Time-dependent diffusion kinetics for an electron beam incident in water vapor

The problem of the time-dependent effects, both transient and steady-state, diffusion kinetics on the spatial yields generated by a monoenergetic, collimated electron beam incident in water vapor is investigated for incident energies below 2 keV. Spatial yields of the primary species created by the electron energy degradation are based on spatial yield spectra generated by Daniel Rio in his Ph.D. thesis (1983). Diffusion is calculated in the spherical approximation. It is observed that proton transfer from ions to H2O and electron transfer to ions from H2O and proton hydration, can be treated as globally instantaneous, while electron-proton recombination can be treated as locally instantaneous only, because of charge separation. Above 500 eV charge separation is fairly negligible, and the approximation of globally instantaneous recombination can be made. This approximation is also employed below 500 eV as a rough estimate, where charge separation becomes increasingly significant