Trapped Electrons in Water and Deuterium Oxide

For each of the alkali metals used the e.s.r. spectrum at 77°K consisted of a single narrow line (Fig. 12a, b). The relevant features of the e.s.r. spectra are summarized in Table 4. The absence of any effect of the cation on the line width or p-factors shows conclusively that the electron has been transferred completely from the alkali metal atom and is therefore not held in an expanded orbital around the cation, as suggested by Jortner and Sharf (1962). The difference in line width between the spectra in D2O (3-2 G) and in water (9-2 G) suggests that there is a hyperline interaction between the electron and the protons in water. This was shown conclusively by the observation of seven equally spaced hyperfine lines when a deposit prepared from water was warmed carefully (Fig. 12c), whereas no hyperfine structure was observed from a sample containing deuterium oxide. The hyperfine structure shows that the electron interacts primarily with six protons and that it is not delocalized over a large number of water molecules but is located in a well-defined trap surrounded by these protons. 

The optical absorption spectrum resembles that of solvated electrons in liquid water and consists of a diffuse featureless band with a broad maximum in the range of 5500-7000 A and possibly an increase in absorption towards the near ultraviolet. Since the good resolution of the e.s.r. spectrum shows that the ground states of all the trapped electrons are very similar, the broad optical absorption band must be caused by large variations in the excited states of different traps. 

The most likely structure of the trap is an octahedral arrangement of the six protons around the electron and it is probably similar to an 

F-centre in an alkali halide crystal but with OH dipoles replacing the cations. The trap may be stabilized further to a varying extent by the orientation of solvent molecules outside this first coordination sheath. 

The fact that the electrons are completely ionized from the alkali metal atoms at 77°K allows a lower limit to be placed on the heat of solvation of the electron in ice. The reaction which may be represented formally as 

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