15min:

NIKESH S. DATTANI AND ROBERT J. LE ROY, Guelph-Waterloo Centre for Graduate Work in Chemistry and Biochemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada; AMANDA J. ROSS, Université Lyon 1; CNRS; LASIM UMR 5579, 43 Bd du 11 novembre 1918, F-69622 Villeurbanne, France; COLAN LINTON, Physics Department, University of New Brunswick, Fredericton, NB E3B 5A3, Canada.

While a number of studies of the weakly bound a ³ _u⁺ state of ^6,6Li₂ and ^7,7Li₂ have been reported, the only potential functions obtained for these systems were based on point-wise semiclassical RKR curves generated from Dunham or near-dissociation expansions for the vibrational energies and B_v constants.\footnote(a) F. Martin et al., Spectrochim. Acta 44A, 1369 (1988); (b) C. Linton et al., J. Chem. Phys. 91, 6036 (1989); (c) W.T. Zemke and W.C. Stwalley, J. Phys. Chem. 97, 2053 (1993); (d) A.J. Moerdijk et al., Phys. Rev. Lett. 72, 40 (1994); (e) C. Linton et al., J. Mol. Spectrosc. 196, 20 (1999).~ Moreover, to date the data for ^6,6Li₂ and ^7,7Li₂ have always been treated independently, so the effect of Born-Oppenheimer breakdown in this system is unknown.

The present work reports a combined-isotopologue direct-potential-fit analysis of all available fluorescence and PAS data for the 1 ³ _g⁺ -a ³ _u⁺ and 2 ³ _g -a ³ _u⁺ systems of ^6,6Li₂ and ^7,7Li₂. The analytic potential energy functions used to characterize the a ³ _u⁺ and 1 ³ _g⁺ states are extended versions of the `Morse-Long-Range' (MLR) potential model which explicitly incorporates the theoretically-known inverse-power long-range behaviour within a unified potential function form.\footnoteR.J. Le Roy, Y. Huang and C. Jary, J. Chem. Phys. 125, 164310 (2006); R.J. Le Roy and R.D.E. Henderson, Mol. Phys. 105, 663 (2007).~ `Adiabatic' Born-Oppenheimer breakdown functions are required to yield a consistent analysis of the data for the two isotopologues, and they yield isotopologue-dependent well depths for these two states. The -C₃/r³ limiting long-range behaviour of the 1 ³ _g⁺-state potential presented a challenge to the use of the MLR potential function form with two or more long-range terms, and this led to better understanding and a significant extension of the MLR model.