Additivity failure in total X-ray production and electron capture cross sections for carbon-bearing gases by ∼MeV/amu H+ and He++

Additivity of atomic cross sections is commonly invoked to approximate the equivalent molecular cross section. Three areas in atomic collisions are examined where this assumption of additivity has been found to fail: (i) C (and O) K x-ray production from inner shell ionization of C- (and O)-bearing molecules; (ii) capture to bound states of H and He projectiles; (iii) capture to continuum states of bar H and He projectiles. Additivity failure in these three cases is examined in terms of entrance effects - expected changes in relevant cross sections due to the atom being in a molecular environment - and exit effects - associated with the projectile (and possibly its companion electron) undergoing a secondary collision as it passes through the remainder of the molecule after the initial ionizing/capture collision. For ∼MeV/amu H and He projectiles the entrance effect is the dominant contributor to additivity failure in x-ray production while the exit effect predominates in capture processes