On corrections to spectroscopically measured Franck–Condon energies arising from motion of the parent molecules and from ion collisions in fusion plasmas: relevant time scales for atomic velocity distributions

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We show that while corrections to spectroscopically measured Franck–Condon energies, arising from motion of the parent molecules at the edge of fusion plasmas, are insignificant in many practical cases, ion collisions play an important role in randomizing and redistributing the velocities of the atomic products of molecular dissociation. Formulae are obtained from which the time scales for randomization and equilibration of the atomic velocity distribution may be estimated, and hence compared with characteristic times for other important atomic processes. Examples of the latter are: spontaneous emission, production of atoms by electron impact-induced dissociation and ionization of molecules, electron collisions of the first and second kinds, charge-exchange recombination and electron impact ionization. We show that the time scales of randomization and equilibration can be comparable with those for some of the other processes listed above, but are much longer than typical times for spontaneous emission. The ion–atom collisional processes considered here therefore affect the atomic velocity distribution mainly after the atoms produced directly by molecular dissociation have arrived in their ground states. The formulae derived for these collisional time scales may well be useful in modelling hydrogen transport and recycling in fusion plasmas.