Richard Jarman - Theoretical Study of the Nonlinear Optical Properties of KTiOPO4: Cooperative Effects in Extended -Ti-O-Ti-O- Chains

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  Publication Details (including relevant citation   information):

  Theoretical   Study of the Nonlinear Optical Properties of KTiOPO4:   Cooperative Effects in Extended -Ti-O-Ti-O- Chains, M. Munowitz,   R.H. Jarman and J.F. Harrison, Chem. Mater., 5, 661 (1993).

  Abstract:

  The sum-over-states perturbative formalism, combined with   extended Huckel wave functions,

  is used in a study of second harmonic generation in model systems   related to KTiOPO4 (KTP).

  Clusters of one, three, and five interconnected TiO6 groups are   constructed to simulate the

  extended --Ti--O-Ti--O- chains present in the crystal. To   facilitate comparison and illuminate

  differences among systems of different sizes, we compute a   specifically local nonlinear response.

  Ground and excited states are determined for the full structure,   but only those portions of the

  molecular orbitals relevant to a designated subsystem (typically   the central TiO6 group) are

  used to evaluate dipole transition moments in the sum over   states. Results show, first, that the

  nonresonant hyperpolarizability is enhanced locally as more TiO6   units are added. An intact

  and terminated structure, whether an isolated TiO6 monomer or   trimer, generally exhibits a

  smaller nonlinearity, with opposite sign, than a fully integrated   structure with the same number

  of TiO6 groups. A second observation is that, despite differences   in magnitude, the changes in

  hyperpolarizability accompanying an axial distortion of each   octahedron follow a nearly uniform

  pattern in chains of different lengths. These various effects are   interpreted by comparing

  relative changes in total electronic density and by a detailed   analysis of the matrix elements

  and molecular orbitals contributing to the perturbative   expression. Indications are that simple

  molecular orbital pictures developed for isolated fragments may   be insufficient to account for

  the cooperative nonlinear response of highly delocalized systems   such as KTP.

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