We measured the stopping of energetic protons in an isochorically heated solid-density Be plasma with an electron temperature of ∼32 eV, corresponding to moderately coupled ½ðe 2 =aÞ=ðk B T e þ E F Þ ∼ 0.3 and moderately degenerate ½k B T e =E F ∼ 2 "warm-dense matter" (WDM) conditions. We present the first highaccuracy measurements of charged-particle energy loss through dense plasma, which shows an increased loss relative to cold matter, consistent with a reduced mean ionization potential. The data agree with stopping models based on an ad hoc treatment of free and bound electrons, as well as the average-atom local-density approximation; this work is the first test of these theories in WDM plasma.

Quasi-chemical theory and electronic structure results on inner-sphere H(H 2 O) n + clusters are used to discuss the absolute hydration free energy of H + (aq). It is noted that this quantity is not thermodynamically measurable, and this leads to some relative misalignment of available tables of absolute hydration free energies of ions in water. The simplest quasi-chemical model produces a reasonable quantitative result in the range of -256 to -251 kcal/mol. The primitive concepts on which the model is based naturally identify the Zundel cation H 5 O 2 + as the principal chemical structure contributing to this hydration free energy. The specific participation of an Eigen cation H 9 O 4 + is not required in this model because the definition of that structure depends on outer-sphere arrangements, and a crude dielectric continuum model is here used for outer-sphere contributions.

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