Electron Solvation in Finite Systems: Femtosecond Dynamics of Iodide·(Water)
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            Anion Clusters

Lehr, L.; Zanni, Martin T.; Frischkorn, Christian; Weinkauf, R.; Neumark, Daniel M.

doi:10.1126/science.284.5414.635

Cited by 235 publications

(184 citation statements)

References 25 publications

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“…47 In our laboratory, we have excited these bands in TRPES experiments to study photoinduced charge transfer in halide-solvent clusters. [48][49][50] Smaller water cluster complexes with n e 10 were pumped at 4.65 eV, and the ensuing dynamics were probed by detachment with a delayed 1.55 eV probe pulse. Exemplary pump-probe spectra of I -(H 2 O) 7 are shown in Figure 8.…”

Section: Photoinduced Charge Transfer In I -(H 2 O) Nmentioning

confidence: 99%

Dynamics of Electron Solvation in Molecular Clusters

Ehrler

Neumark

2009

Acc. Chem. Res.

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S olvated electrons, and hydrated electrons in particular, are important species in condensed phase chemistry, physics, and biology. Many studies have examined the formation mechanism, reactivity, spectroscopy, and dynamics of electrons in aqueous solution and other solvents, leading to a fundamental understanding of the electron-solvent interaction. However, key aspects of solvated electrons remain controversial, and the interaction between hydrated electrons and water is of central interest. For example, although researchers generally accept that hydrated electrons, e aq -, occupy solvent cavities, another picture suggests that the electron resides in a diffuse orbital localized on a H 3 O radical. In addition, researchers have proposed two physically distinct models for the relaxation mechanism when the electron is excited. These models, formulated to interpret condensed phase experiments, have markedly different time scales for the internal conversion from the excited p state to the ground s state.Studies of negatively charged clusters, such as (H 2 O) n -and I -(H 2 O) n , offer a complementary perspective for studying aqueous electron solvation. In this Account, we use time-resolved photoelectron spectroscopy (TRPES), a femtosecond pump-probe technique in which mass-selected anions are electronically excited and then photodetached at a series of delay times, to focus on time-resolved dynamics in these and similar species. In (H 2 O) n -, TRPES gives evidence for ultrafast internal conversion in clusters up to n ) 100. Extrapolation of these results yields a p-state lifetime of 50 fs in the bulk limit. This is in good agreement with the nonadiabatic solvation model, one of the models proposed for relaxation of e aq -. Similarly, experiments on (MeOH) n -up to n ) 450 give an extrapolated p-state lifetime of 150 fs.TRPES investigations of I -(H 2 O) n and I -(CH 3 CN) n probe a different aspect of electron solvation dynamics. In these experiments, an ultraviolet pump pulse excites the cluster analog of the charge-transfer-to-solvent (CTTS) band, ejecting an electron from the iodide into the solvent network. The probe pulse then monitors the solvent response to this excess electron, specifically its stabilization via solvent rearrangement. In I -(H 2 O) n , the iodide sits outside the solvent network, as does the excess electron initially formed by CTTS excitation. However, the iodide in I -(CH 3 CN) n is internally solvated, and both experimental and theoretical evidence indicate that electrons in (CH 3 CN) n -are internally solvated. Hence, these experiments reflect the complex dynamics that ensue when the electron is photodetached within a highly confined solvent cavity.

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Section: Photoinduced Charge Transfer In I -(H 2 O) Nmentioning

confidence: 99%

Dynamics of Electron Solvation in Molecular Clusters

Ehrler

Neumark

2009

Acc. Chem. Res.

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“…[29][30][31][32][33][34][35][36] The valence electrons of CTTS anions are bound by the nucleus in the ground state, but the excited states are bound only by the polarization of the surrounding solvent. Thus, the CTTS label is somewhat of a misnomer; excitation of a CTTS transition (see, e.g., Figure 1a and c, below) does not directly transfer the excess electron to the solvent.…”

Section: Introductionmentioning

confidence: 99%

Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 2. Photoinduced Electron Transfer to Counterions in Solution

Bragg

Schwartz

2008

J. Phys. Chem. A

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Although they represent the simplest possible charge-transfer reactions, the charge-transfer-to-solvent (CTTS) dynamics of atomic anions exhibit considerable complexity. For example, the CTTS dynamics of iodide in water are very different from those of sodide (Na -) in tetrahydrofuran (THF), leading to the question of the relative importance of the solvent and solute electronic structures in controlling charge-transfer dynamics. In this work, we address this issue by investigating the CTTS spectroscopy and dynamics of I -in THF, allowing us to make detailed comparisons to the previously studied I -/H 2 O and Na -/THF CTTS systems. Since THF is weakly polar, ion pairing with the counterion can have a substantial impact on the CTTS spectroscopy and dynamics of I -in this solvent. In this study, we have isolated "counterion-free" I -in THF by complexing the Na + counterion with 18-crown-6 ether. Ultrafast pump-probe experiments reveal that THF-solvated electrons (e THF -) appear 380 ( 60 fs following the CTTS excitation of "free" I -in THF. The absorption kinetics are identical at all probe wavelengths, indicating that the ejected electrons appear with no significant dynamic solvation but rather with their equilibrium absorption spectrum. After their initial appearance, ejected electrons do not exhibit any additional dynamics on time scales up to ∼1 ns, indicating that geminate recombination of e THF -with its iodine atom partner does not occur. Competitive electron scavenging measurements demonstrate that the CTTS excited state of I -in THF is quite large and has contact with scavengers that are several nanometers away from the iodide ion. The ejection time and lack of electron solvation observed for I -in THF are similar to what is observed following CTTS excitation of Na -in THF. However, the relatively slow ejection time, the complete lack of dynamic solvation, and the large ejection distance/lack of recombination dynamics are in marked contrast to the CTTS dynamics observed for I -in water, in which fast electron ejection, substantial solvation, and appreciable recombination have been observed. These differences in dynamical behavior can be understood in terms of the presence of preexisting, electropositive cavities in liquid THF that are a natural part of its liquid structure; these cavities provide a mechanism for excited electrons to relocate to places in the liquid that can be nanometers away, explaining the large ejection distance and lack of recombination following the CTTS excitation of I -in THF. We argue that the lack of dynamic solvation observed following CTTS excitation of both I -and Na -in THF is a direct consequence of the fact that little additional relaxation is required once an excited electron nonadiabatically relaxes into one of the preexisting cavities. In contrast, liquid water contains no such cavities, and CTTS excitation of I -in water leads to local electron ejection that involves substantial solvent reorganization.

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“…Several spectroscopic experiments [62,63] and ab initio calculations [64][65][66][67][68][69][70] have been performed to investigate the CTTS "precursor" states in clusters containing iodide and a small number of water molecules. Optimal geometries of iodide-water clusters correspond to a surface location of the ion and the water cluster possesses a relatively large dipole moment [66,69].…”

Section: Dynamics Upon Photoexcitationmentioning

confidence: 99%

“…Recent time-resolved spectroscopic experiments [58][59][60], as well as mixed quantum/classical molecular dynamics simulations [61], show that a solvated electron is created by electron transfer from the excited anion into a nearby cavity site within 200 fs. These liquid studies are complemented by the 7 advent of cluster experiments [62,63] and ab initio calculations [64][65][66][67][68][69] investigating the structure of the CTTS precursor states in finite size systems. A very recent computational study addressed the connection between the cluster and bulk liquid CTTS states with special emphasis on the air/water interface [70].…”

mentioning

confidence: 99%

Ions at the Air/Water Interface

2002

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We present results from theoretical studies of aqueous ionic solvation of alkali halides aimed at developing a microscopic description of structure and dynamics at the interface between air and salt solutions. The traditional view has depicted the air/solution interface of simple electrolytes as being devoid of ions. However, it is now firmly established that polarizable anions, such as the heavier halides, occupy the surface of small to medium sized water clusters. Using a combination of theoretical calculations, including ab initio quantum chemistry, Car-Parrinello molecular dynamics simulations, and primarily molecular dynamics simulations based on polarizable force fields, we present a unified view of the interfacial structure of aqueous ionic clusters and bulk solutions. Indeed, we demonstrate that the heavier halogen anions have a propensity for the interface that is proportional to their polarizabil- * Electronic mail: jungwirt@jh-inst.cas.cz † Electronic mail: dtobias@uci.edu 2

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Electron Solvation in Finite Systems: Femtosecond Dynamics of Iodide·(Water) _n Anion Clusters

Cited by 235 publications

References 25 publications

Dynamics of Electron Solvation in Molecular Clusters

Dynamics of Electron Solvation in Molecular Clusters

Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 2. Photoinduced Electron Transfer to Counterions in Solution

Ions at the Air/Water Interface

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Electron Solvation in Finite Systems: Femtosecond Dynamics of Iodide·(Water) n Anion Clusters

Cited by 235 publications

References 25 publications

Dynamics of Electron Solvation in Molecular Clusters

Dynamics of Electron Solvation in Molecular Clusters

Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 2. Photoinduced Electron Transfer to Counterions in Solution

Ions at the Air/Water Interface

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Product

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Electron Solvation in Finite Systems: Femtosecond Dynamics of Iodide·(Water) _n Anion Clusters