Y. Gauduel

Some evidence of ultrafast H2O+-water molecule reaction in femtosecond photoionization of pure liquid water: Influence on geminate pair recombination dynamics

Abstract The elucidation of detailed mechanisms of primary events in molecular dynamics, charge transfer or reaction dynamics have been made possible by advances in spectroscopic techniques using ultrashort laser pulse generation. In this paper we will center on femtosecond investigations of the very primary processes occurring in pure liquid water following a photoionization of solvent molecules by ultraviolet femtosecond pulses. We have observed in the near ultraviolet region (460, 410 nm), an instantaneous transient absorption with ultrashort lifetime which appears during the initial energy deposition. This induced absorption rises within the pulse, i.e. in less than 100 fs and faster than the precursor of the fully hydrated electron. Its relaxation can be described by a monoexponential law. This ultrashort transient absorption is tentatively assigned to the water cation H2O+. The relaxation would then correspond to the ion-molecule reaction H2O+ + H2O → H3O+ + OH for which the cleavage rate constant is measured to be 1013 s−1 at 294 K. An H/D isotope effect on the dynamics of the ion-water molecule reaction has been observed. These results are analyzed considering the dynamical properties of the protic solvent. The influence of the local dynamical molecular structure of the fluid on the primary reactions involving an ultrafast geminate recombination (e−hyd…X3O+; e−hyd…OX with X = H or D) will be discussed.

Short-lived charge-transfer-to-solvent-states and multiple electronic relaxations following femtosecond excitation of aqueous chloride ion

Abstract Early charge transfer processes triggered by the photoexcitation of an aqueous sodium chloride solution (molar ratio H 2 O/NaCl = 55) at 294 K have been investigated by using femtosecond absorption UV-IR spectroscopy. The initial UV energy deposition proceeding by one- (4 eV) and/or two-photon (2 × 4 eV) absorption results in the formation of multiple short-lived electronic states which have been discriminated within the spectral range 360–1250 nm (3.44-0.99 eV). Two well-separated spectral signatures involving UV and infrared transitions have been discriminated and assigned to different excited CTTS states (charge transfer to solvent) as recently suggested by quantum simulations of an aqueous halide I − [Sheu and Rossky, Chem. Phys. Letters 202 (1993) 186; 213 (1993) 233]. A short-lived ultraviolet component appearing within the UV pump pulse and exhibiting a monoexponential relaxation time of 190 ± 20 fs would correspond to a lo excited CTTS state of the chloride ion (CTTS ∗ ). The other ultrashort-lived band peaking above 0.99 eV (1250 nm) and characterized by a high deactivation rate (≈2 × 10 13 s −1 ) is tentatively assigned to a high excited CTTS state (CTTS ∗∗ ) triggered by a two-photon absorption process (8 eV). This transient state precedes the ap pearance of a well-defined infrared component peaking around 1250 nm and due to the (p-like) excited hydrated electron (e prehyd − ). The relaxation of this infrared electron occurs with a time constant of 300 fs and leads to the formation of the ground state of the hydrated electron (e hyd − ). Near-infrared spectroscopic investigations performed in the energy range 1.51-1.24 eV (820–1000 nm) have permitted to clearly identify the existence of additional absorption bands peaking around 880 nm. It is the first time that near-infrared bands are directly observed in an aqueous solution of halide ions. These spectral contributions are assigned to inhomogeneous populations of electron-atom pairs ({e − :Cl n H 2 O A,B} ). The involved photochemical channel can compete wit electron hydration channel for which a pre-hydrated state (e prehyd − ) has been identified. The existence of these near-infrared states would be due to local solvent effects which assist or impede an electron localization outside the first hydration shells of the atomic core (Cl). The electronic population absorbing in the near infrared exhibits a dual behavior whose characteristic times are 330 fs ({e − :Cl n H 2 O A} ) and 750 fs (e − : Cl n H 2 O B ) respectivley. The faster relaxation channel due to contribute to the early geminate recombination between the ground state of the hydrated electron and the chlorine atom. The slower deactivation channel (1.29 × 10 12 s −1 ) would be due to an electronic state ({e − :Cl n H f 2 0 B} ) whose interconversion with a ground state of a hydrated electron has been identified in the present study. This electron photodetachment pathway leads to a delayed formation of hydrated electrons (e hyd −′ ) and can be seen as a specific solvent cage effect in the vicinity of the counterion (Na + ). The direct characterization of short-lived semi-ionized states by near-infrared spectroscopy provides new informations on solvent cage effects during ultrafast electron transfer reactions in ionic solutions. These complex photochemical data obtained with aqueous sodium chloride are discussed at the microscopic level considering recent quantum theories on semi-ionized or metastable states in ionic solutions.

Ultra-short electron beams based spatio-temporal radiation biology and radiotherapy

Deeply understanding the basic mechanisms of radiation damage in vitro and on living cells, starting from the early radical and molecular processes to mutagenic DNA lesions, cell signalling, genomic instability, apoptosis, microenvironment and Bystander effects, radio sensitivity should have many practical consequences such as the customization of cancer radiotherapy or radioprotection protocols. In this context, innovative laser-plasma accelerators provide ultra-short particle beams (electrons, protons) with parameters of interest for radiation biology and medical physics. This review article approaches some complex links that exist between radiation physics of new pulsed irradiation sources and potential biomedical applications. These links concern mainly the understanding of spatio-temporal events triggered by a radiation, within a fluctuating lapse of time from the initial energy deposition to primary damages of biological interest.

Femtosecond investigation of single-electron transfer and radical reactions in aqueous media and bioaggregate-mimetic systems

The elucidation of detailed mechanisms of ultrafast complex events that occur in molecular dynamics, charge transfer, or reaction dynamics has been made possible by recent advances in spectroscopy techniques that use ultrashort laser pulse generation. We focus on the implications of femtosecond spectroscopy for the investigation of electron reactivity in homogeneous aqueous solutions and organized assemblies that mimic bioaggregates.

Femtosecond and picosecond time-resolved electron solvation in aqueous and reversed micelles

Abstract Time-resolved electron solvation is studied using femtosecond photolysis of phenothiazine in a micellar system. While hydrated electrons appear in less than 10O fs in an AOT/heptane micelle, this process takes about 50O fs in an anionic aqueous micelle. We find that the nature and the type of micelle forming surfactant influence the lifetime of hydrated electrons.

Ultrafast electronic relaxation dynamics: A comparison between water and ionic aqueous solutions

Femtosecond spectroscopy of aqueous solutions at room temperature allows to investigate electronic dynamics triggered by the energy deposition and a subsequent excess electron photodetachment from solvent or solute (chloride ion). In ionic aqueous solutions, early physicochemical processes are analyzed within the frequency domain 27 103 − 7.7 103 cm−1 by considering multiple time-resolved electronic relaxation channels: electron localization within the solvation shells of chlorine atom, electron-ion pair formation, deactivation of excited charge transfer to solvent states (excited CTTS states). At short time (0.5 ps < t < 4ps) ultrafast electron-chlorine atom recombination reaction (kA ∼ 3 1012 s−1) competes with a second electron hydration channel (kB ∼ 1.3 1012 s−1). The direct characterization of ultrafast electron-atom reactions in polar solutions provide further basis for i) the investigation of ultrashort-lived solvent cage effects at the microscopic level, ii) a better understanding of quantum branching processes during ultrafast electron transfer reactions, iii) a knowledge on the role of reorientational correlation function of solvent molecules around semi-ionized and metastable electronic states.

Femtosecond relativistic electron beam triggered early bioradical events

With the recent advent of table-top terawatt Ti:Sa laser amplifier systems, laser plasma interactions provide high-energy, femtosecond electron bunches, which might conjecture direct observation of radiation events in media of biological interest. We report on the first femtolysis studies using such laser produced relativistic electron pulses in the 2.5-15 MeV range. A real-time observation of elementary radical events is performed on water molecules and media containing an important disulfide biomolecule. The primary yield of a reducing radical produced in clusters of excitation-ionisation events (spurs) has been determined at t~3.5 10-12 s. These data provide important information about the initial energy loss and spatial distribution of early radical events. Femtolysis studies devoted to a disulfide biomolecule is noteworthy as it is the first time that a primary ionisation event can be controlled by an ultrafast radical anion formation in the prethermal regime. This innovating domain foreshadows the development of new applications in radiobiology (microdosimetry at the nanometric scale). In the near future, electron femtolysis studies would clearly enhance the understanding of radiation-induced damages in biological confined spaces (aqueous groove of DNA and protein pockets).

Primary steps of an electron–proton reaction in aqueous electrolyte solutions

Abstract We report infrared and visible femtosecond spectroscopic data on primary steps of an electron–proton reaction in aqueous concentrated solutions ([H2O]/[HCl]=5 and 7, [D2O]/[DCl]=7). After an initial electron photodetachment triggered by a two-photon UV excitation of aqueous chloride ion, a first electronic channel appears with a time constant of 130±10 fs and involves a IR p-like state ({e−IR}p→s). This transient IR state exhibits a deactivation process toward the hydrated electron ground state with a characteristic time of 550±30 fs at 294 K. A H+/Li+ substitution does not modify this IR electronic dynamics. Near-IR spectroscopic investigations provide direct evidence that a specific pathway participates to an ultrafast electron–proton reaction. The elementary process whose the frequency rate is 1.18×10 12 s −1 involves a transient nIR state ({Cl⋯e−⋯H+}aq). This three-body complex is localized ∼1 eV below the level of {e−IR}p→s. We conclude that the 4s-like character of nIR {Cl⋯e−⋯H+}aq would be more favorable for an efficient electron attachment on the hydrated proton than a 2p-like state IR prehydrated electron. A low frequency band (270–560 cm−1:0.0334–0.0694 eV) characterizing a short-lived three-body complex {Cl⋯e−⋯H+}aq is assigned to intermolecular vibrational modes that originate from a stretching of hydrogen-bridge OH⋯O. These modes would assist a complete electron attachment on the hydrated proton. The effects of a H/D isotope substitution on the ultrafast electron–proton reaction emphasize the prevailing role of solvent molecules coordinated to protonated hydrates.

ULTRAFAST ELECTRON TRANSFERS IN AQUEOUS ELECTROLYTE SOLUTIONS

Abstract Femtosecond spectroscopy of aqueous ionic solutions represents a good tool for the study of elementary chemical steps at the microscopic level. Ultrafast electron transfer processes are investigated at 294K.

Electron reactivity in aqueous media: A femtosecond investigation of the primary species

Abstract The electron is universally considered as a primary species in the radiolysis and photolysis of aqueous media. The existence of this simplest aqueous radical implicates fascinating questions concerning the coupling of excess electrons with polar liquids whose chemical and physical properties may have a determinant influence on reactivity in chemistry and biology. The recent experimental results available from the femtosecond spectroscopy of water or ionic aqueous solutions provide unique experimental data for the elucidation of detailed mechanisms of primary processes involved in electron reactivity with an essentially protic solvent.