Effect of water density on the absorption maximum of hydrated electrons in sub- and supercritical water up to 400 °C.

The optical absorption spectra of the hydrated electron (e_aq^-) in supercritical (heavy) water (SCW) are measured by electron pulse radiolysis techniques as a function of water density at three temperatures of 380, 390, and 400 °C, and over the density range of ∼0.2–0.65 g/cm³. In agreement with previous work, the position of the e_aq^- absorption maximum (E_Amax) is found to shift slightly to lower energies (spectral “redshift”) with decreasing density. A comparison of the present E_Amax-density data with other measurements already reported in the literature in subcritical (350 °C) and supercritical (375 °C) water reveals that at a fixed pressure, E_Amax decreases monotonically with increasing temperature in passing through the phase transition at t_c. By contrast, at constant density, E_Amax exhibits a minimum as the water passes above the critical point into SCW. These behaviors are explained in terms of simple microscopic arguments based on the crucial role played by local density and configurational fluctuations (associated with criticality) in providing pre-existing polymeric clusters, which act as trapping sites for electrons. [ABSTRACT FROM AUTHOR]