Helicity of the SmC alpha* phase and its variations in a thiobenzoate series

We have studied by optical means a thiobenzoate series exhibiting SmCA*, Sm C*, SmCFI*, and SmCalpha* phases. We have made pitch measurements on the Sm CA* and SmC* phases using the Grandjean-Cano method. Observing free surface drops in the SmCalpha* phase, we found periodic ellipticity fringes, due t o a helical structure, and measured their optical period versus temperature ; this can reach very small as well as significant values. It does not exhi bit a single evolution and appears incoherent at first sight, but always fo llows one part of a general behaviour, depending on the thermal history of the sample. Lastly we compare experimental results with one of the structur al models proposed for the SmCalpha* phase.