In the realm of energy and atmospheric sciences, understanding the interactions between different particles is crucial for modeling and predicting atmospheric behavior. Researchers Cheikh T. Bop and Marko Gacesa, affiliated with the University of New South Wales, have delved into the quantum mechanics of collisions between hot hydrogen atoms and carbon dioxide molecules. Their work, published in the Journal of Physical Chemistry A, sheds light on processes that are vital for energy transfer and atmospheric escape in planets with CO2-rich atmospheres.
The study focuses on the collisions between hydrogen (H) or deuterium (D) atoms and CO2 molecules at high energies, up to 5 electron volts (eV). The researchers employed quantum mechanical calculations to determine the cross sections and rate coefficients for these collisions. Cross sections measure the likelihood of a particular type of collision occurring, while rate coefficients describe the rate at which these collisions lead to energy transfer or other outcomes.
One of the key findings is that the scattering of H or D atoms off CO2 molecules is strongly forward-peaked, meaning that the particles tend to continue moving in their original direction after the collision. This results in momentum-transfer cross sections that are substantially smaller than previously assumed. The researchers found that mass-scaling from similar systems involving oxygen or carbon atoms overestimates the total cross sections for H–CO2 collisions by factors of 30 to 45. Additionally, existing empirical fits underestimate the cross sections in the low-energy regime by up to 45%.
The study also highlights the importance of isotopic effects. The substitution of hydrogen with its heavier isotope deuterium leads to energy-dependent differences in the cross sections of up to 35% at energies below 0.1 eV. This invalidates uniform scaling approaches for D/H fractionation, which is the process by which different isotopes of hydrogen are separated based on their mass.
Maxwellian-averaged rate coefficients, which are derived from the cross sections and describe the average rate of collisions in a gas at a given temperature, were found to be significantly smaller than mass-scaled values. This implies that the efficiency of energy transfer between hydrogen atoms and CO2 molecules is lower than previously thought.
The implications of these findings for atmospheric escape modeling are substantial. For instance, in the case of Mars, the revised cross sections can shift the exobase altitude—the altitude at which the atmosphere transitions to outer space—by 10 to 20 kilometers. This can lead to order-of-magnitude changes in the rates of thermal escape, where atmospheric particles gain enough energy to escape the planet’s gravity. These revisions have important implications for understanding the hydrogen loss in early CO2-dominated planetary atmospheres, not only on Mars but also on early Earth and CO2-rich exoplanets.
In summary, the research provides essential quantum-mechanical inputs for revisiting atmospheric evolution scenarios on various planets and exoplanets. The findings can help refine models of atmospheric escape and energy transfer, leading to a better understanding of the evolution of planetary atmospheres and the potential for habitability. For the energy sector, this research underscores the importance of accurate quantum mechanical modeling in predicting the behavior of atmospheric particles, which can inform strategies for managing atmospheric escape and energy transfer in industrial processes.
Source: Bop, C. T., & Gacesa, M. (2023). Quantum scattering of hot H/D on CO2: Cross sections and rate coefficients for planetary atmospheres and their evolution. Journal of Physical Chemistry A.
This article is based on research available at arXiv.