Chasing charge localization and chemical reactivity following photoionization in liquid water

Marsalek, Ondrej; Elles, Christopher G.; Pieniazek, Piotr A.; Pluhařová, Eva; VandeVondele, Joost; Bradforth, Stephen E.; Jungwirth, Pavel

doi:doi:10.1063/1.3664746

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Rotational tunneling in CH₄ II: Disorder effects

Transitions within the tunneling multiplet of CH4 in phase II have been measured in an experiment at the backscattering instrument BASIS of the Neutron Source SNS. They all involve transitions from or to T-states. A statistical model is put forward which accounts for local departures from tetrahedral symmetry at the sites of ordered molecules. Different from previous work, in which discrete sets of overlap matrix elements have been studied, now large numbers of elements as well as the ensemble o...

Nuclear quadrupole resonance lineshape analysis for different motional models: Stochastic Liouville approach

A general theory of lineshapes in nuclear quadrupole resonance (NQR), based on the stochastic Liouville equation, is presented. The description is valid for arbitrary motional conditions (particularly beyond the valid range of perturbation approaches) and interaction strengths. It can be applied to the computation of NQR spectra for any spin quantum number and for any applied magnetic field. The treatment presented here is an adaptation of the “Swedish slow motion theory,” [T. Nilsson and J. Kow...

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 H₂O⁺ + H₂O → OH + H₃O⁺. 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 H₂O⁺_(aq) species.

Article Outline

INTRODUCTION
METHODS
RESULTS AND DISCUSSION
1. Ab initio MD following photoionization in bulk water
2. Benchmarking the initial delocalization of the cationic hole
3. Modeling of the absorption spectra
4. Experimental search for the cationic hole and its evolution by femtosecond transient absorption
CONCLUSIONS

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KEYWORDS and PACS

Keywords

ab initio calculations, approximation theory, biochemistry, biological tissues, chemical exchanges, coupled cluster calculations, density functional theory, high-speed optical techniques, molecular biophysics, molecular dynamics method, molecule-photon collisions, optical pumping, photochemistry, photoionisation, quantum theory, reaction kinetics theory, spectral line shift, ultraviolet spectra, visible spectra, water

PACS

87.15.R-

Reactions and kinetics
87.15.ap

Molecular dynamics simulation
82.50.-m

Photochemistry
82.39.Jn

Charge (electron, proton) transfer in biological systems
82.30.Hk

Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Wt

Computational modeling; simulation

ARTICLE DATA

Digital Object Identifier

http://dx.doi.org/10.1063/1.3664746

PUBLICATION DATA

ISSN

0021-9606 (print)

1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef

American Institute of Physics

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