Experimental study on rotational energy transfer in LiH (X¹ ∑ + v) + Ar collisions

Using high-resolution transient laser spectroscopy, the rotational energy transfer between LiH (12, 8) and Ar through collisions was studied. LiH (12, 8) was generated via degenerate stimulated hyper-Raman scattering. The population of LiH (12, J″ ≠ 8) generated during collisions were obtained using transient laser-induced fluorescence. According to the rate equation, the rate coefficients for the transfer from (12, 8) to (12, J″ ) states are between 7.1 × 10 −12 and 3.5 × 10 −13 cm 3 molecule −1 s −1 within 2 μ s of the collision occurring. Above 2 μ s, the rate coefficient is no longer constant. Meanwhile, no vibrational relaxation occurs before 10 μ s. The rotational energy E rot is the sum of the rotational energies of rotational states (12, J″ ). Within the period of 0–10 μ s, the value of E rot decreases from 541 cm −1 to 390 cm −1 . The distribution of translational energy E trans at different delay time of LiH (12, 8) is obtained by measuring the Doppler broadened line widths. It increases from 707 cm −1 at 0 μ s to 852 cm −1 at 10 μ s. Therefore, when rotational relaxation occurs, the decrease in rotational energy is approximately equal to the increase in translational energy.