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 cm³ molecule⁻¹ s⁻¹ within 2 μs of the collision occurring. Above 2 μs, the rate coefficient is no longer constant. Meanwhile, no vibrational relaxation occurs before 10 μs. The rotational energy E_rot is the sum of the rotational energies of rotational states (12, J″). Within the period of 0–10 μs, 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 μs to 852 cm⁻¹ at 10 μs. Therefore, when rotational relaxation occurs, the decrease in rotational energy is approximately equal to the increase in translational energy.