Isotopic effects in rotational de-excitation of H ₂ CO isotopologues (HDCO and D ₂ CO) by He at low temperatures

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 (H 2 CO), such data are available; however, no collisional data have been reported for its deuterated isotopologues, HDCO and D 2 CO. Aims . The main goal of this work is to study isotopic effects in the collision of formaldehyde with He, particularly on HDCO and D 2 CO, and to report new sets of collisional rate coefficients for these systems at low temperatures. Methods . New potential energy surfaces were developed for the H 2 CO+He and HDCO+He systems using ab initio calculations at the CCSD(T)-F12a/aug-cc-pVTZ level of theory. The H 2 CO+He potential energy surface was also shifted to the centre of mass of D 2 CO to study its collision with He. These potential energy surfaces were employed in close-coupling scattering calculations. Results . The potential energy surfaces for H 2 CO+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-H 2 CO, ortho-H 2 CO, para-D 2 CO, ortho-D 2 CO, and HDCO by collision with He. A comparison with available collisional data for H 2 CO+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 H 2 CO+He but allowed in HDCO+He are reported for the first time.