Isotopic effects in rotational de-excitation of H2CO isotopologues (HDCO and D2CO) by He at low temperatures

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. For formaldehyde (), such data are available; however, no collisional data have been reported for its deuterated isotopologues, and. H2 H2CO HDCO D2CO 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. New potential energy surfaces were developed for the + and + systems using ab initio calculations at the CCSD (T) -F12a/aug-cc-pVTZ level of theory. The + potential energy surface was also shifted to the centre of mass of to study its collision with +. These potential energy surfaces were employed in close-coupling scattering calculations. H2CO He HDCO He H2CO He D2CO He The potential energy surfaces for + and + 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-, ortho-, para-, ortho-, and HDCO by collision with. A comparison with available collisional data for + 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 +He but allowed in +He are reported for the first time. H2CO He HDCO He H2CO H2CO D2CO D2CO He H2CO He H2CO HDCO