Ocko, B. M.Pershan, PeterSafinya, C. R.Chiang, L. Y.2013-03-011987Ocko, B. M., Peter S. Pershan, C. R. Safinya, and L. Y. Chiang. 1987. Incommensurate smectic order at the free surface in the nematic phase of \(4-n-heptylphenyl-4'-(4"-nitrobenzoyloxy)benzoate (DB7NO_2)\). Physical Review A 35(4): 1868-1872.1050-29471094-1622http://nrs.harvard.edu/urn-3:HUL.InstRepos:10357480We 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.en-USJournal Article2013-03-0110.1103/PhysRevA.35.1868