98 research outputs found
Recommended from our members
New Insights into the Charge-Transfer-to-Solvent Spectrum of Aqueous Iodide: Surface versus Bulk.
Liquid phase charge-transfer-to-solvent (CTTS) transitions are important, as they serve as photochemical routes to solvated electrons. In this work, broadband deep-ultraviolet electronic sum frequency generation (DUV-ESFG) and two-photon absorption (2PA) spectroscopic techniques were used to assign and compare the nature of the aqueous iodide CTTS excitations at the air/water interface and in bulk solution. In the one-photon absorption (1PA) spectrum, excitation to the 6s Rydberg-like orbital (5p → 6s) gives rise to a pair of spin-orbit split iodine states, 2P3/2 and 2P1/2. In the 2PA spectra, the lower-energy 2P3/2 peak is absent and the observed 2PA peak, which is ∼0.14 eV blue-shifted relative to the upper 2P1/2 CTTS peak seen in 1PA, arises from 5p → 6p electronic promotion. The band observed in the ESFG spectrum is attributed to mixing of excited states involving 5p → 6p and 5p → 6s promotions caused by both vibronic coupling and the external electric field generated by asymmetric interfacial solvation
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
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
Recommended from our members
Electronic structure of aqueous solutions: Bridging the gap between theory and experiments
Predicting the electronic properties of aqueous liquids has been a long-standing challenge for quantum mechanical methods. However, it is a crucial step in understanding and predicting the key role played by aqueous solutions and electrolytes in a wide variety of emerging energy and environmental technologies, including battery and photoelectrochemical cell design. We propose an efficient and accurate approach to predict the electronic properties of aqueous solutions, on the basis of the combination of first-principles methods and experimental validation using state-of-the-art spectroscopic measurements. We present results of the photoelectron spectra of a broad range of solvated ions, showing that first-principles molecular dynamics simulations and electronic structure calculations using dielectric hybrid functionals provide a quantitative description of the electronic properties of the solvent and solutes, including excitation energies. The proposed computational framework is general and applicable to other liquids, thereby offering great promise in understanding and engineering solutions and liquid electrolytes for a variety of important energy technologies
Ultrafast Dynamics for Electron Photodetachment from Aqueous Hydroxide
Charge-transfer-to-solvent (CTTS) reactions of hydroxide induced by 200 nm
monophotonic or 337 nm and 389 nm biphotonic excitation of this anion in
aqueous solution have been studied by means of pump-probe ultrafast laser
spectroscopy. Transient absorption kinetics of the hydrated electron, eaq-,
have been observed, from a few hundred femtoseconds out to 600 ps, and studied
as function of hydroxide concentration and temperature. The geminate decay
kinetics are bimodal, with a fast exponential component (ca. 13 ps) and a
slower power "tail" due to the diffusional escape of the electrons. For the
biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8
times higher than for the monophotonic 200 nm excitation (31% vs. 17.5% at 25
oC, respectively), due to the broadening of the electron distribution. The
biphotonic electron detachment is very inefficient; the corresponding
absorption coefficient at 400 nm is < 4 cm TW-1 M-1 (assuming unity quantum
efficiency for the photodetachment). For [OH-] between 10 mM and 10 M, almost
no concentration dependence of the time profiles of solvated electron kinetics
was observed. At higher temperature, the escape fraction of the electrons
increases with a slope of 3x10-3 K-1 and the recombination and
diffusion-controlled dissociation of the close pairs become faster. Activation
energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The
semianalytical theory of Shushin for diffusion controlled reactions in the
central force field was used to model the geminate dynamics. The implications
of these results for photoionization of water are discussed.Comment: 44 pages, 9 figures; supplement: 4 pages, 7 figures; to be submitted
to J. Chem. Phy
Excitation-energy dependence of the mechanism for two-photon ionization of liquid H2O and D2O from 8.3to12.4eV
This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/125/4/10.1063/1.2217738.Transient absorption measurements monitor the geminate recombination kinetics of solvated electrons following two-photonionization of liquid water at several excitation energies in the range from 8.3to12.4eV. Modeling the kinetics of the electron reveals its average ejection length from the hydronium ion and hydroxyl radical counterparts and thus provides insight into the ionization mechanism. The electron ejection length increases monotonically from roughly 0.9nm at 8.3eV to nearly 4nm at 12.4eV, with the increase taking place most rapidly above 9.5eV. We connect our results with recent advances in the understanding of the electronic structure of liquid water and discuss the nature of the ionization mechanism as a function of excitation energy. The isotope dependence of the electron ejection length provides additional information about the ionization mechanism. The electron ejection length has a similar energy dependence for two-photonionization of liquid D(2)O, but is consistently shorter than in H(2)O by about 0.3nm across the wide range of excitation energies studied
Exploring autoionization and photo-induced proton-coupled electron transfer pathways of phenol in aqueous solution
The excited state dynamics of phenol
in water have been investigated
using transient absorption spectroscopy. Solvated electrons and vibrationally
cold phenoxyl radicals are observed upon 200 and 267 nm excitation,
but with formation time scales that differ by more than 4 orders of
magnitude. The impact of these findings is assessed in terms of the
relative importance of autoionization versus proton-coupled electron
transfer mechanisms in this computationally tractable model system
- …