State-to-state collision energy transfer in the 2 (3) Pi(g), 3 (3) Pi(g), 3 (3) Sigma(g)(-), and 1 (3) Delta(g) states and from singlet to triplet states of Li-7(2) has been studied experimentally by continuous wave optical-optical double resonance (OODR) resolved fluorescence spectroscopy. Propensity rules of collision-induced transitions within a triplet state have been observed. When a rovibrational (upsilon,J) level of the F (1) Sigma(g)(+) or G (1) Pi(g) state was excited by OODR transition, fluorescence from high-lying triplet gerade states to the a (3) Sigma(u)(+) and/or b (3) Pi(u), states was always observed. This indicates that population can transfer from F (1) Sigma(g)(+) and G (1) Pi(g) states to triplet states by collision. Irrespective of the J of the initially excited singlet level, the most intense fluorescence from the triplet state will occur at the N or J corresponding to that of the F (1) Sigma(g)(+) (and/or G (1) Pi(g)) similar to (3) Lambda(g) mixed levels. Molecular population moves to the mixed levels and then transfers to the tripler state. This clearly indicates that singlet-triplet mixed levels played the role of a gateway, through which the population moves from singlet to triplet states. The gateway levels can be perturbed levels in the same vibrational level of the F (1) Sigma(g)(+) or G (1) Pi(g) state, or the perturbed levels in different vibrational levels.