In Situ Orientation-Sensitive Observation of Molecular Adsorption on a Liquid/Zeolite Interface by Second-Harmonic Generation

Abstract

The inherently surface-specific technique of second-harmonic generation was employed to probe the adsorption of an organic molecule, a hemicyanine dye, on b-oriented silicalite-1 films in situ. Measurements were performed in a purpose-built cell for solution experiments. By measuring at two different polarization combinations of the fundamental and second-harmonic light, the orientation of the adsorbed molecules was measured continuously. It has been observed that the adsorbed molecules gradually align themselves with the straight pores of the zeolite crystallites, thus adsorbing into the pores.

Probing microporous materials with second-harmonic generation

“…We studied the adsorption of the stilbazolium dye DAMPI from the liquid phase onto the surface of b-oriented silicalite-1 films in real time [56]. Due to steric hindrance this molecule can only enter the b-pores (5.6 Â 5.3 Å) with its stilbazolium moiety, ensuring that we studied pore-mouth adsorption (see Scheme 1).…”

“…Alternative surface analysis techniques like ATR-IR would detect significant contributions of the stilbazolium molecules present in the liquid phase. For the mathematical background to interpret the observations, the reader is referred to van der Veen et al [56].…”

Acoustic effects on nonlinear optical processes

“…Moreover, generating and detecting specific polarizations of second-harmonic light using laser light with a known polarization can be used to probe the orientation of adsorbed molecules. [19][20][21] Hence this technique could be of great use to probe chemical reactions at surfaces. 22 In case the first hyperpolarizability of the adsorbed molecule has one dominant contributing tensor element, such as βzzz in the case of the studied chromophores 21,23 , this orientation angle can be determined from the SHG caused by two perpendicular polarizations of laser light.…”

“…[19][20][21] Hence this technique could be of great use to probe chemical reactions at surfaces. 22 In case the first hyperpolarizability of the adsorbed molecule has one dominant contributing tensor element, such as βzzz in the case of the studied chromophores 21,23 , this orientation angle can be determined from the SHG caused by two perpendicular polarizations of laser light. 20 The orientation of molecules has a great influence in chemical reactions.…”

Simplified bond-hyperpolarizability model of second harmonic generation, group theory, and Neumann’s principle

“…In the surface or interface however, the inversion symmetry is broken and is represented by a nonzero ( ) 2 χ , thus second order nonlinear processes such as SHG and SFG are allowed for example at solid/air, liquid/liquid, liquid/air, semiconductors, polymer and biological films [3][4][5][6]. Hence, the main SHG contribution in centrosymmetric crystals only comes from a few layers near the surface, which feel the disturbance for the deviation of periodicity.…”