Hydrated electrons were prepared by multi-photon ionization of neat water with 266 nm light. Using femtosecond pump-probe spectroscopy the dynamics of geminate recombination of the solvated electrons were studied over a wide temperature (296 K ≤T≤ 660 K) and density (0.18 g cm(-3)≤ρ≤ 1.00 g cm(-3)) range extending from the liquid well into the supercritical phase of water. The probability that hydrated electrons escape an initial recombination was found to strongly decrease with increasing temperature. In contrast, the isothermal density-dependence of this survival probability above the critical temperature was surprisingly weak. The peculiar dependence of the initial electron annihilation process on the thermodynamic state variables is discussed in terms of the Onsager model for initial recombination of ion pairs and an effective shielding of the electrostatic interactions of the recombining partners. A finite escape probability for a dielectric constant approaching unity can be interpreted by the existence of a minor fraction of highly mobile electrons created via autoionization.

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.

Give me five: Time-resolved Fourier-transform IR spectroscopy is used to time-resolve the formation and the reaction dynamics of a fourfold symmetrical nitrido iron(V) complex (light blue C, red Fe, blue N) in liquid solution under physiological and technologically relevant conditions.

Independent pairs (IP) and Monte Carlo (MC) simulations are employed to model experimental femtosecond time-resolved pump-probe spectroscopic data on the geminate recombination dynamics of solvated electrons in liquid-to-supercritical water. The hydrated electron was created by two-photon ionization of the neat fluid with a total ionization energy of 9.3 eV. In both numerical approaches, the ejection length, , (i.e. the distance from the ionization core, at which the electron is thermally and spatially localized) is used as the primary adjustable fitting parameter that can bring both model simulations into quantitative agreement with the ultrafast kinetic experiment. The influence of the thermodynamic conditions on the ejection length and on the recombination mechanism is discussed. Whereas in the compressed liquid associated with a high dielectric constant (ε ≥ 20), the electron recombines predominantly with the OH radical, the dissociative recombination via charge neutralization with the hydronium cation takes over at small dielectric constants (ε < 20). The importance of charge-dipole interactions for Monte-Carlo simulations of the recombination reactions of the hydrated electrons in the low-permittivity region is stressed.

The photochemistry of iron azido complexes is quite challenging and poorly understood. For example, the photochemical decomposition of [Fe(III)N(3)(cyclam-ac)]PF(6) ([1]PF(6)), where cyclam-ac represents the 1,4,8,11-tetraazacyclotetradecane-1-acetate ligand, has been shown to be wavelength-dependent, leading either to the rare high-valent iron(V) nitrido complex [Fe(V)N(cyclam-ac)]PF(6) ([3]PF(6)) after cleavage of the azide N(α)-N(β) bond, or to a photoreduced Fe(II) species after Fe-N(azide) bond homolysis. The mechanistic details of this intriguing reactivity have never been studied in detail. Here, the photochemistry of 1 in acetonitrile solution at room temperature has been investigated using step-scan and rapid-scan time-resolved Fourier transform infrared (FTIR) spectroscopy following a 266 nm, 10 ns pulsed laser excitation. Using carbon monoxide as a quencher for the primary iron-containing photochemical product, it is shown that 266 nm excitation of 1 results exclusively in the cleavage of the Fe-N(azide) bond, as was suspected from earlier steady-state irradiation studies. In argon-purged solutions of [1]PF(6), the solvent-stabilized complex cation [Fe(II)(CH(3)CN)(cyclam-ac)](+) (2red) together with the azide radical (N(3)(.)) is formed with a relative yield of 80%, as evidenced by the appearance of their characteristic vibrational resonances. Strikingly, step-scan experiments with a higher time resolution reveal the formation of azide anions (N(3)(-)) during the first 500 ns after photolysis, with a yield of 20%. These azide ions can subsequently react thermally with 2red to form [Fe(II)N(3)(cyclam-ac)] (1red) as a secondary product of the photochemical decomposition of 1. Molecular oxygen was further used to quench 1red and 2red to form what seems to be the elusive complex [Fe(O(2))(cyclam-ac)](+) (6).

Femtosecond mid-infrared pump-probe and two-dimensional mid-infrared spectroscopy have been used to investigate the dynamics of vibrational relaxation and vibrational spectral diffusion of the asymmetric N3-stretching vibration of pseudo-octahedral azidoiron(III) complexes, [L6-nFe(N3)n](+) with n = 1 or 2 and L being an auxiliary ligand of denticity 6-n, in acetonitrile at room temperature. Compared to the free azide anion in acetonitrile solution, the vibrational relaxation dynamics are considerably accelerated. Vibrational energy transfer to the solvent is accelerated by virtue of a resonance with an overtone transition of the solvent. Intramolecular vibrational redistribution is found to be accelerated by virtue of a coupling between the initial azide stretching vibration and the torsional modes involving the axial ligands. Vibrational spectral diffusion within the asymmetric N3-stretching resonance was found to be insensitive to solvent fluctuations because the axial azide ligands are only partially accessible to the solvent. The particular role of intramolecular structural relaxations of the complex for shaping the linear and nonlinear two-dimensional infrared spectra is discussed in terms of ultrafast symmetry-breaking torsional fluctuations and on the basis of density functional theory calculations.

Recently, the complex cation [(cyclam-ac)Fe III (N 3 )] + 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.

Species containing iron at an oxidation state higher than +III are often termed "high-valent iron" and are considered to be key catalytic intermediates in biochemistry. Here, we report the direct time-domain probing of the photochemical formation of an octahedral nitrido iron(V) complex through dinitrogen cleavage from an diazido iron(III) precursor by using femtosecond mid-infrared (MIR) spectroscopy. From the time-resolved vibrational spectra, a mechanism is suggested for the photooxidation of the metal within 10 ps. This mechanism involves an initial ultrafast non-adiabatic transition, followed by a quasithermal N-N bond rupture on the ground-state surface.