Collisional Quenching of Highly Excited H2 due to H2 Collisions

Rate coefficients for pure rotational quenching in H-2(nu(1) = 0, j(1)) + H-2(nu(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) -1K) 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.