Structure of freely suspended chiral smectic films as determined by x-ray reflectivity and optical ellipsometry

Abstract

We report on a combined x-ray reflectivity and optical ellipsometry study of freely suspended smectic (Sm) films of a chiral liquid crystalline compound with the phase sequence $\mathrm{Sm}\ensuremath{-}A--{\mathrm{S}\mathrm{m}\ensuremath{-}\mathrm{C}}_{\ensuremath{\alpha}}^{*}--{\mathrm{S}\mathrm{m}\ensuremath{-}\mathrm{C}}^{*}--{\mathrm{S}\mathrm{m}\ensuremath{-}\mathrm{C}}_{\ensuremath{\gamma}}^{*}--{\mathrm{S}\mathrm{m}\ensuremath{-}\mathrm{C}}_{A}^{*}.$ Using tilt magnitude profiles from x-ray reflectivity as input to model the average optical properties obtained by ellipsometry, tilt direction profiles are also obtained. In this way realistic models can be elaborated for the various types of chiral Sm-C films. We find that the surface layers are more tilted than the interior layers due to surface interactions and finite size effects. For the ferrielectric ${\mathrm{S}\mathrm{m}\ensuremath{-}\mathrm{C}}_{\ensuremath{\gamma}}^{*}$ phase the tilt direction profile corresponds to a three-layer helix, in agreement with the clock model of chiral Sm-C phases. In thin films the surface interactions suppress the bulk helix structure of the ${\mathrm{S}\mathrm{m}\ensuremath{-}\mathrm{C}}_{\ensuremath{\alpha}}^{*}$ phase and a ${\mathrm{S}\mathrm{m}\ensuremath{-}\mathrm{C}}_{A}^{*}$-like structure is formed with an anticlinic layer-by-layer alternation of the tilt directions.