Reactions associated with ionization in water: A direct <i>ab initio</i> dynamics study of ionization in (H2O)17

Furuhama, Ayako; Dupuis, Michel; Hirao, Kimihiko

doi:10.1063/1.2194904

Cited by 45 publications

(55 citation statements)

References 34 publications

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“…Small water clusters will not contain a fully solvated water molecule in the interior, but for n ജ 17 an internally solvated water molecule is possible. 19 Consequently, the observation of He͑H 2 O͒ n + clusters for up to n = 27 suggests that, at least for a subset of the cluster ions, migration of the positive charge to an interior water molecule does not occur.…”

Section: Discussionmentioning

confidence: 99%

“…The He͑H 2 O͒ n + peaks are noticeably broader than other peaks in the mass spectrum, particularly for low n. This peak width has proved advantageous in identifying He͑H 2 O͒ n + . For example, the n = 4 cluster has the same mass as the more abundant He 19 + cluster, but He͑H 2 O͒ 4 + can still be identified as a broader feature underlying the much sharper and stronger He 19 + peak. We have not been able to confirm or disprove the presence of He͑H 2 O͒ n + clusters for n =1-3 due to severe interference from other mass spectral features in this region, so we can only say for certain that He͑H 2 O͒ n + ions are formed for n ജ 4.…”

Section: A Mass Spectrometrymentioning

confidence: 99%

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Electron impact ionization of water-doped superfluid helium nanodroplets: Observation of He(H2O)n+ clusters

Yang¹,

Brereton²,

Nandhra³

et al. 2007

The Journal of Chemical Physics

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Electron impact mass spectra have been recorded for helium nanodroplets containing water clusters. In addition to identification of both H + ͑H 2 O͒ n and ͑H 2 O͒ n + ions in the gas phase, additional peaks are observed which are assigned to He͑H 2 O͒ n + clusters for up to n = 27. No clusters are detected with more than one helium atom attached. The interpretation of these findings is that quenching of ͑H 2 O͒ n + by the surrounding helium can cool the cluster to the point where not only is fragmentation to H + ͑H 2 O͒ m ͑where m ഛ n −1͒ avoided, but also, in some cases, a helium atom can remain attached to the cluster ion as it escapes into the gas phase. Ab initio calculations suggest that the first step after ionization is the rapid formation of distinct H 3 O + and OH units within the ͑H 2 O͒ n + cluster. To explain the formation and survival of He͑H 2 O͒ n + clusters through to detection, the H 3 O + is assumed to be located at the surface of the cluster with a dangling O-H bond to which a single helium atom can attach via a charge-induced dipole interaction. This study suggests that, like H + ͑H 2 O͒ n ions, the preferential location for the positive charge in large ͑H 2 O͒ n + clusters is on the surface rather than as a solvated ion in the interior of the cluster.

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Section: Discussionmentioning

confidence: 99%

Section: A Mass Spectrometrymentioning

confidence: 99%

Electron impact ionization of water-doped superfluid helium nanodroplets: Observation of He(H2O)n+ clusters

Yang¹,

Brereton²,

Nandhra³

et al. 2007

The Journal of Chemical Physics

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“…Transition times in the fs range have been found. This is even faster than the proton rearrangement associated with the ionization of water 17 . Autoionization via ICD therefore will dominate over alternative relaxation channels, such as fluorescence or nuclear rearrangement.…”

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confidence: 96%

A hitherto unrecognized source of low-energy electrons in water

et al. 2010

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Because ICD is expected to take place universally in weakly bound aggregates containing light atoms between carbon and neon in the periodic table 2,3 , these results could have implications for our understanding of ionization damage in living tissues. NPHYS-2009-06-00979a 2 Electronic vacancy states can be produced in matter by ionizing radiation, such as X-ray photons or fast charged particles. When a state with a high electronic excitation energy has been produced by impact of such particles, electron correlation can cause the ejection of electrons. Auger decay is the best known representative of this class of secondary processes that is more generally termed autoionization. In other words, the mechanism is a concerted transition in which a single hole in an inner shell is replaced by two vacancies in the outer valence shells of two adjacent molecules, and a free electron. This decay channel was termed Intermolecular (Interatomic, in the case of atomic clusters) Coulombic Decay and was subsequently observed in rare gas clusters 4-7 .The process is shown schematically in Fig. 1. A resonant variant of ICD, which may take place after photoexcitation into an unoccupied orbital, has also been discussed [7][8][9] . In the present paper, we consider ICD of inner valence vacancy states, for which case the ejected electrons have a low kinetic energy.On the basis of energetic considerations, ICD can take place whenever the binding energy of the ionized state lies above the double ionization threshold of the corresponding cluster or liquid. This prerequisite for ICD is fulfilled in hydrogen-bonded systems 2,10 , but so far the process has not been seen. Calculations of the energy spectrum of electrons ejected by ICD of small water clusters give a hint as to why it has escaped observation: A broad, rather unstructured distribution of energies is expected, which peaks at zero eV 10 . Ifwe consider an experiment with a conventional electron energy analyser on a bulk or liquid NPHYS-2009-06-00979a 3 sample, an electron spectrum with this shape can hardly be distinguished from the "universal curve" 1 for secondary electrons (Fig. 2). In this respect our work differs from earlier experiments, which were either restricted to dimers 5-7 , or dealt with simpler cases where an ICD feature appears from simple electron kinetic energy spectra 4,8,9 . Producing primary electrons of a well-defined energy by photoionization and detecting them in coincidence with the ICD electron has allowed us to overcome the aforementioned problem. Here, we demonstrate that ICD follows the photoionization of medium-sized water clusters and show that -above the corresponding photoionization threshold -ICD electrons make an important contribution to the low kinetic energy spectrum.In our experiment, a jet of water clusters with a mean size 〈N〉 of 40 or 200 was used.Such clusters are believed to form amorphous structures, which resemble the hydrogenbonded network of liquid water rather than that of crystalline ice 11 . Inner valence vacancies were p...

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“…25 Additionally, an ab initio molecular dynamics study of ionization in a cluster with 17 water molecules was performed recently which, however, due to the use of the Hartree-Fock (HF) method leads to a localized charge already at the instant of ionization. 27 The previously studied ionized water clusters allow investigation of the onset of the medium effects; nevertheless, they are still very far from the condensed phase. A natural question therefore arises as to what is the degree of charge delocalization upon ionization in liquid water (i.e., what is the nature of the nascent bulk "H 2 O + ") and whether this delocalization can play a role in slowing reaction from the cationic hole toward H 3 O + and OH products in the aqueous bulk.…”

Section: Introductionmentioning

confidence: 99%

Chasing charge localization and chemical reactivity following photoionization in liquid water

Maršálek

Elles

Pieniazek

et al. 2011

The Journal of Chemical Physics

105

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The ultrafast dynamics of the cationic hole formed in bulk liquid water following ionization is investigated by ab initio molecular dynamics simulations and an experimentally accessible signature is suggested that might be tracked by femtosecond pump-probe spectroscopy. This is one of the fastest fundamental processes occurring in radiation-induced chemistry in aqueous systems and biological tissue. However, unlike the excess electron formed in the same process, the nature and time evolution of the cationic hole has been hitherto little studied. Simulations show that an initially partially delocalized cationic hole localizes within ∼30 fs after which proton transfer to a neighboring water molecule proceeds practically immediately, leading to the formation of the OH radical and the hydronium cation in a reaction which can be formally written as H 2 O + + H 2 O → OH + H 3 O + . The exact amount of initial spin delocalization is, however, somewhat method dependent, being realistically described by approximate density functional theory methods corrected for the self-interaction error. Localization, and then the evolving separation of spin and charge, changes the electronic structure of the radical center. This is manifested in the spectrum of electronic excitations which is calculated for the ensemble of ab initio molecular dynamics trajectories using a quantum mechanics/molecular mechanics (QM/MM) formalism applying the equation of motion coupled-clusters method to the radical core. A clear spectroscopic signature is predicted by the theoretical model: as the hole transforms into a hydroxyl radical, a transient electronic absorption in the visible shifts to the blue, growing toward the near ultraviolet. Experimental evidence for this primary radiation-induced process is sought using femtosecond photoionization of liquid water excited with two photons at 11 eV. Transient absorption measurements carried out with ∼40 fs time resolution and broadband spectral probing across the near-UV and visible are presented and direct comparisons with the theoretical simulations are made. Within the sensitivity and time resolution of the current measurement, a matching spectral signature is not detected. This result is used to place an upper limit on the absorption strength and/or lifetime of the localized H 2 O + (aq) species.

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Reactions associated with ionization in water: A direct ab initio dynamics study of ionization in (H2O)17

Cited by 45 publications

References 34 publications

Electron impact ionization of water-doped superfluid helium nanodroplets: Observation of He(H2O)n+ clusters

Electron impact ionization of water-doped superfluid helium nanodroplets: Observation of He(H2O)n+ clusters

A hitherto unrecognized source of low-energy electrons in water

Chasing charge localization and chemical reactivity following photoionization in liquid water

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