Isotopic fractionation of Oxygen due to tunneling.
Abstract
At the low temperatures (T < 10 K) characteristic of dense molecular cloud cores, diffusion on dust grain surfaces via thermal hopping slows significantly and the prevailing mechanism for diffusion of atoms becomes quantum tunneling. Using a computational model written in Matlab to simulate oxygen isotope energies and wavefunctions, we show that the tunneling behavior of isotopes of oxygen atoms may be significant and may lead to patterns that can be described as mass-independent fractionation. To show this, we solved the 1-dimensional Schrodinger equation for the energy eigenstates and eigenfunctions using a matrix representation of the Hamiltonian and imposing Dirichlet boundary conditions on oxygen atoms trapped in a double potential well system on a surface. Using this system, we determined the eigenfunctions, eigenenergies and evaluated the time-evolution of the probability density for each isotope of oxygen. The tunneling time was defined as the period of each isotope oscillating between wells. We determine the instantaneous fractionation factors (α18, α17) by taking the ratios of the tunneling time of the minor isotopes (17O and 18O) compared to 16O. We studied the behavior of this system and corresponding isotopic fractionations by varying the potential well depths and barrier widths. We examine the simulated instantaneous fractionations due to tunneling through the barrier and compare these timescales to thermal hopping rate expected using transition state theory. We found that the minor isotopes of oxygen bound to a weak double well with a potential barrier of 500K and a barrier width ranging from 0.7 Å to 1.2 Å have longer tunneling timescales than 16O, ranging from ~(1.6-2.13) and ~(2.6-4.4) for 17O and 18O respectively. These tunneling timescales may represent a potentially signigcant source of isotopic fractionation on cold dust grains in molecular clouds.
- Publication:
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American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23513707L