Lifetimes of very high Rydberg states of aromatic molecules

Abstract

Recent experimental results reporting lifetimes of very high Rydberg states of phenanthrene and deuterated phenanthrene together with a theoretical analysis are examined critically. Weaknesses in the theoretical model are pointed out, in particular the proposal that lifetimes of isolated field-free molecules vary as n6 up to values of n≂100 above which very rapid decay occurs by autoionization. Another model, in which the effects of external fields and collisions result in lifetimes which vary in an ideal statistical limit as n5 at lower values of n and rapid destruction at higher values of n, is shown to explain the experimental results equally well. To the extent that such a statistical limit is not attained, nonexponential decay is expected. Decay processes of high Rydberg states of large polyatomic molecules are discussed. Especially in the case of a fused polynuclear aromatic, the isolated molecule with lower values of n is proposed to decay by an internal conversion mechanism in which the rate-determining initial step involves a transfer of energy from the Rydberg electron to the ion core amounting to one or a very few vibrational quanta. The experimentally observed rapid decrease of lifetimes for the highest values of n is attributed to destruction, primarily by ionization, due to such external effects as collisions and fields. A continuity principle requires that such collisional ionization as well as autoionization be associated with excitation of a range of values of n (and therefore of lifetimes for isolated molecules) approaching infinity.