The presence of the TD component in the Ar TOF distributions (Figure ) is a consequence of the three-dimensional nature of the FiberForm "surface". In previous studies of hyperthermal Ar scattering on planar surfaces of HOPG and vitreous carbon at elevated temperatures, only an IS component was present in the TOF distributions, suggesting that hyperthermal Ar atoms do not trap on high-temperature carbon surfaces long enough to come into thermal equilibrium before they exit to the vacuum. The presence of a TD component thus suggests that multiple bounces of Ar atoms in the porous three-dimensional carbon fiber network may cause loss of incidence energy and drive the atoms into thermal equilibrium before they eventually desorb from a carbon fiber and escape from the network. The fact that the TD components in the TOF distributions can consistently be fit well by a MB distribution at the surface temperature using tzero as the time at which the incident beam pulse struck the sample is strong evidence that the Ar atoms do not diffuse out of the FiberForm microstructure over long times. As will be discussed in more detail below, we refer to such TD behavior as "TD-prompt." If the Ar atoms resided in the microstructure for more than ~100 microseconds (see Supplementary Material), then there would be a slow tail in the TOF distributions that could not be fit by a MB distribution ("TD-slow"). This observation is critical in analyzing the TOF distributions of scattered products following bombardment of FiberForm by O and O2, which will be discussed below. Additional evidence for multiple-bounce scattering dynamics is seen in the representative IS P(ET) distribution shown in Figure . The IS Ar atoms have a bimodal P(ET) distribution, with a higher-energy peak corresponding to final energies >100 kJ mol -1 (IS-fast) and a lower-energy peak corresponding to final energies <100 kJ mol -1 (IS-slow). Such bimodal translational energy distributions have been observed in several studies of hyperthermal atoms scattering from a surface . The bimodal behavior could arise from backward scattering in the center-of-mass frame or multiple bounces on a rough surface , although we prefer the latter explanation. Note that the P(ET) distribution shown in Figure drops to zero before 600 kJ mol -1 and we found that the probability of translational energies above the incidence energy of Ar (〈Ei〉 = 773 kJ mol -1 ) was negligible for all combinations of incidence and final angles. This finding suggests that the monoenergetic beam approximation is valid for the analysis of the data presented here. Previous work in our laboratory has shown that the monoenergetic beam approximation breaks down if the P(ET) distribution extends well beyond 〈Ei〉 for the incident atom or molecule, in which case a forward convolution approach is required to fit the TOF distributions .