Thermodynamics amorphous ices

The low threshold energies for the production of D( S), 0( P), and 0( D2) show the importance of valence excited states in the BSD of neutral fragments [47]. The pathway for D( S) desorption probably involves D O D -I- OD. Ffowever, the thresholds for producing 0( P2) and 0( D2), which are the same within experimental error, are lower than the 9.5-and 11.5-eV thermodynamic energies required to produce 0( P2) + 2D( S) and 0( D2) + 2D( S), respectively. The low threshold values therefore indicate that the formation of 0( P2) and 0( D2) must occur by a pathway which involves simultaneous formation of D2. Kimmel et al. have in fact reported [46] a threshold for the production of D2 from D2O ice at — 6 to 7 eV, which supports this conclusion. Above the ionization threshold of amorphous ice, these excited states can be formed directly or via electron-ion recombination. 

Many experiments have been performed to test the various hypotheses discussed in the previous section, but there is as yet no widespread agreement on which physical picture, if any, is correct. The connection between liquid and the two amorphous forms predicted by the LLPT hypothesis is difficult to prove experimentally because supercooled water freezes spontaneously below the nucleation temperature Tw, and amorphous ice crystallizes above the crystallization temperature Tx [32,33]. Crystallization makes experimentation on the supercooled liquid state between Th and Tx almost impossible. However, comparing experimental data on amorphous ice at low temperatures with those of liquid water at higher temperatures allows an indirect discussion of the relationship between the liquid and amorphous states. It is found from neutron diffraction studies [10] and simulations that the structure of liquid water changes toward the LDA structure when the liquid is cooled at low pressures and changes toward the HDA structure when cooled at high pressures, which is consistent with the LLPT hypothesis. Because their entropies are small, the two amorphous states are presently considered to be smoothly connected thermodynamically to the liquid state [34]. 

The difficulty of the experimental proof of the LLCP hypothesis, apart from the crystallization in NML, was that the amorphous ices were solid and not in the thermodynamical equilibrium. As for liquid, it was in the equilibrium and could be a sole state once pressure and temperature were fixed. Therefore, it would be possible to prove the discontinuity of the transition between two different liquids. However, regarding liquid water, LDL and HDL would crystallize immediately in NML, and we could not observe the LLT directly. In contrast, although we could observe the LDA HDA transition, the nonequilibrium nature of LDA and HDA threw doubt on the discontinuity of the transition logically. That is, the existence of the barrier in Fig. 5b was doubted the potential surface between LDA and HDA might be flat by nature. Then, the apparently discontinuous LDA-to-HDA transition (Fig. 7) might be, correctly, continuous or caused by unknown sticky relaxation of a nonequUibrium LDA state. If so, LLT of water would be continuous and LLCP would not exist. 

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