Genesis of Polyatomic Molecules in Dark Clouds: CO₂ Formation on Cold Amorphous Solid Water

Abstract

Understanding the formation of molecules under conditions relevant to interstellar chemistry is fundamental to characterize the chemical evolution of the universe. Using reactive molecular dynamics simulations with model-based or high-quality potential energy surfaces provides a means to specifically and quantitatively probe individual reaction channels at a molecular level. The formation of CO2 from collision of CO­(1Σ) and O­(1D) is characterized on amorphous solid water (ASW) under conditions typical in cold molecular clouds. Recombination takes place on the subnanosecond time scale and internal energy redistribution leads to stabilization of the product with CO2 remaining adsorbed on the ASW on extended time scales. Using a high-level, reproducing kernel-based potential energy surface for CO2, formation into and stabilization of CO2 and COO are observed.

Energy Redistribution Following CO2 Formation on Cold Amorphous Solid Water

“…Finally, heating of the water molecules occurs on the 10 ps time scale following the recombination reaction. Consistent with earlier work, ( Fredon et al, 2021 ; Upadhyay et al, 2021 ), no CO 2 desorption is found from the simulations carried out here.…”

“…Relaxation of the angle occurs within the following 50 ps and the CO 2 molecule remains in an internally excited state on much longer time scales, see Figure 2C . ( Upadhyay et al, 2021 ). Concomitantly, the average internal energy of the surrounding water molecules increases by about 10%, see black, red and green traces in Figures 2D,E .…”

“…The simulation system, Figure 1 , consisted of an equilibrated cubic box of amorphous solid water with dimension 31 × 31 × 31 Å 3 containing 1000 water molecules. As all bonds and angles are flexible, the simulations were run with a time step of Δ t = 0.1 fs and the non-bonded cutoff was at 13 Å. Simulations were started from an existing, equilibrated ASW structure ( Pezzella et al, 2018 ; Pezzella and Meuwly, 2019 ; Upadhyay et al, 2021 ) by adding CO A and O B inside ( Figure 1A ) or on top of ( Figure 1B ) ASW.…”

“…To describe CO A + O B recombination to form CO 2 the Morse-Morse-Harmonic (MMH) parametrization was employed. ( Upadhyay et al, 2021 ). This model treats the two CO bonds with a Morse potential and the OCO bend as a harmonic function.…”

“…This model treats the two CO bonds with a Morse potential and the OCO bend as a harmonic function. MMH is a computationally efficient model (fitted to MRCI/aug-cc-pVTZ data), which yields results for recombination probabilities on ASW comparable to a more elaborate reproducing kernel Hilbert space (RKHS) representation with an exothermicity of –7.27 eV ( Veliz et al, 2021 ; Upadhyay et al, 2021 ).…”