Incommensurate smectic order at the free surface in the nematic phase of 4-n-heptylphenyl-4’-(4’’-nitrobenzoyloxy)benzoate (DB7NO2)

We report x-ray reflectivity measurements on the free surface of 4-n-heptylphenyl-4’-(4’ ’-nitrobenzoyloxy)benzoate (${\mathrm{DB}7\mathrm{NO}}_2$) at the nematic to smectic-A phase transition, $T_{\mathrm{NA}}$=99.9 °C. The free surface in the nematic phase exhibits smecticlike ordering at two q vectors, one which is commensurate with the smectic-A monolayer q vector $q_2$. The other q vector is incommensurate corresponding to ordering at ∼0.59$q_2$. The commensurate peak constructively interferes with the air-liquid interface while the incommensurate peak destructively interferes. These results are compared with bulk-phase x-ray scattering measurements.