Using an energy selective mass spectrometer, ion energy distribution functions (IEDFs) in a low pressure radio frequency (rf) argon discharge have been measured. This experiment has been done in the diffusion chamber of a helicon source. Two kinds of IEDF have been observed depending on the gas pressure: classical "one-peak" spectra at high pressure (>1 mubar) and "two-peak" spectra at low pressure (< 1 mubar). These results have been correlated with the plasma potential and density profiles along the axis of the reactor obtained with a Langmuir probe and the IEDF have been fitted using a simple analytical model. In this way, noncollisional diffusion of energetic ions from the source chamber to the diffusion chamber has been shown.