Theory of deviations from the limiting near-dissociation behavior of diatomic molecules

The nature of the deviations from the limiting near-dissociation behavior of diatomic molecule properties is investigated. It is shown that for strongly bound species the leading deviations from the limiting behavior associated with the asymptotically dominant C_n/rⁿ potential energy term can be quantitatively attributed to the higher inverse power terms contributing to the long-range potential. The properties of the derived expressions show that experimental vibrational energies should often obey the limiting near-dissociation equation even when the term C_n/rⁿ is responsible for only a fraction of the potential strength at the levels' outer turning points. In contrast, B_v values (and other properties) are quite sensitive to the presence of additional contributions to the long-range potential, and deviations from their predicted limiting behavior should provide a sensitive new means of determining values of higher-order potential coefficients. The theory is illustrated by and tested against results for B(³Π⁺_0u)-state I₂ and for simple model potentials.