Abstract

Context. In typical interstellar molecular clouds, densities are low; therefore, the analysis of observed molecular spectra requires using non-LTE models. Critical inputs for these models are collisional rate coefficients of the detected molecule with He and H-2. For formaldehyde (H2CO), such data are available; however, no collisional data have been reported for its deuterated isotopologues, HDCO and D2CO.
Aims. The main goal of this work is to study isotopic effects in the collision of formaldehyde with He, particularly on HDCO and D2CO, and to report new sets of collisional rate coefficients for these systems at low temperatures.
Methods. New potential energy surfaces were developed for the H2CO+He and HDCO+He systems using ab initio calculations at the CCSD(T)-F12a/aug-cc-pVTZ level of theory. The H2CO+He potential energy surface was also shifted to the centre of mass of D2CO to study its collision with He. These potential energy surfaces were employed in close-coupling scattering calculations
.Results. The potential energy surfaces for H2CO+He and HDCO+He show differences that arise mainly from symmetry effects, leading to additional non-zero expansion coefficients in the HDCO+He case. The cross sections and rate coefficient are computed for para-H2CO, ortho-H2CO, para-D2CO, ortho-D2CO, and HDCO by collision with He. A comparison with available collisional data for H2CO+He is presented. Similarities in the collisional rate coefficients are found for the same transition for the three isotopologues. However, rotational de-excitation rate coefficients for transitions forbidden in H2CO+He but allowed in HDCO+He are reported for the first time.