Rotational excitation of ortho-H₂O by para-H₂ (j₂ = 0, 2, 4, 6, 8) at high temperature

Abstract

Aims. Our objective is to obtain the best possible set of rotational (de)-excitation state-to-state and effective rate coefficients for temperatures up to 1500 K. We present state-to-state rate coefficients among the 45 lowest levels of o-H 2 O with H 2 ( j 2 = 0) and Δ j 2 = 0, +2, as well as with H 2 ( j 2 = 2) and Δ j 2 = 0, −2. In addition and only for the 10 lowest energy levels of o-H 2 O, we provide state-to-state rate coefficients involving j 2 = 4 with Δ j 2 = 0, −2 and j 2 = 2 with Δ j 2 = +2. We give estimates of effective rate coefficients for j 2 = 6, 8. Methods. Calculations are performed with the close coupling (CC) method over the whole energy range, using the same 5D potential energy surface (PES) as the one employed in our latest publication on water. We perform comparisons with coupled states (CS) calculations, with thermalized quasi-classical trajectory (QCT) calculations using the same PES and with previous quantum calculations obtained between T = 20 K and T = 140 K with a different PES. Results. We find that the CS approximation fares extremely badly even at high energy for j 2 different from zero. Comparisons with thermalized QCT calculations show large factors at intermediate temperatures and factors from 1 to 3 at high temperature for the strongest rate coefficients. Finally we stress that scaled collisional rate coefficients obtained with He cannot be used in place of collisional rate coefficients with H 2 .

Water distribution in shocked regions of the NGC 1333-IRAS 4A protostellar outflow

“…To characterize the two gas components at R2 in terms of excitation conditions, we ran the radex non-LTE molecular LVG radiative transfer code (van der Tak et al 2007) in plane-parallel geometry, with collisional rate coefficients from Dubernet et al (2006Dubernet et al ( , 2009 and Daniel et al (2010Daniel et al ( , 2011 and molecular data from the Leiden Atomic and Molecular Database (LAMDA 4 , Schöier et al 2005). A grid of models with density ranging between 10 4 cm −3 and 10 8 cm −3 and temperatures ranging between 100 K and 1600 K was built.…”

Rotational excitation of 20 levels of para-H2O by ortho-H2(j2= 1, 3, 5, 7) at high temperature

“…Our previous paper (Dubernet et al 2009) provided state-to-state rate coefficients among the 45 lowest levels of ortho-H 2 O with para-H 2 ( j 2 = 0) and Δ j 2 = 0, +2, as well as with j 2 = 2 and Δ j 2 = 0, −2. In addition to and only for the 10 lowest energy levels of ortho-H 2 O did Dubernet et al (2009) obtain state-tostate rate coefficients involving j 2 = 4 with Δ j 2 = 0, −2 and j 2 = 2 with Δ j 2 = +2.…”

“…Present and past Dubernet et al ( , 2009 quantum dynamical calculations were carried out with the 5D potential energy surface (PES) determined by Faure et al (2005). This accurate 5D PES, suitable for inelastic rotational calculations, was obtained from a 9D PES by averaging over H 2 and H 2 O ground vibrational states.…”

“…For the de-excitation of ortho-H 2 O by para-H 2 ( j = 0), Dubernet et al (2009) compared the effective rate coefficients of Phillips et al (1996) for the first 10 levels of ortho-H 2 O with temperatures in the range 20 K to 140 K. It was shown that the ratios of effective rate coefficients converge slowly towards unity with temperature, certainly reflecting the decreasing influence of the difference between the two different PES (Phillips et al 1994;Faure et al 2005) as temperature increases.…”

“…In addition, Dubernet et al (2009) carried out comparisons with thermalized QCT calculations of Faure et al (2007) that showed large factors at intermediate temperature and factors from 1 to 3 at high temperature for the strongest rate coefficients. We showed also that scaled collisional rate coefficients obtained with He could not be used in place of collisional rate coefficients with para-H 2 .…”