Further studies are reported which attempt to identify the transient species (X) produced from hydrated electrons which may be photodissociated to re-form [Formula: see text] The method used involves the production of [Formula: see text] by u.v. flash photolysis of aqueous OH−/H2 solutions followed, after any specified delay, by a second and third photoflash of restricted wavelengths. Photoregeneration of [Formula: see text] is effected by light in the spectral range 320 to 400 nm, not at λ > 700 nm as earlier reported (1). The species X is much longer lived than [Formula: see text] having a mean lifetime of ∼5 ms at pH 11, but this is very dependent on the lifetime of [Formula: see text] in the system. Two conditions seem to be necessary in order to detect the presence of X, (i) [Formula: see text] must decay to a considerable extent through the bimolecular dimerisation reaction and (ii) the lifetime of [Formula: see text] with respect to all other reactions must be >ca. 200 μs. Thus it is not expected to be observable in most pulse radiolysis studies. Its extinction coefficient in that part of its absorption band which leads to photodissociation (320–400 nm) is estimated to average >2000 M−1 cm−1, may be as high as 6000 M−1 cm−1 at 347 nm but does not exceed 12 000 M−1 cm−1 at any wavelength. Its identity is discussed and it is concluded that X is probably the hydrated electron dimer, [Formula: see text] or a product formed there from by association with a cation.
