Ultrafast relaxation dynamics of solvated electrons in water

Hertwig, Andreas; Hippler, H.; Unterreiner, Andreas‐Neil; Vöhringer, Peter

doi:10.1002/bbpc.19981020603

Cited by 64 publications

(80 citation statements)

References 31 publications

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“…3 Since liquid water contains no preexisting cavities, the ejected electron must force significant local solvent reorganization, explaining the marked spectral shifts observed following both CTTS excitation of aqueous iodide 4,9 and the multiphoton ionization of neat water. 4,65,68 We propose that the CTTS dynamics of I -in THF are sensibly explained if the single s-like CTTS excited state lies relatively high in the manifold of disjoint states energetically, as suggested at the center of Figure 7. Thus, CTTS excitation of I -in THF leads to rapid nonadiabatic coupling to disjoint states (as depicted by the wiggly green arrow in Figure 7), such that few electrons relax to their ground states near their iodine atom partners.…”

Section: Discussion: Understanding the Roles Of The Solute And Somentioning

confidence: 81%

The Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 1. Exploring the Roles of Solvent and Solute Electronic Structure in Condensed-Phase Charge-Transfer Reactions

Bragg

Schwartz

2007

J. Phys. Chem. B

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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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Section: Discussion: Understanding the Roles Of The Solute And Somentioning

confidence: 81%

The Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 1. Exploring the Roles of Solvent and Solute Electronic Structure in Condensed-Phase Charge-Transfer Reactions

Bragg

Schwartz

2007

J. Phys. Chem. B

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“…A recent femtosecond study on ultrafast relaxation dynamics of electrons produced by UV-photolysis in a water jet has shown that at present no other transient states than 'hot' solvated electrons are necessary to understand ultrafast dynamics for times t Ն 300 fs [23]. In other words there is a continuous transition from the 'hot' hydrated electron to the wellknown equilibrated hydrated electron.…”

Section: Some Comments On the 'Incompletely Relaxed Electron'mentioning

confidence: 98%

Is there any Correlation between the Mobility and the Absorption Spectra of Solvated Electrons in Polar Solvents?

Krebs¹

2000

Zeitschrift Für Physikalische Chemie

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Some years ago Jay-Gerin and Ferradini attempted to establish a correlation between the optical absorption spectrum and the mobility of excess electrons in various polar solvents (J. Chem. Phys. 91 (1989) 3275). To explain this correlation they postulated two types of electrons, i.e., highly localized solvated electrons and so-called incompletely relaxed electrons. This was done, however, without any physical justification. According to this correlation and its interpretation just the stable ammoniated electron (surprisingly not considered by Jay-Gerin and Ferradini) should be a quite typical representative of these incompletely relaxed electrons. Therefore, we analyzed the theoretical relationship between the electron mobility and the zero-frequency absorption cross section with the aid of the experimentally observed absorption spectra. We found out that there is no support for a linear relationship between the mobility and the position of the maximum, បωmax, of the optical absorption band and that the contribution of the cross section to the electron mobility is practically zero. Therefore, the proposed correlation has no physical background. We show, however, that there exists a rough correlation between the excess electron mobility and the self-diffusion coefficient of the polar solvent. Finally we critically consider the so-called incompletely relaxed electron state and the results of quantum statistical simulations of the diffusion coefficient of solvated electrons (e Ϫ solv ).

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“…Values for the larger isomer I clusters are proportional to 20 The observed fast relaxation, dynamical isotope effects, absence of solvent dynamics upon photoexcitation, and extrapolation to ultrafast IC rate of 50 fs are consistent with the "nonadiabatic" relaxation mechanism of the bulk hydrated electron. 11,12 According to this mechanism, IC occurs on an ultrafast 50 fs time scale, followed by a two-step solvent relaxation on the longer ∼300 fs and ∼1 ps time scales. Further support for the assignment of the latter lifetimes to solvent dynamics comes from measurements by Paik et al, 37 who found biexponential ground-state relaxation with lifetimes of ∼380 fs and 2-10 ps following IC in clusters with 15 e n e…”

Section: Dynamics Of Electron Solvation In Molecular Clusters Ehrler mentioning

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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Ultrafast relaxation dynamics of solvated electrons in water

Cited by 64 publications

References 31 publications

The Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 1. Exploring the Roles of Solvent and Solute Electronic Structure in Condensed-Phase Charge-Transfer Reactions

The Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 1. Exploring the Roles of Solvent and Solute Electronic Structure in Condensed-Phase Charge-Transfer Reactions

Is there any Correlation between the Mobility and the Absorption Spectra of Solvated Electrons in Polar Solvents?

Dynamics of Electron Solvation in Molecular Clusters

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