[en] This work presents the results for the irradiation of pure SO_2 sample that was condensed in a preevacuated chamber, from Laboratório de Astroquímica e Astrobiologia (LASA/UNIVAP), at low temperature (12 K) and irradiated by ionizing photons which simulate Solar photons in the vacuum ultraviolet (VUV) and soft X-rays range. The infrared spectra of irradiated sample have presented the formation of SO_3. Experimental formation cross section was determined. Theoretical investigations were performed at Second-order Moller- Plesset perturbation theory (MP2) level and indicate the most likely SO_3 formation channels vary with the reaction supporting medium. (paper)