We are studying the vacuum ultraviolet spectroscopy of the lowest-lying electronic state of sub- and supercritical water to glean information on changes to the hydrogen bonding environment as a function of temperature and density. Above the critical point, water is thought to exist as a dynamic system consisting of single water molecules and nanoclusters of dimers, trimers, etc. Computational studies indicate that the electronic states of these various species are highly energetically sensitive to the local hydrogen bonding structure, where an increasing degree of hydrogen bonding causes the lowest-lying electronic state energy gap to blue shift up to 0.7 eV from that of the water monomer.We have designed a unique high-sensitivity vacuum ultraviolet absorption measurement to directly probe these changes, and have done so from room temperature up to supercritical conditions, and then as a function of density for supercritical water. The lowest-lying energy gap is seen to gradually red shift as a function of increasing temperature, likely confirming the purported weakening of the hydrogen bond network. Above the critical point, the gas-phase water monomer spectrum can be replicated in the low-density limit, where hydrogen bonding can be considered negligible. With increasing density, the energy gap gradually blue shifts, presumably with an increase in the extent of hydrogen bonding. We suggest that these VUV measurements of the water absorption are the most extensive to date, and we will be able to provide reliable extinction coefficients from 1450-2200 � over the entire temperature and density range examined.We also reveal new information on the temperature dependence of the charge transfer to solvent transition for aqueous hydroxide and iodide, extending information from many previous studies from < 100 �C up to supercritical conditions. Results likely confirm predicted changes in the properties of the anion solvent shell as a function of temperature.

Insights into the structure of water under sub- and supercritical conditions

246th National Meeting (2013)

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