Below-Band-Gap Ionization of Liquid-to-Supercritical Ammonia: Geminate Recombination via Proton-Coupled Back Electron Transfer

Abstract

Femtosecond multiphoton ionization experiments have been conducted on ammonia over a wide range of temperature (225 K ≤ T ≤ 490 K) and density (0.18 g/cm(3) ≤ ρ ≤ 0.7 g/cm(3)), thereby covering the liquid and supercritical phases. The experiments were carried out with excitation pulses having a wavelength of 400 nm, and the ionization was found to involve two photons. Therefore, the total ionization energy in this study corresponds to 6.2 eV, which is roughly 2 eV below the valence-to-conduction band gap of the fluid. The ionization generates solvated electrons, which have been detected through their characteristic near-infrared resonance, and must be facilitated through a coupling to nuclear degrees of freedom of the liquid. The recombination of the solvated electron with the geminate fragments was found to obey predominantly single-exponential kinetics with time constants between 500 fs and 1 ps. Only a very minor fraction of the photogenerated electrons is able to escape from the geminate recombination. The results indicate that the majority of electrons are injected into suitable trapping sites located between the first and second solvation shells of the initially ionized ammonia molecules. Such configurations can be considered as instantly reactive and facilitate an ultrafast barrierless electron annihilation. This process is found to exhibit a pronounced kinetic isotope effect, which indicates that the electronic decay is accompanied by the transfer of a proton. The sequence of ionization and recombination events can therefore be described appropriately as a proton-coupled electron transfer (PCET) followed by a proton-coupled back electron transfer (PCBET).

Ultrafast primary processes of the stable neutral organic radical, 1,3,5-triphenylverdazyl, in liquid solution

“…Since acetonitrile is transparent at the pump wavelength, it must be concluded that solvated electrons can only be generated from the solvent through a multi-photon ionization process. [56][57][58][59] Yet, the pump-probe signals scale linearly with the pump intensity as was discussed in the context of Fig. 4d.…”

“…[56][57][58][59] Yet, the pump-probe signals scale linearly with the pump intensity as was discussed in the context of Fig. Since acetonitrile is transparent at the pump wavelength, it must be concluded that solvated electrons can only be generated from the solvent through a multi-photon ionization process.…”

“…However, it has to be emphasized that solvated electrons and verdazylium cations are destined to suffer from geminate recombination [56][57][58][59] thereby most likely reforming the radical's ground electronic state. Consequently, the positive and negative features in Fig.…”