SECOND HARMONIC GENERATION ON DISPERSE $SiO_{2}$ SURFACE WITH ADSORBED MOLECULAR ANIONS ${CrO_{4}}^{2-}$
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Date
1995
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Ohio State University
Abstract
Second harmonic generation (SHG) has been used for structural and phase diagnosties of such a complex heterogeneous system as disperse $SiO_{2}$ surface modified with molecular ions ${CrO_{4}}^{2-}$ (MI) for the first time. It was established surface structural defects caused weak SHG in non-modified samples. The SHG dipole surface mechanism is supposed because symmetry violation on inversion center is characteristic for surface defects. The SHG efficiency increases by ${\sim} 10$ times under thermal coagulation of the samples (heat treatment temperature $T_{M} > 1200 K$). It is due to the formation of bridging peroxy linkages under association of surface nonbridging oxygen (NBO) atoms into $O_{2}$ - quasimolecules where O - O bond is realized with $\pi$ -electrons ($p_{x}$ and $p_{y}$ orbitals of oxygen). These $\pi$ - electrons are delocalized in the entire space of peroxy linkages and have a high value of hyperpolarizability. Surface NBO atoms can be active sites in the MI ${CrO_{4}}^{2-}$ adsorption [1]. The formation of such adsorption complexes leads to the SHG increasing too. The adsorption bonds are formed by $\pi$ - electrons of MI and surface NBO (O - O bonds) and can be also described within the limits of $O_{2}$ - quasimolecule model. This allows to affirm SHG on disperse $SiO_{2}$ surface modified with ${CrO_{4}}^{2-}$ anions is generally caused by the formation of strong adsorption $\pi$ - bonds between MI and surface NBO atoms.
Description
1. Yu. D. Glinka, S.N. Naumenko, V.M. Ogenko, and A.A. Chujko. Fundamentals of Adsorption 1993. Proc, Ivth Int. Conf. on Fundamentals of Adsorption, Kyoto, Japan, May 17 - 22, 1992. P. 217 - 225.
Author Institution: Kiev University, Glushkov Ave, 6, 252127 Kiev, UKRAINE
Author Institution: Kiev University, Glushkov Ave, 6, 252127 Kiev, UKRAINE