Dramatic effect of an additional ${\mathrm{CH}}_2$ group on the temperature variation of the $\mathrm{Sm}{-C}_{\alpha }^{*}$ short helical pitch

High-resolution differential optical reflectivity was used to study the temperature evolution of the short helical pitch in the $\mathrm{Sm}-C_{\alpha }^{*}$ phase of successive members from two liquid-crystal homologous series. With the addition of one ${\mathrm{CH}}_2$ group, the magnitude and temperature evolution of the pitch change dramatically, and the molecular arrangements between consecutive surface layers found in free-standing films change from being anticlinic to synclinic.