Enhanced Access to the Dark Triplet States of (7)Li(2) through New Singlet-Triplet A(1)Sigma(+)(u) approximately b(3)Pi(u) Perturbation Window Levels: Perturbation-Facilitated Optical-Optical Double Resonance Study of the 2(3)Sigma(+)(g) State

Two new pairs of singlet-triplet A(1)Sigma(+)(u) approximately b(3)Pi(u) mixed levels of (7)Li(2) have been observed and used here as "window" levels in cw perturbation-facilitated optical-optical double-resonance (PFOODR) experiments. Previously, only one b(3)Pi(u) vibrational level, v = 19, was known to mix with the singlet A(1)Sigma(+)(u) v = 13 level, resulting in three perturbed A approximately b pairs L. Li, T. An, T.-J. Whang, A. M. Lyyra, W. C. Stwalley, R. W. Field, and R. A. Bernheim, J. Chem. Phys. 96, 3342 (1992)]. The scarcity of window levels and the resulting difficulty in accessing the dark triplet states of Li(2) is caused by the weak spin-orbit interaction of Li(2). The two new mixed b(3)Pi(u) v = 15 and 22 levels reported here enhance access to the dark triplet state manifold through expansion of the Franck-Condon overlap factor range. Furthermore, the earlier range of accessible rotational levels, N = 5, 7, and 10, is now expanded to include N = 8 and N = 16, thereby allowing for more reliable determination of the excited triplet states rotational structure. To demonstrate the importance of the new A(1)Sigma(+)(u) approximately b(3)Pi(u) mixed levels, we have studied the 2(3)Sigma(+)(g) state by cw PFOODR fluorescence excitation spectroscopy. New molecular constants and RKR potential curve have been determined. As previously reported L. Li, G. Lazarov, and A. M. Lyyra, J. Mol. Spectrosc. 191, 387 (1998)], the 2(3)Sigma(+)(g) state interacts with the repulsive 1(3)Pi(g) state by L-uncoupling and predissociates. We show that some 2(3)Pi(g) levels predissociate accidentally by the 1(3)Pi(g) state via the 2(3)Sigma(+)(g) state through L-uncoupling. Copyright 2001 Academic Press.