Using H2O and/or CO as a projectile

In cometary comas and in some protoplanetary and debris disks, collisional excitation can be dominated by H2O and/or CO molecules. Just like H2, water molecules exist in two isomeric forms: para-water with total nuclear spin I = 0 and ortho-water with I = 1. The rotational energy levels of H2O are labeled by three numbers: the angular momentum j and the pseudo-quantum numbers ka and kc (corresponding to the projection of j along the principal inertia a and c axes). In the ground electronic and vibrational state1, the rotational levels of para-H2O have even values of ka + kc while the levels of ortho-H2O have odd values of ka + kc. The rotational energy levels of CO are labelled by the angular momentum j.

In contrast to H2, even in cold environments where the kinetic temperature Tk is lower than 50 K, several rotational states of H2O and CO can be significantly populated due to the relatively small rotational spacings in these species (compared to those of para- and ortho-H2). As a result, several excited levels of H2O and CO must be considered as collision partners in radiative transfer calculations.

EMAA provides de-excitation rate coefficients (in cm3s-1) due to collisions with 'para-H2O' and 'ortho-H2O' and 'CO' which in practice correspond (for each species) to 'thermalized' rate coefficients, >i.e. summed over all possible final states and averaged over a thermal distribution (at the kinetic temperature) of initial states (see Faure et al. 2020 and Żółtowski and al. 2023).

Note: H2O and CO are not handled as a colliding partner in the public version of the RADEX2 code. In order to use RADEX with para-H2O and or ortho-H2O as projectiles, please select 'p-H2' and/or 'o-H2' as collision partners in the RADEX input file (corresponding to id=2 and id=3 respectively, where 'id' is the code of collison partners). This is obviously only possible if H2 and H2O are simultaneous collision partners. In order to use RADEX with CO as projectile, please select 'H' as collision partners (id=5) in the input file. Again, this is only possible if H and CO are not simultaneous collision partners.


Faure A., Lique F., Loreau J., MNRAS 493 776-782 (2020)
Żółtowski M., Lique F., Loreau J, Faure A., Cordiner M., MNRAS 520 3887-3894 (2023)

1The first vibrational level of H2O opens at 1594.7 cm-1