Incommensurate Smectic Order at the Free Surface in the Nematic Phase of \(4-N-Heptylphenyl-4'-(4"-Nitrobenzoyloxy)Benzoate (DB7NO_2)\)

We report x-ray reflectivity measurements on the free surface of \(4-n-heptylphenyl-4'-(4"-nitrobenzoyloxy)benzoate (DB7NO_2)\) at the nematic to smectic-A phase transition, \(T_{NA}=99.9^{\circ}C\). 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 \(\sim 0.59q_2\). 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.Engineering and Applied Science