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Recombination Spectra: II. Collisional Transitions Between States of Degenerate Energy Levels
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EngineeringNuclear PhysicsExcitation Energy TransferElectronic Excited StateCollisional TransitionsRecombination SpectraDegenerate Energy LevelsQuantum SciencePhotometryHigh-energy Nuclear ReactionPhysicsNuclear TheoryAtomic PhysicsProtoplanetary DiskNuclear AstrophysicsAstrophysicsExcited State PropertyNatural SciencesNl ′Applied PhysicsPlanetary Nebulae
Previous work has shown that calculated recombination spectra may be sensitive to collisional transitions of the type nl → nl ′ ( l ′ = l ± 1). With equal energies for nl , nl ′, the usual collision theory expressions give infinite cross-sections, but finite results are obtained on introducing a cut-off at large impact parameters. At very low densities the cut-off is determined by the radiative lifetimes of the excited states and at higher densities it is determined by the Debye radius. For conditions in planetary nebulae ( N e ∼ 10 4 cm −3 , Te ∼ 10 4 °K), collisional transitions, nl → nl ′, are faster than radiative transitions, nl → n ′ l ′ ( n ′ < n ), for n ≳ 15 for H I and for n ≳ 22 for He II .