To investigate the reaction layer dynamics of ion-assisted etching we have measured the time response of product formation in an ultrahigh vacuum beam-surface experiment on a time scale ranging from 100 mus to 1000 s. Both the step function response and the delta function response are investigated. For the latter the pseudorandom cross correlation method is used. The system investigated is the classic Si(100)/XeF2/Ar+ example, at low flux conditions of 0.6 ML/s XeF2 and 0.04 ML/s Ar+ ions at 1 keV energy. We observe a consistent picture of the fourfold action of the bombarding ions. First, on a 1 ms time scale and shorter, the release of tightly bound intermediate radical species such as SiF and SiF2 by physical sputtering, i.e., by momentum of the impinging ions, is the main effect. Second, on a time scale of 40 ms, we observe ion-enhanced formation of SiF4, most likely by the influence of ion bombardment on a rate limiting step in the reaction chain such as the formation of SiF4 from SiF3. Third, on a time scale of 4 s, there is a redistribution of intermediate SiF(x) products in the reaction chain, both in depth profile and in absolute density. Finally, on an even longer time scale, detectable after some 10 s at an ion bombardment of 0.04 ML/s, the production of vacancies and/or broken bonds as reactive sites deep in the substrate becomes important, as observed by a long-term enhanced etch rate after switching off the ion beam. The results are consistent with a model that for low temperatures (T<600 K) the disproportionation reaction, 2 SiF3-->SiF4+SiF2, is the rate limiting step, while at high temperatures (T>700 K) the reaction step leading from SiF2 to SiF3 plays this role.