Electron Photodetachment from Aqueous Anions. III. Dynamics of Geminate Pairs Derived from Photoexcitation of Mono- vs. Poly- atomic Anions

Photostimulated electron detachment from aqueous inorganic anions is the simplest example of solvent-mediated electron transfer. Here we contrast the behavior of halide anions with that of small polyatomic anions, such as pseudohalide anions (e.g., HS-) and common polyvalent anions (e.g., SO32-). Geminate recombination dynamics of hydrated electrons generated by 200 nm photoexcitation of aqueous anions (I-, Br-, OH-, HS-, CNS-, CO32-, SO32-, and Fe(CN)64-) have been studied. Prompt quantum yields for the formation of solvated, thermalized electrons and quantum yields for free electrons were determined. Pump-probe kinetics for 200 nm photoexcitation were compared with kinetics obtained at lower photoexcitation energy (225 nm or 242 nm) for the same anions, where possible. Free diffusion and mean force potential models of geminate recombination dynamics were used to analyze these kinetics. These analyses suggest that for polyatomic anions (including all polyvalent anions studied) the initial electron distribution has a broad component, even at relatively low photoexcitation energy. There seem to be no well-defined threshold energy below which the broadening of the distribution does not occur, as is the case for halide anions. Direct ionization to the conduction band of water is the most likely photoprocess broadening the electron distribution. Our study suggests that halide anions are in the class of their own; electron photodetachment from polyatomic, especially polyvalent, anions follows a different set of rules.Comment: to be submitted to J. Phys. Chem. A; 28 pages, 5 figs + Supplemen

Transient x-ray absorption spectroscopy of hydrated halogen atom

Time-resolved x-ray absorption spectroscopy monitors the transient species generated by one-photon detachment of an electron from aqueous bromide. Hydrated bromine atoms with a lifetime of ca. 17 ns were observed, nearly half of which react with excess Br- to form Br2-. The K-edge spectra of the Br atom and Br2- anion exhibit distinctive resonant transitions that are absent for the Br- precursor. The absorption spectra indicate that the solvent shell around a Br0 atom is defined primarily by hydrophobic interactions, in agreement with a Monte Carlo simulation of the solvent structure.Comment: 6 pages, 4 figures + supplement, will be submitted to PR

Geminate recombination of electrons generated by above-the-gap (12.4 eV) photoionization of liquid water

The picosecond geminate recombination kinetics for hydrated electrons generated by 200 nm two photon absorption (12.4 eV total energy) has been measured in both light and heavy water. The geminate kinetics are observed to be almost identical in both H2O and D2O. Kinetic analysis based upon the independent reaction time approximation indicates that the average separation between the electron and its geminate partners in D2O is 13% narrower than in H2O (2.1 nm vs. 2.4 nm). These observations suggest that, even at this high ionization energy, autoionization of water competes with direct ionization.Comment: 10 pages + 2 figures, submitted to Chem. Phys. Letter

Frequency-domain "single-shot" (FDSS) ultrafast transient absorption spectroscopy using compressed laser pulses. Part I. Basic treatment

Single-shot ultrafast absorbance spectroscopy based on the frequency encoding of the kinetics is analyzed theoretically and implemented experimentally. The kinetics are sampled in the frequency domain using linearly chirped, amplified 33 fs FWHM pulses derived from a Ti:sapphire laser. A variable length grating pair compressor is used to achieve the time resolution of 500-1000 channels per a 2-to-160 ps window with sensitivity > 5x10-4. In terms of the acquisition time, FDSS has an advantage over the pump-probe spectroscopy in a situation when the "noise" is dominated by amplitude variations of the signal, due to the pump and flow instabilities. The possibilities of FDSS are illustrated with the kinetics obtained in multiphoton ionization of water and aqueous iodide and one-photon excitation of polycrystalline ZnSe and thin-film amorphous Si:H. Unlike other "single-shot" techniques, FDSS can be implemented for fluid samples flowing in a high-speed jet and for thin solid samples that exhibit interference fringes; no a priori knowledge of the excitation profile of the pump across the beam is needed. Another advantage is that due to the interference of quasimonochromatic components of the chirped probe pulse, an oscillation pattern near the origin of the FDSS kinetics emerges. This pattern is unique and can be used to determine the complex dielectric function of the photogenerated species.Comment: 29 pages, 10 figure

Title Excited state dynamics of liquid water: Insight from the dissociation reaction following two-photon excitation

We use transient absorption spectroscopy to monitor the ionization and dissociation products following two-photon excitation of pure liquid water. The two decay mechanisms occur with similar yield for an excitation energy of 9.3 eV, whereas the major channel at 8.3 eV is dissociation. The geminate recombination kinetics of the H and OH fragments, which can be followed in the transient absorption probed at 267 nm, provide a window on the dissociation dynamics at the lower excitation energy. Modeling the OH geminate recombination indicates that the dissociating H atoms have enough kinetic energy to escape the solvent cage and one or two additional solvent shells. The average initial separation of H and OH fragments is 0.7+-0.2 nm. Our observation suggests that the hydrogen bonding environment does not prevent direct dissociation of an O-H bond in the excited state. We discuss the implications of our measurement for the excited state dynamics of liquid water and explore the role of those dynamics in the ionization mechanism at low excitation energies.Comment: 25 pages, 5 figs, submitted to J Chem Phy

Excited state dynamics of liquid water: Insight from the dissociation reaction following two-photon excitation

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/126/16/10.1063/1.2727468.The authors use transient absorption spectroscopy to monitor the ionization and dissociation products following two-photon excitation of pure liquid water. The primary decay mechanism changes from dissociation at an excitation energy of 8.3eV to ionization at 12.4eV. The two channels occur with similar yield for an excitation energy of 9.3eV. For the lowest excitation energy, the transient absorption at 267nm probes the geminate recombination kinetics of the H and OH fragments, providing a window on the dissociation dynamics. Modeling the OH geminate recombination indicates that the dissociating H atoms have enough kinetic energy to escape the solvent cage and one or two additional solvent shells. The average initial separation of H and OH fragments is 0.7±0.2nm. Our observation suggests that the hydrogen bonding environment does not prevent direct dissociation of an O–H bond in the excited state. We discuss the implications of our measurement for the excited state dynamics of liquid water and explore the role of those dynamics in the ionization mechanism at low excitation energies

Security of attachment to spouses in late life: Concurrent and prospective links with cognitive and emotional wellbeing

Social ties are powerful predictors of late-life health and well-being. Although many adults maintain intimate partnerships into late life, little is known about mental models of attachment to spouses and how they influence aging. A total of 81 elderly heterosexual couples (162 individuals) were interviewed to examine the structure of attachment security to their partners; respondents also completed measures of cognition and well-being concurrently and 2.5 years later. Factor analysis revealed a single factor for security of attachment. Higher security was linked concurrently with greater marital satisfaction, fewer depressive symptoms, better mood, and less frequent marital conflicts. Greater security predicted lower levels of negative affect, less depression, and greater life satisfaction 2.5 years later. For women, greater security predicted better memory 2.5 years later and attenuated the link between frequency of marital conflict and memory deficits. Late in life, mental models of attachment to partners are linked to well-being concurrently and over time