Enhanced access to the dark triplet states of Li-7(2) through new singlet-triplet A(1)Sigma(+)(u) similar to 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(i)Sigma (+)(u)similar to b(3)Pi (u) mixe d levels of Li-7(2) have been observed and used here as "window" levels in cw perturbation-facilitated optical-optical double-resonance (PFOODR) exper iments. Previously, only one b(3)Pi (u) vibrational level, v = 19, was know n to mix with the singlet A(1)Sigma (+)(u) v = 13 level, resulting in three perturbed A similar to 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 (199 2)]. The scarcity of window levels and the resulting difficulty in accessin g the dark triplet states of Li-2 is caused by the weak spin-orbit interact ion of Li-2. The two new mixed b(3)Pi (u) v = 15 and 22 levels reported her e enhance access to the dark triplet state manifold through expansion of th e Franck-Condon overlap factor range. Furthermore, the earlier range of acc essible 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 exci ted triplet states rotational structure. To demonstrate the importance of t he new A(1)Sigma (+)(u) similar to b(3)Pi (u) mixed levels, we have studied the 2(3)Sigma (+)(g) state by cw PFOODR fluorescence excitation spectrosco py. New molecular constants and RKR potential curve have been determined. A s previously reported [L. Li, G. Lazarov, and A. M. Lyyra, J. Mel. Spectros c. 191, 387 (1998)]. the 2(3)Sigma (+)(g) State interacts with the repulsiv e 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. (C) 2001 Academic Press.