Molecular orientation in Langmuir monolayers studied by surface second harmonic generation

Presentation Date: 

Tuesday, March 23, 1999

Location: 

APS Centennial Meeting 1999 (Atlanta, GA)

Presentation Slides: 

We used surface second harmonic generation (SHG) to study molecular orientation in a smectic-C liquid-crystal Langmuir monolayer at an air/water interface. The molecules under investigation are 4-octyl-4'-(3-carboxytrimenthyleneoxy) azobenzene (8AZ3). Because water is inversion symmetric, the SHG signal comes only from the surface. Furthermore, the magnitude of the surface SHG signal depends on the orientation of molecules, which makes SHG a sensitive probe of the phase of the monolayer. We measured the response of the SHG signal as we compressed the monolayer from a smectic-C phase to a more condensed phase and determined the average polar angle in each phase. The SHG probe samples an area roughly the size of a monolayer domain. As a result, the SHG technique combined with the depolarized reflected-light microscope enables us to extract further information about the local molecular orientation.