Do H atoms stick to PAH cations in the interstellar medium?

Recent experimental work has shown that under relatively high density condi tions, selected PAH cations associate efficiently with H atoms to form prot onated PAH ions. If such association reactions occur efficiently under low density conditions, the protonated PAH ions can be considered as candidate molecular species in diffuse interstellar clouds. The high density and low density processes are not the same, however, since at:high density associat ion proceeds by collisional stabilization while at low density it proceeds by radiative stabilization. In this paper, we show: using theoretical techn iques that for the cation C6H6+, the laboratory results do not prove that t he association with H is efficient at low densities, whereas for C10H8+ and most probably larger PAH cations, the radiative association with H is very efficient. For these latter cations, the competitive Ii-atom transfer chan nels (e.g. H + C10H8+ --> C10H7+ + H-2), which can lead to catalytic format ion of Hz by regeneration of the PAN cations, are probably not important.