Fluorocarbon films were grown on polystyrene in vacuum from 25- to 100-eV mass-selected C3F5+ ion beams. The films were analyzed by X-ray photoelectron spectroscopy, atomic force microscopy, and X-ray reflectivity after exposure to the atmosphere for 4-8 weeks. The X-ray reflectivity indicates films that range from similar to30 to 60-Angstrom thick. The thinner films form at lower ion energies, where the ion penetration depth and efficiency of film formation are lowest. X-ray reflectivity estimates air-fluorocarbon film roughness values of similar to6 Angstrom for 25- and 50-eV films but similar to20 Angstrom for the 100-eV films. The fluorocarbon-polystyrene-buried interface displays similar roughness and trends with ion energy. The AFM roughness trends are similar, but the absolute AFM roughnesses are only similar to(1)/(4) of the X-ray reflectivity values. This discrepancy is attributed to tip effects and the method of determining roughness by AFM. The AFM images and power spectral densities of the 100-eV films displayed quasi-periodic cones spaced 300-700 A apart. Such features are either absent or of much lower amplitude in the 25- and 50-eV films. Classical molecular dynamics simulations of C3F5+ deposition on polystyrene at energies of 50 and 100 eV/ion reveal that etching at the higher energy is largely responsible for the dissimilar film structures obtained experimentally. These results demonstrate that deposition of the fluorocarbon polyatomic ion C3F5+ allows control of film nanostructure at the surface and buried interface.