How thick is a liquid-vapor interface?

The structure of the interface of an argonlike fluid in equilibrium with its vapor near the triple point is studied using Monte Carlo simulation. By referring particle coordinates to the capillary waves and calculating the transverse structure factor in that reference frame, one can determine the penetration depth of the capillary waves into the bulk as a function of the wave vector. The penetration depth of the lowest capillary mode is consistent with the interfacial thickness as determined from the intrinsic density profile, and the data for the higher modes suggest that the penetration depth might be independent of k.