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The coincidence Doppler broadening (CDB) technique was applied to study the momentum distribution of positronium (Ps) and its interactions with oxygen molecules in free spaces of silica aerogels filled with nitrogen and oxygen mixtures of various ratios. The deconvoluted CDB spectra become narrower and narrower with increasing oxygen partial pressure, which is due to electron exchange collision of ortho-Ps (o-Ps) with oxygen molecules. The momentum distribution of para-Ps (p-Ps) was successfully derived from deconvoluted CDB spectra by a two-Gaussian-function fitting. The bimodal distribution of Ps momentum reveals that o-Ps atoms (with enough kinetic energy) can be moderated effectively by exciting oxygen molecules from the ground state to the excited state (a'Δg or b'Σg+) through inelastic collisions. It is interesting to find that the energy difference ΔE between two components of p-Ps momentum distributions decreases gradually because more and more o-Ps atoms lose energy through elastic collisions prior to inelastic collisions as oxygen partial pressure increases. [ABSTRACT FROM AUTHOR]