Application of direct potential fitting to line position data for the X1Σg+ and A1Σu+ states of Li2

A collection of 15 222 spectroscopic Li2 line positions, all with measurement precision in the range 0.005-0.02 cm−1, and which sample 99.97 and 99.98% of the A and X state well depths, respectively, have been employed in a direct least squares fit of the effective potential energy and Born-Oppenheimer breakdown functions for the two states. The fully analytical potential function for the A state of 6Li2 is determined out to R ≈ 55 Å, well into the long-range region. For the three isotopologues, 6Li2, 6Li7Li, and 7Li2, most of the data are comprised of rotationally resolved transitions in the A-X system recorded by Fourier transform spectroscopies. For 6Li2 and 7Li2, complementary fluorescence data are also included for high vibrational levels of both the A and X states. The reduced standard deviation of the fit was close to unity, indicating that the quantum mechanical eigenvalues calculated from the analytical functions of the Hamiltonians of the two states, which are described by a total of only 45 fitted parameters, represent the line positions, on average, to within the estimated uncertainties. The dissociation energies for 6Li2 are estimated precisely as De(X) = 8516.74(1) cm−1 and De(A) = 9351.96(1) cm−1.