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 D_2CO, 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