Potential traps for an excess electron in liquid water. Geometry, energy distributions and lifetime

The possibilities of trapping of an excess electron by potential fluctuations in liquid water have been investigated by means of the computer simulation method. The equilibrium configurations of 200 water molecules were generated by the molecular dynamics method. Given an interaction potential between a negative test charge and a water molecule, the molecular configurations generated by the simulation are searched for local minima of the potential energy. The analysis of a large set of the minima allows us to obtain an extensive statistical description of the microscopic trapping sites, including the distributions of the trap energy, volume and the trapping cross section. The estimated concentration of the electron traps in liquid water is about 0.5 mol/dm3. The possiblity of electron trapping depends very strongly on the lifetime of the potential traps. The simulations yielded the distribution of the trap lifetime with the average of 84 fs. A substantial fraction (20%) of the traps live longer than 100 fs, a small fraction (0.2%) live as long as 1 ps. These values can be compared with experimental measurements of the electron hydration time of the order of 100 fs.