C(1s) region of x-ray photoelectron spectra of native polystyrene (PS) thin film (bottom), 25-eV CF3+ modified PS, 25-eV C3F5+ modified PS, 50-eV CF3+ modified PS, and 50-eV C3F5+ modified PS (top). Components of C(1s) peak are labeled, with detected carbon atom underlined. All ion fluences are equivalent to 1.5 × 1016 F atoms/cm2.