Direct potential fit analysis of the X 1Σg+ state of Rb2:: Nothing else will do!

High resolution A-X emission data involving vibrational levels of the ground X (1)Sigma(g)(+) electronic state up to v' = 113, spanning 99.8% of the potential well, have been acquired for three isotopomers of Rb-2. While a good fit (<(sigma)over bar>(f) = 1.03) to the 12 148 transition frequencies (with uncertainties +/- 0.001 cm(-1)) is obtained from an unconstrained combined-isotopomer Dunham-type analysis, it requires a large number (62) of expansion parameters, and the resulting empirical centrifugal distortion constants (CDCs) are unreliable for extrapolation to higher-J. Moreover, Dunham expansion fits using constrained theoretical values of the first six CDCs (up to O-v) fail to properly represent the data, as even higher-order CDCs are required. However, a direct fit of these data to an analytical modified Lennard-Jones potential energy function involving only 16 fitted parameters yields essentially the same quality of fit as did the unconstrained Dunham fit, and should be reliable for extrapolation to arbitrarily high J. This potential form incorporates the proper R-6 asymptotic behavior of the potential, and is constrained to have the theoretically predicted C-6 dispersion coefficient. Although the dataset involves the three isotopomers Rb-85,85(2), Rb-85,87(2), and Rb-87,87(2), none of the present analyses was able to determine any Born-Oppenheimer breakdown effects. (C) 2000 American Institute of Physics. [S0021-9606(00)01932-2].