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

Solid CO 2 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 CO 2 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 H 2 CO, CH 3 OH, and a water ice cap on the CO 2 film. The rate of production of CO from direct X-ray photolysis of CO 2 is measured to be 5.4 × 10 2 molecule photon -1 , corresponding to a formation cross section of 4.7 × 10 -20 cm 2 . 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 CO 2 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 CO 2 of 5 × 10 -12 and 1 × 10 -12 cm 2 s -1 , for O and CO, respectively.