Pulse radiolysis of liquids at high pressures. V. Absorption spectrum and yield of the solvated electron in liquid ammonia at 23 °C and pressures up to 6.7 kbar

The absorption spectrum and primary yield of the solvated electron in liquid ammonia ${\mathrm{(e}}_{\mathrm{am}}^{-})$ have been determined by nanosecond pulse radiolysis of liquid ammonia at 23°C and pressures up to 6.7 kbar. With increase in pressure from 0.009 kbar (vapor pressure at 23°C) to 6.7 kbar, the following changes occur in the absorption spectrum: The transition energy at the absorption maximum (E_max) increases from 0.67 to 0.91 eV; the observable portion W of the bandwidth at half‐maximum, from E_max to the high‐energy side of the spectrum, increases by 35%; and optical density at the absorption maximum (OD_max) decreases by 32%. The pressure and temperature dependences of E_max are attributed to changes in (1) size of the electron cavity and (2) the local dielectric constant associated with the solvation shell of the electron. At 23°C and 0 kbar, (∂lnE_max/∂P)_T for $e_{\mathrm{am}}^{-}$ exceeds that for solvated electrons in water, methanol, and ethanol by a factor of ∼4. Such a result indicates that the cavity of $e_{\mathrm{am}}^{-}$ (V̄=98 ml mol⁻¹) is more compressible than that of the electron in water (V̄ = 7 ml mol⁻¹ for $e_{\mathrm{aq}}^{-}$ ) and the alcohols. From the changes in W and OD_max with increase in pressure from 0.009 to 6.7 kbar, the extinction coefficient at the absorption maximum is estimated to decrease by 19% (similar to the result for $e_{\mathrm{aq}}^{-}$ ) and the primary yield is estimated to decrease from 3.2 to 2.0 solvated electrons per 100 eV (in contrast with invariance of the $e_{\mathrm{aq}}^{-}$ yield). The decrease in primary yield is attributed to an increase in the probability of neutralization relative to separation of geminate ${\mathrm{NH}}_4^{+}{\mathrm{-e}}_{\mathrm{am}}^{-}$ pairs owing to the large negative activation volume of the neutralization reaction as a consequence of the large volume of $e_{\mathrm{am}}^{-}$ .