The influence of rare gas dilution on absolute oxygen atom density in O-2 radio frequency (RF) capacitively coupled plasma (CCP) was investigated by vacuum ultraviolet absorption spectroscopy (VUVAS) with pulse modulation of main plasma. Helium dilution is effective for raising 0 atom density at high total pressure (5 Torr) due to the Penning ionization by metastable He. Argon and Krypton dilution keeps 0 atom density same to pure O-2 case at 0.5 Torr while helium dilution lowers the 0 atom density by the enhanced diffusion flux of 0 atoms toward walls. The trend is supported by a diffusion model of 0 atoms. (c) 2005 Elsevier B.V. All rights reserved.