Laboratory studies on the formation of ozone (O3) on icy satellites and on interstellar and cometary ices

The formation of ozone (O-3) in neat oxygen ices was investigated experimentally in a surface-scattering machine. At 11 K, solid oxygen was irradiated with energetic electrons; the chemical modification of the target was followed on-line and in situ via Fourier transform infrared spectroscopy (FTIR; solid state) and quadrupole mass spectrometry (QMS; gas phase). The dominant product identified was the ozone molecule in the bent, C-2v symmetric structure, O-3(X(1)A(1)); the cyclic D-3h isomer was not observed. The associated van der Waals complexes [O-3...O] and [O-3...O-3] could also be detected via infrared spectroscopy, verifying explicitly the existence of oxygen atoms in the matrix at 11 K. Three different formation mechanisms of ozone were revealed. Two pathways involve the reaction of suprathermal oxygen atoms with molecular oxygen [O-2(X-3 Sigma(-)(g))] at 11 K. Once the sample was warmed after the irradiation to about 38 K, a third, thermal reaction pathway involving the barrierless reaction of ground-state oxygen atoms with molecular oxygen sets in. During the warm-up phase, the inherent sublimation of oxygen and ozone was monitored by mass spectrometry and occurs in the ranges 28-43 and 58-73 K, respectively. Our data are of help to understand the mechanisms of ozone formation within apolar interstellar and cometary ices and could also be applicable to outer solar system icy bodies, such as the moons of Jupiter (Ganymede, Europa, and Callisto) and Saturn (Rhea and Dione), where ozone and/or condensed oxygen has been observed.