X-ray reflectivity measurements have been carried out at the liquid/vapor interface of normal alkanes. The reflectivities over a large temperature range of different chain lengths (C20 and C36) provide a critical test of the various capillary wave models. Our data are most consistent with the hybrid model which allows for a molecular size dependent cutoff ${\mathit{q}}_{\max }$ for the capillary waves and an intrinsic interface width ${\mathrm{\sigma }}_0$.

• Received 8 October 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.242