SECOND HARMONIC GENERATION ON DISPERSE $SiO_{2}$ SURFACE WITH ADSORBED MOLECULAR ANIONS ${CrO_{4}}^{2-}$

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.