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

¹ Departamento de Física, Facultad de Ciencias, Universidad de Chile, Av. Las Palmeras 3425, 7800003 Ñuñoa, Santiago, Chile
² Departamento de Física, Facultad de Ciencias, Universidad de Chile, Cedenna, Av. Las Palmeras 3425, 7800003 Ñuñoa, Santiago, Chile

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Received: 28 January 2026
Accepted: 24 February 2026

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 H₂. 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.