Collision-induced population distribution of the energy acceptor H2(v = 1, J = 1–6) in D2–H2 mixtures and its temperature dependence

Abstract

D₂(1,12) molecules were selectively excited using a stimulated Raman pumping technique, and time-resolved coherent anti-Stokes Raman scattering (CARS) signals were recorded in both pure D₂ and D₂–H₂ mixed systems. By analyzing the measurements within the Stern–Volmer framework, the collisional relaxation rate coefficients of the D₂(1,12) state at 295 K were determined. In the pure D₂ system, the D₂–D₂ self-relaxation rate coefficient was found to be 1.76 × 10⁻¹⁴ cm³ s⁻¹. In contrast, in the D₂–H₂ system, the relaxation rate coefficient for D₂–H₂ collisions was 2.77 × 10⁻¹⁴ cm³ s⁻¹, while the D₂–D₂ self-relaxation rate coefficient decreased to 0.45 × 10⁻¹⁴ cm³ s⁻¹. These results indicate that the overall collisional relaxation rate in the D₂–H₂ system is approximately 1.8 times larger than that in pure D₂. The introduction of H₂ therefore significantly enhances near-resonant rotational–vibrational (R–V) energy transfer between D₂(1,12) and H₂, leading to more efficient quenching of the excited D₂(1,12) state.Furthermore, within the temperature range of 295–453 K, systematic scanning CARS measurements were performed to probe H₂ molecules in the v = 1 vibrational states following D₂–H₂ collisions. With increasing temperature, the populations of the H₂(v = 1, J = 3, 4) levels increase, whereas that of H₂(v = 1, J = 1) decreases. In contrast, the populations of the H₂(v = 1, J = 2, 5, 6) levels exhibit relatively weak temperature dependence, providing insight into collision-induced R–V energy transfer mechanisms in the D₂–H₂ system.

Graphical abstract

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Population distribution on rotational energy levels of H₂ molecules after collision with ro-vibrationally excited D₂(v = 1, J = 12) molecules as a function of temperature in the range of 295–453 K, when the mole fraction of H₂ is 0.5. 2）