The sweeping rate in diffusion-mediated reactions on dust grain surfaces

Lohmar, Ingo; Krug, Joachim

doi:10.1111/j.1365-2966.2006.10541.x

Cited by 23 publications

(51 citation statements)

References 27 publications

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“…It is important to note that the k/k 0 values all lie in the range 0.3-1.0, showing that the simple rate method is not strongly in error for the flat surfaces considered here. This difference is mainly due to back diffusion, and the factors we found are in agreement with the predicted values of Lohmar & Krug (2006) in the high temperature end of the efficiency peak. Lohmar & Krug (2006) derived an analytical expression for the actual sweeping rate of an atom as compared with the one used in rate equations (and master equations).…”

Section: Results For Flat Surfacessupporting

confidence: 90%

“…This difference is mainly due to back diffusion, and the factors we found are in agreement with the predicted values of Lohmar & Krug (2006) in the high temperature end of the efficiency peak. Lohmar & Krug (2006) derived an analytical expression for the actual sweeping rate of an atom as compared with the one used in rate equations (and master equations). They found a value of 0.33 for olivine at 10 K and 0.37 for amorphous carbon at 18 K. The agreement is much less for lower surface temperatures of carboneous grains (1.0 versus 0.21 for 10 K).…”

Section: Results For Flat Surfacessupporting

confidence: 90%

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Effective rate coefficients for molecular hydrogen formation in diffuse interstellar clouds

Chang¹,

Cuppen²,

Herbst

2006

A&A

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Aims. A method to determine effective rate coefficients for H 2 formation on grain surfaces is proposed based on the continuous-time, random-walk Monte Carlo simulation. Methods. Our Monte Carlo simulation is used to calculate the efficiency of molecular hydrogen formation via recombination of H atoms on flat and rough surfaces of olivine and amorphous carbon under a variety of diffuse cloud conditions. The results are then fitted to two types of rate laws to determine effective rate coefficients for use in rate equation treatments. These rate coefficients can be utilized in pure gas-phase models, where the rate of molecular formation is associated with a grain collision rate for H atoms multiplied by an efficiency factor, and in gas-grain models, where the actual diffusion of H atoms on a grain surface is considered. Results. The effective rate coefficients are tabulated as a function of incoming atomic hydrogen flux and surface temperature, over temperature ranges for which the surfaces show a reasonable efficiency of molecular hydrogen formation. For the flat surfaces studied, corrections to the standard rate treatment do not exceed a factor of three.

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Section: Results For Flat Surfacessupporting

confidence: 90%

Section: Results For Flat Surfacessupporting

confidence: 90%

Effective rate coefficients for molecular hydrogen formation in diffuse interstellar clouds

Chang¹,

Cuppen²,

Herbst

2006

A&A

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“…When an atoms scans the grain surface, it can visit a site more than once, which is called backdiffusion. This effect can cause the rate equations to overestimate the molecular hydrogen formation rate up to a factor of 3 depending on the physical conditions (Lohmar & Krug 2006).…”

Section: Site Dependencementioning

confidence: 99%

Modelling of surface chemistry on an inhomogeneous interstellar grain

Cuppen

Garrod

2011

A&A

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Context. Many interstellar molecules are formed through grain surface reactions. These reactions are usually modelled using rate equations, while considering a homogeneous grain with only one type of binding site for each species. However, amorphicity and the irregular character of interstellar dust grains make inhomogeneous grain surfaces much more likely. Aims. The aim of this study is to investigate the effect of surface inhomogeneity on surface reaction rates. The formation of molecular hydrogen is here taken as an example. Methods. The continuous-time random-walk Monte Carlo method is used to study the dependence of the H 2 formation rate on surface size, fraction of strong binding sites and the distribution of these sites for different temperatures and bond strengths. Classical rate equations and modified rate equations are applied to try to reproduce these results. Results. The H 2 formation efficiency is strongly affected by the introduction of a second type of binding site. This effect depends on the strength of the binding energy and the fraction of strong binding sites. The way in which the sites are distributed can change the formation rate by as much as four orders of magnitude. Classical rate equations fail to reproduce the formation rate for all tested situations. Modified rate equations are able to obtain a reasonable agreement with the Monte Carlo results for inhomogeneous surfaces with randomly distributed sites consisting of less than 30% strong sites. Conclusions. Since the different types of binding sites on interstellar grains are probably randomly distributed, we recommend the use of modified rate equations in gas grain models to include the effect of surface inhomogeneity. This method reproduces the most important features while being computationally inexpensive.

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“…A more accurate expression for A i in the case of spherical grains appears in Ref. [20], where it is shown to be reduced by a logarithmic factor.…”

Section: The Rate Equations and The Master Equationmentioning

confidence: 99%

Evaluation of the multiplane method for efficient simulations of reaction networks

2007

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Reaction networks in the bulk and on surfaces are widespread in physical, chemical and biological systems. In macroscopic systems, which include large populations of reactive species, stochastic fluctuations are negligible and the reaction rates can be evaluated using rate equations. However, with suitable couplings between them. In this paper we test the multiplane method and examine its applicability. We show that the method is accurate in the limit of small domains, where fluctuations are strong. It thus provides an efficient framework for the stochastic simulation of complex reaction networks with strong fluctuations, for which rate equations fail and direct integration of the master equation is infeasible. The method also applies in the case of large domains, where it converges to the rate equation results.

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The sweeping rate in diffusion-mediated reactions on dust grain surfaces

Cited by 23 publications

References 27 publications

Effective rate coefficients for molecular hydrogen formation in diffuse interstellar clouds

Effective rate coefficients for molecular hydrogen formation in diffuse interstellar clouds

Modelling of surface chemistry on an inhomogeneous interstellar grain

Evaluation of the multiplane method for efficient simulations of reaction networks

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