Femtosecond two-photon ionization and solvated electron geminate recombination in liquid-to-supercritical ammonia.

The first-ever femtosecond pump-probe study is reported on solvated electrons that were generated by multiphoton ionization of neat fluid ammonia. The initial ultrafast ionization was carried out with 266 nm laser pulses and was found to require two photons. The solvated electron was detected with a femtosecond probe pulse that was resonant with its characteristic near-infrared absorption band around 1.7 μm. Furthermore, the geminate recombination dynamics of the solvated electron were studied over wide ranges of temperature (227 K ≤ T ≤ 489 K) and density (0.17 g cm(-3) ≤ ρ ≤ 0.71 g cm(-3)), thereby covering the liquid and the supercritical phase of the solvent. The electron recombines in a first step with ammonium cations originating from the initial two-photon ionization thereby forming transient ion-pairs (e(am)(-)·NH(4)(+)), which subsequently react in a second step with amidogen radicals to reform neutral ammonia. The escape probability, i.e., the fraction of solvated electrons that can avoid the geminate annihilation, was found to be in quantitative agreement with the classical Onsager theory for the initial recombination of ions. When taking the sequential nature of the ion-pair-mediated recombination mechanism explicitly into account, the Onsager model provides a mean thermalization distance of 6.6 nm for the solvated electron, which strongly suggests that the ionization mechanism involves the conduction band of the fluid.