Knowledge Bank

Second harmonic generation (SHG) has been used for structural and phase diagnosties of such a complex heterogeneous system as disperse $SiO2$ surface modified with molecular ions $CrO42-$ (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 $TM>1200K$). It is due to the formation of bridging peroxy linkages under association of surface nonbridging oxygen (NBO) atoms into $O2$ - quasimolecules where O - O bond is realized with $\pi$ -electrons ($px$ and $py$ 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 $CrO42-$ 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 $O2$ - quasimolecule model. This allows to affirm SHG on disperse $SiO2$ surface modified with $CrO42-$ anions is generally caused by the formation of strong adsorption $\pi$ - bonds between MI and surface NBO atoms.