Therefore, the heating laser power was immediately decreased to stabilize F z , in this case dropping T NP to energies are lower for NPs with lower initial charges (scales like Q 2 ) or for the heavy fragment in asymmetric fission. Here, a light fragment from a highly charged NP remained trapped, leading to the conclusion that this NP must have shattered into many fragments, each carrying only a small fraction of the total recoil energy. Fig. gives the area-normalized mass loss rates in different T NP ranges obtained by fitting the data in Fig. , plotted in Arrhenius format, in which rates that vary like exp(-E a /kT) appear as straight lines, with slopes that give the activation energy, E a . Note that at 1500 and 1600 K, slow-but-non-zero mass loss rates were observed, without the systematic dependence on T NP expected for an endothermic reaction. This slow mass loss is attributed to NP reactions with background species such as O 2 and H 2 O, which can etch Hf (see below) and which are present at partial pressures in the 10 -8 Torr range. These background reactions set a lower limit on the sensitivity for measuring very slow mass changes. In this case, the background reactions remove one Hf atom every few seconds from the initial total of ~1.5 million atoms. At higher temperatures, the mass loss rates increased rapidly, as expected for sublimation. The increase is fit well by two straight lines with different slopes, for 1700 to 2000K and for 2100 K to 2400 K. Similar slope changes have been observed in sublimation experiments on other Hf NPs, suggesting that some structural change occurs for Hf NPs between 2000 and 2100 K, increasing E a for sublimation.