New vibrational numbering and potential energy curve for the 3(3)Pi(g) electronic state of the Li-2 molecule

An experimental study of the 3(3)Pi g electronic state of Li-7(2), using th e Perturbation-Facilitated Optical-Optical Double Resonance (PFOODR) techni que, was recently reported [A. Yiannopoulou et al., J. Chem. Phys. 103, 589 8, (1995)]. However, due to the very small number of known Li-7(2) A (1)Sig ma(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 upsilo n(x) + 3 in that reference) could be observed. Dunham coefficients, based o n 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 th e 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 ab solute vibrational numbering and the 3(3)Pi(g) RKR potential curve obtained from the Dunham coefficients, must be considered to be uncertain. In the p resent work, we show that the previously reported 3(3)H(g) RKR curve is una ble 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 r eport 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) (upsilo n(x) +/- 1, N = 11) --> b(3)Pi(u) spectra obtained using the PFOODR experim ental technique. We demonstrate that the correct vibrational numbering and an improved RKR potential curve can be obtained by analyzing the experiment al term values in combination with all observed bound-free and discrete spe ctra. Finally, term values for four Li-6(2) 3(3)Pi(g) ro-vibrational levels were obtained using PFOODR spectroscopy. The measured isotope shifts confi rm the absolute vibrational numbering obtained from the present analysis. ( C) 1999 Academic Press.