In this example, INP was initially observed to increase significantly, starting before O2 was added, when there was essentially mass loss, and continuing for the first few minutes of fast etching after O2 was added. Thereafter, INP was roughly constant during most of the mass loss, with no obvious change as the NP passivated at long times. The laser power required to hold TNP at 1300 K increased substantially, before, during, and after the period of initial fast etching. Thus, the optical properties were already changing prior to starting oxidation and continued to evolve as the NP was heated and exposed to O2. Decreasing PO2 did clearly slow the passivation process compared to the NPs in Fig. , where PO2 was held constant at 4.3 x 10 -5 Torr. EEO2 for those NPs peaked after ~30 to 40 minutes of etching and had decreased by > two orders of magnitude after 2.5 hours of etching. EEO2 for the NP in Fig. also peaked after ~40 minutes of etching, but decreased more slowly, dropping by only a factor of five after >3.5 hours of oxidation. At that point, PO2 was doubled, which clearly resulted in a faster decrease in EEO2. Thus, the conclusions of this experiment are that the raw mass loss rate during the initial phase of fast etching is linear in PO2 (EEO2 is PO2 independent), but that passivation is slowed by decreasing PO2. Once the passivation process starts, increasing PO2