New vibrational numbering and potential energy curve for the 33Πg electronic state of the Li2 molecule

An experimental study of the 3(3)Pi g electronic state of Li-7(2), using the Perturbation-Facilitated Optical-Optical Double Resonance (PFOODR) technique, was recently reported [A. Yiannopoulou et al., J. Chem. Phys. 103, 5898, (1995)]. However, due to the very small number of known Li-7(2) A (1)Sigma(u)(+) similar to b(3)Pi(u) window levels, only 13 ro-vibrational levels (spanning a range of vibrational levels designated upsilon(x) - 1 to upsilon(x) + 3 in that reference) could be observed. Dunham coefficients, based on the assignment upsilon(x) = 7, were found to fit the observed term values and give a qualitative fit to the intensities of the first six lines of the 3 (3)Pi(g) (upsilon = upsilon(x), N = 11) --> b(3)Pi(u) emission spectrum. However, due to the limited number of levels used in the fit, both the absolute vibrational numbering and the 3(3)Pi(g) RKR potential curve obtained from the Dunham coefficients, must be considered to be uncertain. In the present work, we show that the previously reported 3(3)H(g) RKR curve is unable to reproduce the experimental intensity distribution in the Li-7(2) 3(3)Pi(g) (upsilon(x)= 7, N = 11) --> a(3)Sigma(u)(+) emission continuum. We report new experimental data for the Li-7(2) 3(3)Pi(g) (upsilon(x) + 1, N = 11) --> a(3)Sigma(u)(+) bound-free continuum and discrete 3(3)Pi(g) (upsilon(x) +/- 1, N = 11) --> b(3)Pi(u) spectra obtained using the PFOODR experimental technique. We demonstrate that the correct vibrational numbering and an improved RKR potential curve can be obtained by analyzing the experimental term values in combination with all observed bound-free and discrete spectra. Finally, term values for four Li-6(2) 3(3)Pi(g) ro-vibrational levels were obtained using PFOODR spectroscopy. The measured isotope shifts confirm the absolute vibrational numbering obtained from the present analysis. (C) 1999 Academic Press.