Photolysis of pure solid O3 and O2 films at 193 nm

TL;DR

This study investigates the photochemistry of solid O3 and O2 films at 193 nm and 22 K, revealing enhanced ozone formation in solid state due to photoabsorption by O2 dimers and providing quantitative reaction yields and probabilities.

Contribution

It provides the first quantitative analysis of photolysis yields and reaction probabilities for solid O3 and O2 films at 193 nm, highlighting the role of solid-state effects in photochemistry.

Findings

Ozone destruction in O3 films with residual ozone at high fluence.

Ozone formation in O2 films increases with photon fluence to a saturation level.

Enhanced ozone production in solid state due to O2 dimer photoabsorption.

Abstract

We studied quantitatively the photochemistry of solid O3 and O2 films at 193 nm and 22 K with infrared spectroscopy and microgravimetry. Photolysis of pure ozone destroyed O3, but a small amount of ozone remained in the film at high fluence. Photolysis of pure O2 produced O3 in an amount that increased with photon fluence to a stationary level. For both O2 and O3 films, the O3:O2 ratio at saturation is 0.03, nearly 10-30 times larger than those obtained in gas phase photolysis. This enhancement is attributed to the increased photodissociation of O2 due to photoabsorption by O2 dimers, a process significant at solid state densities. We obtain initial quantum yield for ozone synthesis from solid oxygen, {\Phi} (O3) = 0.18 and for destruction of ozone and oxygen in their parent solids, {\Phi} (- O3) = 1.7 and {\Phi} (-O2) = 0.28. Combined with known photoabsorption cross sections, we estimate probabilities for germinate recombination of 0.15 for O3 fragments and 0.90 for oxygen atoms from O2 dissociation. Using a two-parameter kinetic model, we deduce the average probabilities for the reaction of an O atom with O2 and O3 to be 0.10 and 1, respectively. These probabilities are the same for both O2 and O3 films, even though the distribution of kinetic and internal energy of the photofragments is very different in both cases. This finding suggests efficient energy relaxation of photofragments in the solid occur prior to their reactions with other species.