Photoexcitation of Hydrated Electrons Using a Q-Switched Ruby Laser

The lifetime of the photoexcited state of the hydrated electron in liquid water at room temperature is shown to be less than 6 × 10⁻¹² sec following photoexcitation at 694 nm, a wavelength very close to ${\lambda }_{\max }$ . This has implications concerning the nature of the optical transition involved and may suggest that the absorption band is a photoionisation efficiency profile rather than an electronic transition to a bound excited state. The absorbance of hydrated electrons (produced by uv flash photolysis) was measured at very high light fluxes using a Q‐switched ruby laser under conditions where cryptocyanine and methylene blue solutions showed marked nonlinear absorption due to saturation of optical transitions. Whereas the dye solutions showed diminished absorbance and a shortening of the laser pulse at high light intensities, hydrated electrons gave identical absorbance at all intensities, varied over 7 orders of magnitude, up to 200 photons per hydrated electron per cm² of the light beam.