Collisional Quenching of Highly Excited H₂ due to H₂ Collisions

Rate coefficients for pure rotational quenching in H₂(ν ₁ = 0, j ₁) + H₂(ν ₂ = 0, j ₂) collisions from initial levels of j ₁ = 2-31 (j ₂ = 0 or 1) to all lower rotational levels are presented. We carried out extensive quantum mechanical close-coupling calculations based on a recently published H₂-H₂ potential energy surface (PES) developed by Patkowski et al. that has been demonstrated to be more reliable than previous work. Rotational transition cross sections with initial levels of j ₁ = 2-14, 18, 19, 24, and 25 were computed for energies ranging from 10^-6 to 1000 cm^-1, while the coupled-states approximation was adopted from 2000 to 20,000 cm^-1. The corresponding rate coefficients were calculated for the temperature range 10^-5 ≤ T ≤ 10,000 K. Scaling methods based on the ultra-cold data (10^-5-1 K) were used to estimate rate coefficients for all other intermediate rotational states. Comparisons with previous work that adopted different PESs show small discrepancies at high temperatures and in low-energy resonance regions. The astrophysical applications of the current results are briefly discussed, including the rotational H₂ critical densities due to para-H₂ and ortho-H₂ collisions.