu2+ (3d9) EPR spectrum was observed and the luminescence had changed significantly. The emission then peaked near 510 nm and showed structure identical to that reported by Dingle [Phys. Rev. Lett. 23, 579 (1969)]. Our data reaffirm that the structured green emission in ZnO is associated with Cu2+ ions. We suggest that the unstructured green emission (observed before the high-temperature anneal) is donor–acceptor pair recombination involving the Cu+ acceptors. © 2002 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 80 (2002), S. 1334-1336 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Nitrogen acceptors are formed when undoped single crystals of zinc oxide (ZnO) grown by the chemical-vapor transport method are annealed in air or nitrogen atmosphere at temperatures between 600 and 900 °C. After an anneal, an induced near-edge absorption band causes the crystals to appear yellow. Also, the concentration of neutral shallow donors, as monitored by electron paramagnetic resonance (EPR), is significantly reduced. A photoinduced EPR signal due to neutral nitrogen acceptors is observed when the annealed crystals are exposed to laser light (e.g., 364, 442, 458, or 514 nm) at low temperature. The nitrogens are initially in the nonparamagnetic singly ionized state (N−) in an annealed crystal, because of the large number of shallow donors, and the light converts a portion of them to the paramagnetic neutral acceptor state (N0). © 2002 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2818-2828 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper presents an approach based on the saddle-point approximation to study the equilibrium interactions between small molecules and macromolecules with a large number of sites. For this case, the application of the Darwin–Fowler method results in very simple expressions for the stoichiometric equilibrium constants and their corresponding free energies in terms of integrals of the binding curve plus a correction term which depends on the first derivatives of the binding curve in the points corresponding to an integer value of the mean occupation number. These expressions are simplified when the number of sites tends to infinity, providing an interpretation of the binding curve in terms of the stoichiometric stability constants. The formalism presented is applied to some simple complexation models, obtaining good values for the free energies involved. When heterogeneous complexation is assumed, simple expressions are obtained to relate the macroscopic description of the binding, given by the stoichiomeric constants, with the microscopic description in terms of the intrinsic stability constants or the affinity spectrum. © 1999 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 92 (2002), S. 1221-1226 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Thermoluminescence (TL), optical absorption, and electron paramagnetic resonance (EPR) have been used to characterize point defects in Mg-doped stoichiometric LiNbO3. A broad TL emission, peaking at 440 nm, is observed near 94 K when these crystals are irradiated at 77 K and then rapidly warmed. X rays and below-band-gap lasers (325 and 355 nm) are equally effective in producing the TL peak. During excitation, holes are trapped on oxygen ions adjacent to lithium vacancies and electrons are trapped on niobium ions at regular lattice sites. These defects both have characteristic EPR spectra, and the trapped electron center has an optical absorption band peaking at 1200 nm. Upon warming, the electrons become thermally unstable near 94 K and migrate to the trapped-hole sites where radiative recombination occurs. We suggest that the near-edge charge-transfer absorption band from Fe3+ impurities, nominally present, provides the mechanism by which below-band-gap light initially produces the separately trapped holes and electrons. © 2002 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 1354-1358 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) have been used to characterize the dominant hole trap in undoped Czochralski-grown yttrium–orthovanadate (YVO4) crystals. A silicon impurity, present inadvertently, replaces a vanadium ion and allows a hole to be trapped on one of the four adjacent oxygen ions. The unpaired spin resides in an oxygen p orbital oriented perpendicular to the plane defined by the silicon ion, the electron-deficient oxygen, and the two yttrium ions nearest the oxygen. Principal values of the g matrix (2.0033, 2.0090, and 2.0771) were obtained from EPR data taken at 15 K. Direct verification of the participation of silicon was obtained from ENDOR data taken at 12 K. We have found that this trapped-hole center appeared in large concentrations in all of our Czochralski-grown YVO4 crystals that were exposed to ionizing radiation (i.e., x rays or an ultraviolet laser beam) while the crystal was at 77 K. Interestingly, a small concentration of this trapped-hole center was present in some as-grown YVO4 crystals before exposure to ionizing radiation. © 2002 American Institute of Physics.
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