Molecular Oxygen Formation in Interstellar Ices Does Not Require Tunneling

Abstract

The formation of molecular oxygen in and on amorphous ice in the interstellar medium requires oxygen diffusion to take place. Recent experiments suggest that this process involves quantum tunneling of the oxygen atoms at sufficiently low temperatures. Fitting experimental diffusion rates between 6 and 25 K to an expression that accounts for the roughness of the surface yields excellent agreement. The molecular dynamics of adsorbed oxygen is characterized by rapid intrasite dynamics, followed by intersite transitions over distances of ∼10 Å. Explicit simulations using a realistic free-energy surface for oxygen diffusion on amorphous ice down to 10 K show that quantum tunneling is not required for mobility of adsorbed oxygen. This is confirmed by comparing quantum and classical simulations using the same free-energy surface. The ratio of diffusional and desorption energy E/ E = 275/1082 ≈ 0.3 is at the lower end of typically used values but is still consistent with the assumptions made in models for interstellar chemistry.

Vibrational Spectroscopy of N3– in the Gas and Condensed Phase

“…Independently, using a suitable flexible, spectroscopically accurate water model such as the Kumagai, Katwamura, Yokokawa (KKY) model [56][57][58] it will be possible to probe the coupling between the intramolecular degrees of freedom of solute and solvent as their fundamentals and overtones lead to potentially interesting dynamics. The present model can be further improved by using higher order multipoles [59][60][61][62] or polarization 63 to treat the electrostatic interactions.…”

O2 formation in cold environments

“…In this work formation of molecular oxygen on cold ASW surfaces is examined both inside amorphous solid water and on its surface. Oxygen ( 3 P) migration in and on ASW has been found from simulations 4 and in experiments using neon matrices. 21 If two oxygen atoms O( 3 P) approach one another sufficiently closely the process is rapid as it is barrierless.…”

“…20 It has been previously shown by experiment and simulations that migration of atomic species, such as oxygen, in ASW is readily possible on the sub-microsecond time scale over distances of several ten Angstroms. 2,4 Similarly, oxygen atom diffusion in Neon matrices at temperatures as low as 4 K has recently been established. 21 Diffusing oxygen atoms have also been implicated in the formation of CO 2 through O + CO -CO 2 .…”

“…The mobility of atomic and molecular species on solid surfaces is a primary driver for surface-induced chemistry at low temperatures. It has been found from experiments 1,2 and atomistic simulations 3,4 that H and O atoms can diffuse on amorphous water surfaces down to low temperatures (B10 K). This is relevant for the possibility to form diatomic or larger molecules (such as O 2 , O 3 or CO 2 ) in laboratory experiments 2 and may also play a role for the conditions in interstellar space.…”

New insights into decomposition characteristics of nanoscale methane hydrate below the ice point

“…Water molecules of the ASW structure were described by the TIP3P force eld. [34][35][36] The O-H bond length of water molecules was set at 0.096 nm and the H-O-H bond angle was set at 104.52 . The non-bond effect of van der Waals between molecules was calculated using a Lennard-Jones potential energy function.…”