The etch rate of Si by XeF2 can be enhanced by more than a factor of 8 by ion bombardment. This enhancement is studied in a multiple-beam setup by looking at the response of reaction product signals upon ion pulses on time scales of 1-100 s in a multiple-beam setup. On a time scale of 100 s, it is found that ion bombardment causes fluorine depletion of the reaction layer and changes the structure of the reaction layer. This lower fluorine content results in a lower contribution of the spontaneous SiF4 production during ion bombardment. For the enhanced SiF4 production two processes are found from pulse measurements on the time scale of 1-10 s. First, ion bombardment creates weakly bound surface species, e.g., SiF2, that can react in the reaction layer to SiF4. Second, XeF2 reacts with these species with a higher reaction probability, thus enhancing the SiF4 production. The relative importance of both mechanisms is determined. Further, the limiting steps during spontaneous and ion-assisted etching are discussed, revealing that the creation of dangling bonds is the reason for the higher sticking probability of XeF2 during ion-assisted etching.