  Photochemical escape is an important process for oxygen escape from present
Mars. In this work, a 1-D Monte-Carlo Model is developed to calculate escape
rates of energetic oxygen atoms produced from O2+ dissociative recombination
reactions (DR) under 1, 3, 10, and 20 times present solar XUV fluxes. We found
that although the overall DR rates increase with solar XUV flux almost
linearly, oxygen escape rate increases from 1 to 10 times present solar XUV
conditions but decreases when increasing solar XUV flux further. Analysis shows
that atomic species in the upper thermosphere of early Mars increases more
rapidly than O2+ when increasing XUV fluxes. While the latter is the source of
energetic O atoms, the former increases the collision probability and thus
decreases the escape probability of energetic O. Our results suggest that
photochemical escape be a less important escape mechanism than previously
thought for the loss of water and/or CO2 from early Mars.
