Absolutely calibrated vacuum ultraviolet (VUV) spectroscopy has been used to determine the energy fluxes of VUV photons at an electrically floating substrate in a low-pressure 13.56-MHz radiofrequency plasma reactor used for polymer surface treatments. These fluxes have been compared with the positive ion flux that was reported in an earlier study. At the typical operating parameters of 10-mTorr pressure and 10-W power, the total VUV energy flux is 2.2 mW cm(-2), compared with a value of 3.3 mW cm(-2) from the ions. With increasing power (from 0.5 to 12 W), both the ion and VUV energy fluxes increase monotonically. However, as the pressure increases. (1-100 mTorr), the ion energy flux declines, while the VUV component increases. At discharge powers of 10 W, and pressures greater than 25 mTorr, the greater part of the energy flux to the surface is from the VUV photons. These measurements are used to determine which of the plasma components, VUV or ions, will be most effective in the treatment of polystyrene surfaces in a pure Ar plasma. Because of the low VUV absorption coefficient of polystyrene most of the photon flux penetrates through the outermost 2 nm Of the polymer Surface without attenuation. Consequently, we calculate that under typical plasma operating parameters, the ions play the major role in the modification of polystyrene surfaces because of their shorter stopping distances.