Initial State Dependence of Collisional Energy Transfer in Excited Dilithium-Neon

We have investigated collision induced rotational and vibrational energy transfer in the Li_2 ^*- Ne system. Our experiments were performed under single-collision conditions at an effective temperature of 691K^circ using dispersed, spontaneous, fluorescence techniques. We studied the Li _2 molecules at ten different initial rovibrational levels which covered three different electronic states, A^1Sigma_sp{u }{+}, E^1Sigma _sp{g}{+}, and F ^1Sigma_sp{g}{+} and measured over eight hundred inelastic rate constants. Specifically, for Li_2 A^1Sigma_sp{u}{+ } with an initial rotational level of j _{rm i} = 30, we looked at the initial vibrational level v_{rm i} ranging from 2 to 32 and -2 <= Deltav <= +2. For the Li_2 E ^1Sigma_sp{g}{+} we made measurements for j_{ rm i} = 29, and v_{ rm i} = 4. Finally, for the Li _2 F^1Sigma_sp {g}{+} state, we studied the initial level of j_{rm i} = 31, and v_{rm i} = 10. The dependence of collisional energy transfer on initial vibrational levels is studied systematically in our Li^2 A^1Sigma _sp{u}{+} experiments. We find that increasing v_{rm i } results in a linear increase in the vibrational rate constants, which is accompanied by a decrease in the rotationally inelastic rate constant. The total inelastic rate constant increases with v_{rm i} only at the highest values of v _{rm i}. The net vibrational energy transfer, DeltaE, calculated using rotationally summed rate constants is consistent with a simple model. However, explicit inclusion of rotation gives quite different values of DeltaE. We also compare our experimental results with three-dimensional trajectory calculations using both an ab initio potential surface and a simple repulsive potential surface. The measured rate constant distributions as functions of the final rotational level j_{rm f} from electronic states A^1 Sigma_sp{u}{+}, E^1Sigma_sp{g}{+ }, and F^1Sigma_sp {u}{+} are compared. We find that the distribution of vibrationally-elastic rate constants have a wider spread with respect to j_{ rm f} in the Li_2 A^1Sigma_sp{u}{+ } state than in the Li_2 E^1Sigma_sp{g}{+ } or F^1Sigma_sp {g}{+} states. The vibrationally inelastic rate constant distributions for the three different electronic states are very similar.