Da/collision, then gradually falling to ~zero about 8 hours into the final etching period. EEN2O then remained near zero for the ~4 remaining hours before the experiment was ended, during which an additional ~0.5 MDa sublimed. ~2 orders of magnitude, then gradually dropped below our sensitivity limit (EEO2 <10 -6 Da/O2 collision) over the next ~16 hours, with little additional mass loss (sublimation is slow at 1500 K). Our interpretation is that the initial abrupt ~99% rate decrease occurred when the NP surface layer had isomerized to a fullerene-like structure, and then the rate continued to decrease slowly as further etching-driven isomerization removed defects and produced a fully-coordinated surface. The passivated NP was then held overnight at room temperature, before starting the experiment shown in Fig. , at which point the mass was ~31.5 MDa.