Single and double addition of oxygen atoms to propyne on surfaces at low temperatures

Abstract

Experiments designed to simulate the low temperature surface chemistry occurring in interstellar clouds provide clear evidence of a reaction between oxygen atoms and propyne ice. The reactants are dosed onto a surface held at a fixed temperature between 14 and 100 K. After the dosing period, temperature programmed desorption (TPD), coupled with time-of-flight mass spectrometry, are used to identify two reaction products with molecular formulae C3H4O and C3H4O2. These products result from the addition of a single oxygen atom, or two oxygen atoms, to a propyne reactant. A simple model has been used to extract kinetic data from the measured yield of the single-addition (C3H4O) product at surface temperatures from 30-100 K. This modelling reveals that the barrier of the solid-state reaction between propyne and a single oxygen atom (160 +/- 10 K) is an order of magnitude less than that reported for the gas-phase reaction. In addition, estimates for the desorption energy of propyne and reaction rate coefficient, as a function of temperature, are determined for the single addition process from the modelling. The yield of the single addition product falls as the surface temperature decreases from 50 K to 30K, but rises again as the surface temperature falls below 30 K. This increase in the rate of reaction at low surface temperatures is indicative of an alternative, perhaps barrierless, pathway to the single addition product which is only important at low surface temperatures. The kinetic model has been further developed to characterize the double addition reaction, which appears to involve the addition of a second oxygen atom to C3H4O. This modelling indicates that this second addition is a barrierless process. The kinetic parameters we extract from our experiments indicate that the reaction between atomic oxygen and propyne could occur under on interstellar dust grains on an astrophysical time scale.

Direct measurement of desorption and diffusion energies of O and N atoms physisorbed on amorphous surfaces

“…Simultaneously, two experimental groups (Ward et al 2011;Kimber et al 2014;He et al 2015) proposed a rather high value for the adsorption energy of O atoms (i.e., 1500-1800 K) on different substrates, consistently with theoretical estimations by Bergeron et al (2008). On the other hand, no experimental data have existed so far that cover N-atom desorption and diffusion barriers.…”

Binding energies: New values and impact on the efficiency of chemical desorption

“…The largest impact of these updates seems to come from the new experimental binding energy of atomic oxygen. In previous models, we assumed E D (O) to be 800 K based on Tielens & Hagen (1982) whereas new estimates seem to indicate that atomic oxygen is strongly bound to the surface (He et al 2005, Kimber et al 2014, Minissale et al 2016. As a consequence, the diffusion of O is slower and surface reactions with atomic oxygen are less efficient.…”

Methylacetylene (CH3CCH) and propene (C3H6) formation in cold dense clouds: A case of dust grain chemistry

“…From this model, using the binding energy of methylacetylene on amorphous water? ice measured by (Kimber et al 2014) (equal to 2500 K) for methylacetylene and propene, leads to about 1 % of chemical desorption. This efficiency is highly dependent on the binding energy as well as the semi-empirical parameter ε of (Minissale et al 2016) which corresponds to the fraction of kinetic energy retained by the product colliding with the surface.…”

“…The binding energies of methylacetylene on icy surfaces have been measured equal to 2500 K (Kimber et al 2014) and the one for propene should be relatively close.…”