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)

Vibrational Energy Relaxation of Thiocyanate Ions in Liquid-to-Supercritical Light and Heavy Water. A Fermi’s Golden Rule Analysis

The vibrational relaxation dynamics following an ultrafast nitrile stretching (ν₃) excitation of thiocyanate anions dissolved in light and heavy water have been studied over a wide temperature and density range corresponding to the aqueous liquid up to the supercritical phase. In both solvents, the relaxation of the ν₃ = 1 state of the anion leads to a direct recovery of the vibrational ground state and involves the resonant transfer of the excess vibrational energy onto the solvent. In light water, the energy-accepting states are provided by the bending–librational combination band (ν_b + ν_L), while in heavy water, the relaxation is thermally assisted by virtual acceptor states derived from the stretching–librational/restricted translational hot band (ν_S – ν_L,T). The relaxation rate is found to strictly obey Fermi’s Golden Rule when the density of resonant solvent states is estimated from the linear infrared spectra of the solute and the pure solvents

The Photochemical Route to Octahedral Iron(V). Primary Processes and Quantum Yields from Ultrafast Mid-Infrared Spectroscopy

Recently, the complex cation [(cyclam-ac)­Fe^III(N₃)]⁺ has been used in solid matrices under cryogenic conditions as a photochemical precursor for an octahedral iron nitride containing the metal at the remarkably high oxidation state +5. Here, we study the photochemical primary events of this complex cation in liquid solution at room temperature using femtosecond time-resolved mid-infrared (fs-MIR) spectroscopy as well as step-scan Fourier-transform infrared spectroscopy, both of which were carried out with variable-wavelength excitation. In stark contrast to the cryomatrix experiments, a photooxidized product cannot be detected in liquid solution when the complex is excited through its putative LMCT band in the visible region. Instead, only a redox-neutral dissociation of azide anions is seen under these conditions. However, clear evidence is found for the formation of the highly oxidized iron nitride product when the photolysis is carried out in liquid solution with UV light. Yet, the photooxidation must compete with photoreductive Fe–N bond cleavage leading to azide radicals and an iron­(II) complex. Both, redox-neutral and photoreductive Fe–N bond breakage as well as photooxidative N–N bond breakage occur on a time scale well below a few hundred femtoseconds. The majority of fragments suffer from geminate recombination back to the parent complex on a time scale of 10 ps. Upper limits of the primary quantum yield for photooxidation are derived from the fs-MIR data, which increase with increasing energy of the photolysis photon