The reduced values of n for thermal desorption from carbon fibers in FiberForm may arise from the relationship between the orientation of the fiber at the point of impact and the nominal surface normal that defines 0° in the experiment. Each fiber in FiberForm has a surface normal associated with it which may not be parallel to the nominal surface normal. Strictly speaking, an effective surface normal is parallel to the nominal surface normal only if the fibers are isotropic and randomly oriented. However, the manufacturing process causes the fibers to orient with the effective surface normal being ±15 o relative to the nominal surface normal (see Introduction). As a result, the effective surface normal is not parallel to the nominal normal from which the experimental scattering angles are measured. The cosine distribution would appear rotated by angle α in the lab frame where α is the angle between the effective surface normal and the nominal surface normal. Using the cosine-power fit, we can compute a range for the angle α by equating cos 0.7 θf = cos (θf -α). The angle α lies between 1° and 7°, which is within the uncertainty of the FiberForm orientation. Thus, the cosine-power distribution of the thermal component for Ar scattering could be a result of the preferred orientation of the fibers. As will be discussed below, angular distributions of other thermally-desorbed species (O, O2, and CO) also require a cosinepower fit. It is likely that the deviations from a pure cos(θf) distribution are a result of the threedimensional network of FiberForm. Material), the similarities in the ratios of IS to TD-prompt flux for Ar and O2 resulting from the three-dimensional porous geometry, and the invariance of flux with surface temperature. This result appears to disagree with a recent molecular beam-surface scattering experiment with a higher flux of lower-energy O atoms on vitreous carbon , where O-atom recombination to produce O2 was reported to be significant. However, the recombination probability may depend strongly on the incident O-atom flux, which was 2-3 orders of magnitude higher in the recent experiment. In addition, the continuous beam of the recent experiment would be expected to maintain a high surface oxygen coverage, which might also help to promote O-O recombination.