X-ray photoelectron spectroscopy and mass spectrometry studies of X-ray-processed solid CO₂

Solid CO₂ films have been grown on a stainless steel substrate and processed by X-ray bombardment for up to 6 hr.. The reactions induced were monitored using X-ray photoelectron spectroscopy (XPS) and mass spectrometry. The XPS results are twofold: direct X-ray photolysis of the CO₂ ice produced CO and an unidentified O product, possibly atomic O; secondary effects resulting from surface reactions between CO, O, and residual H from the vacuum environment produced H₂CO, CH₃OH, and a water ice cap on the CO₂ film. The rate of production of CO from direct X-ray photolysis of CO₂ is measured to be 5.4 × 10² molecule photon^-1, corresponding to a formation cross section of 4.7 × 10^-20 cm². The growth rate for the water cap is calculated to be 2.6 × 10^-4 monolayers s^-1 for a partial pressure of H equal to 2 × 10^-10 Torr. The appearance of gas-phase products from the film showed a time lag which indicates that the diffusion of the product species in the bulk CO₂ is affected by some time-dependent process, possibly the creation of defects in the film. A model for the observed time dependence of the dissociation products in the gas phase yields diffusion coefficients in the CO₂ of 5 × 10^-12 and 1 × 10^-12 cm² s^-1, for O and CO, respectively.