| A simulation technique, continuous-time random-walk(CTRW) Monte Carlo simulation, is introduced to model surface kinetics on interstellar grains. The new technique is able to solve problems studied previously with rate equation and master equation approaches. Inhomogeneous and rough surface models are also introduced, and the CTRW Monte Carlo method is found to be very suitable for kinetics simulations on these surfaces. We first try to solve the famous molecular hydrogen formation problem with an inhomogeneous surface model and with this simulation technique. We introduce the concept of effective rate coefficients to couple this technique into full gas-grain network simulations. The dependence of effective rate coefficients on temperature and H flux is studied. We also introduce a new mathematical model to use the CTRW for full gas-grain network simulation in dense clouds. The surface network is a well studied H, O and CO system. We find that rough surface models can help to explain observation. |
