Collisional excitation of H$_2$CO by He: Experimental validation of state-of-the-art scattering calculations

arXiv:2606.07039v1 Announce Type: cross Abstract: Context. Non-local thermodynamic equilibrium conditions in the interstellar medium require collisional rate coefficients to model astronomical observations; these are usually determined from theoretical scattering calculations. Aims. The aim of this study is to measure experimentally low-temperature pressure-broadening cross-sections for the H2CO-He system in order to validate the theoretical methodology involved in determining new collisional rate coefficients. Methods. The experiments employed the chirped-pulse in uniform supersonic flow method, and H2CO is generated in situ by 193 nm excimer laser photolysis of tetrahydrofuran in cold He flows. State-of-the-art calculations are performed by computing a new potential energy surface for the H2CO-He system which is subsequently implemented in scattering calculations using the close-coupling method to derive collisional rate coefficients and pressure-broadening cross-sections. Results. Excellent agreement between theory and experiment is obtained, with the calculated values falling within the 95% confidence intervals of the experimental measurements. Such agreement validates the high accuracy of the theoretical data. Conclusions. Helium constitutes about 20% relative to H2 in the interstellar medium. The inclusion of collisional rate coefficients for H2CO with He in radiative transfer modelling leads to variations in the excitation temperature of frequently detected rotational lines of up to 12% in warm regions such as protostars.