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Scientists experimentally validated quantum mechanical calculations for collisions between formaldehyde (H₂CO) and helium atoms at low temperatures relevant to interstellar space. Using chirped-pulse spectroscopy in supersonic flows, they measured pressure-broadening cross-sections that matched theoretical predictions within 95% confidence intervals. The validated calculations show that including helium collisions in models of interstellar chemistry can alter predicted excitation temperatures by up to 12% in warm regions like protostars.
Why it matters
This validation enables more accurate modeling of formaldehyde observations in space, improving our understanding of interstellar chemistry and star formation regions. The confirmed theoretical methodology can now be applied with confidence to predict collision rates for other molecules where direct experimental measurement is difficult or impossible.
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⚠️ Preprint – Noch nicht peer-reviewed
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Abstract: 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. The aim of this study is to measure experimentally low-temperature pressure-broadening cross-sections for the H$_2$CO-He system in order to validate the theoretical methodology involved in determining new collisional rate coefficients. The experiments employed the chirped-pulse in uniform supersonic flow method, and H$_2$CO 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 H$_2$CO-He system which is subsequently implemented in scattering calculations using the close-coupling method to derive pressure broadening cross-sections and collisional rate coefficients. 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. Helium constitutes about 20 % relative to H$_2$ in the interstellar medium. The inclusion of collisional rate coefficients for H$_2$CO 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.