Collisional Quenching of Highly Excited H₂ due to H₂ Collisions

Rate coefficients for pure rotational quenching in H 2 ( ν 1 = 0, j 1 ) + H 2 ( 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 H 2 –H 2 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 H 2 critical densities due to para-H 2 and ortho-H 2 collisions.