X-Ray Photoelectron Spectroscopy and Mass Spectrometry Studies of X-Ray-processed Solid CO2

Solid CO2films 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 CO2ice 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 CO2film. The rate of production of CO from direct X-ray photolysis of CO2is measured to be 5.4 × 102molecule photon-1, corresponding to a formation cross section of 4.7 × 10-20cm2. The growth rate for the water cap is calculated to be 2.6 × 10-4monolayers s-1for a partial pressure of H equal to 2 × 10-10Torr. 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 CO2is 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 CO2of 5 × 10-12and 1 × 10-12cm2s-1, for O and CO, respectively.