Multiphoton Ionization and Recombination Dynamics in Liquid-to-Supercritical Ammonia

This thesis reports on the first-ever femtosecond transient absorption study of solvated electrons that were produced by multiphoton excitation of neat fluid ammonia. To obtain insight into the ionization mechanism below and above the optical valence-to-conduction band gap of the solvent, the initial ultrafast ionization was carried out with a 400 nm, respectively a 266 nm laser pulse, both of which were found to require two photons corresponding to a total excitation energy of 6.2 eV, respectively 9.3 eV. Subsequently, the solvated electron was monitored with femtosecond probe pulses that was resonant with its characteristic near-infrared absorption band around 1700 nm. A primary goal of the experiments was to explore the role of structural and electronic properties of the solvent network in the photoionization pathways below and above the band-gap. For this purpose, the ammoniated electron’s geminate recombination dynamics was systematically studied over wide ranges of temperature (227 K ≤ T ≤ 489 K) and density (0.17 g/cm3 ≤ ρ ≤ 0.71 g/cm3), thereby covering the dense liquid and the dilute supercritical phase of the solvent. A kinetic analysis of the electron’s survival probability was carried out to determine the temperature and density-dependent average thermalization distance of the solvated electron from its primary ionization site, , which offers insight into the nature of the electronic state of the liquid from which the electron is initially injected. i) At 9.3 eV total excitation energy, vertical ionization was found to initially produce highly mobile electrons in the conduction band of the liquid which only subsequently become localized by the solvent. A pronounced dependence of on the thermodynamic state variables T, ρ was related to a T, ρ-induced energy level shift of the valence-to-conduction band-gap which gives rise to a variation of the energy initially imparted on the photoejected electron. ii) A total excitation energy of 6.2 eV is about 2 eV below the optical band-gap band in liquid ammonia, hence, an ionization of the neat fluid requires solvent nuclear rearrangement. The solvated electron’s geminate recombination is strongly accelerated compared to the data at 9.3 eV and indicates that the majority of electrons is injected into suitable trapping sites located between the first and second solvation shell of the initially ionized ammonia molecules. Such configurations can be considered as instantly reactive and facilitate an ultrafast barrierless electron annihilation

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

Femtosecond multiphoton ionization experiments have been conducted on ammonia over a wide range of temperature (225 K ≤ <i>T</i> ≤ 490 K) and density (0.18 g/cm³ ≤ ρ ≤ 0.7 g/cm³), 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)

Femtosecond two-photon ionization of fluid NH3 at 9.3 eV

Liquid and supercritical ammonia (NH3) is photo-ionized at an energy of 9.3 eV with 100-fs duration pulses at a wavelength of 266 nm. The ionization involves two photons and generates fully solvated electrons via the conduction band of the solvent within the time resolution of the experiment. The dynamics of their ensuing geminate recombination is followed in real time with femtosecond near-infrared (IR) probe pulses. The recombination mechanism can be understood as an ion-pair mediated reaction. The electron survival probability is found to be in quantitative agreement with the classical Onsager theory for the initial recombination of ions