Cationic polymerization is induced at the gas-solid interface by hyperthermal organic cations coincident on a surface with a thermal beam of organic monomers. This process, termed surface polymerization by ion-assisted deposition (SPIAD), produces films that maintain the chemical structure of the monomer. A polythiophene film is produced here by SPIAD with 100 eV thiophene ions and terthiophene monomers coincident on Si and indium tin oxide (ITO) substrates held under vacuum. X-ray photoelectron spectroscopy observes enhancement in film growth for SPIAD compared with either thiophene ion or terthiophene exposure alone. Polythiophene films grown by both mass-selected and nonmass-selected ions with coincident terthiophene dosing both display similar fluorescence intensities at two wavelengths characteristic of emission from films of the terthiophene monomer. Raman spectra of films from nonmass-selected ions display several vibrations also observed in terthiophene films. Ions therefore play a critical role in film growth from nonmass-selected ions, in addition to any radical or photochemically driven processes that may also occur.