We study the dynamics of the reaction layer during Ar+ ion-assisted Si etching by XeF2 in the temperature range T=150-800 K. Depending on temperature, the etch rate can be enhanced a factor of 8 by ion bombardment. The dynamics are studied with ion-pulse measurements on a time scale of 1-100 s in a molecular beam setup. A reaction layer with a submonolayer fluorine coverage and dangling bonds is found to be formed on the Si(100) surface during ion bombardment. The dangling bond concentration increases with ion flux and is independent of temperature in the range 150-600 K. Chemisorption on these dangling bonds results in a higher reaction probability of XeF2. The temperature dependence of the reaction probability of XeF2 is fully determined by the temperature dependence of the XeF2 precursor state. A simple model gives a very good description of the reaction probability as a function of both temperature and ion flux. The model description of the behavior of the precursor concentration as a function of ion flux and temperature is confirmed by ion pulse measurements on a time scale of 1 s. Further, it is concluded that the mechanisms for enhanced SiF4 formation during ion bombardment are the same over the temperature range studied.