Formation Of Hydrated Electrons Research Articles

Ultrafast Dynamics of Water Radiolysis: Hydrated Electron Formation, Solvation, Recombination, and Scavenging.

The ultrafast formation, solvation, and geminate recombination of hydrated electrons upon vacuum ultraviolet photoexcitation of liquid water and the static and dynamic scavenging by NO3- are investigated using femtosecond time-resolved photoelectron spectroscopy. The solvation time constant for excess electrons is typical of that for liquid water but increases slightly with increasing excitation energy. The electron survival probability for geminate recombination is found to be much lower than the literature values owing to previously unobserved ultrafast geminate recombination in a period of 5 ps. NO3- induces the ultrafast (static) scavenging of photoexcited electronic states of liquid water and the dynamic scavenging of detached electrons with a reaction rate that is dependent on the excitation energy. The formation of hydrated electrons at 7.7 eV is ascribed to a H-atom-transfer process, but it is plausible that additional formation channels open at higher energies.

Real-time observation of water radiolysis and hydrated electron formation induced by extreme-ultraviolet pulses.

The dominant pathway of radiation damage begins with the ionization of water. Thus far, however, the underlying primary processes could not be conclusively elucidated. Here, we directly study the earliest steps of extreme ultraviolet (XUV)-induced water radiolysis through one-photon excitation of large water clusters using time-resolved photoelectron imaging. Results are presented for H2O and D2O clusters using femtosecond pump pulses centered at 133 or 80 nm. In both excitation schemes, hydrogen or proton transfer is observed to yield a prehydrated electron within 30 to 60 fs, followed by its solvation in 0.3 to 1.0 ps and its decay through geminate recombination on a ∼10-ps time scale. These results are interpreted by comparison with detailed multiconfigurational non-adiabatic ab-initio molecular dynamics calculations. Our results provide the first comprehensive picture of the primary steps of radiation chemistry and radiation damage and demonstrate new approaches for their study with unprecedented time resolution.

Efficient Singlet‐State Deactivation of Cyano‐Substituted Indolines in Protic Solvents via CNHO Hydrogen Bonds

The photophysical properties of indoline (I) and three of its derivatives, namely, N-methylindoline (MI), 5-cyanoindoline (CI), and 5-cyano-N-methylindoline (CMI), are studied in H-donating solvents of varying polarity. Based on measurements of fluorescence yield and lifetime, and of triplet yield and hydrated-electron formation, two distinct mechanisms of solvent-induced fluorescence quenching are evidenced. The first mechanism involves the cyano substituent and leads to an increase in the rate constant of internal conversion of one order of magnitude in ethanolic solution and of more than two orders of magnitude in water, as compared to solutions in n-hexane or acetonitrile. A similar trend had previously been observed in the case of 4-N,N-dimethylaminobenzonitrile (DMABN). The second mechanism reduces the fluorescence lifetimes of the non-cyanated derivatives in aqueous solution by one order of magnitude and is related to the formation of hydrated electrons. Neither of these mechanisms is influenced by methylation at the ring nitrogen. Quantum chemical calculations are performed on the ground and excited states of the hydrogen-bonded complexes between protic solvents and MI as well as CMI. Stable hydrogen-bonded configurations involving the CN substituent and a solvent OH group are found; these configurations are stable both in the ground and the first excited singlet states, whereas the corresponding complex at the ring amino nitrogen is stable in the ground state only. The CN--HO configuration is therefore a prime candidate for a mechanistic explanation of the observed quenching by the first mechanism. These findings may have useful applications for the design of fluorescence probes for water in biological systems.

Dynamical polaron theory of the hydrated electron

A dynamical polaron theory of the hydrated electron is developed. It explains experimental data on excited electron relaxation in water. The theory fairly precisely reproduces the absorbance curves for the ground and wet states of the hydrated electron as well as time dependencies of the hydrated electron formation for absorbance band at λ = 625 nm and at isobestic point. An analytical expression is obtained for the time required for the formation of the solvated electron from the initially delocalized state in a polar medium. It is shown that when temperature tends to zero this time logarithmically increases as temperature decreases. This result is general and for a homogeneous polar medium it does not depend on the molecular nature of a polar medium.

On the role of the parent cation in the dynamics of formation of laser-induced hydrated electrons

A short account is given of the continuum and molecular descriptions of the formation of hydrated electrons. Starting with a scheme of hydration dynamics of laser-induced electrons that accounts for the formation of hydrated electrons without invoking the "direct" ionization of water, a new model explaining the dynamics of electron hydration in terms of a molecular description is proposed. According to this model, the parent cation plays an active role in the trapping of electrons, deepening electron traps that preexist in the liquid before excitation. Consequences of this description to the transient absorption spectra are briefly discussed. Keywords: laser-induced electron – cation pairs, hydrated electron formation, liquid water.

HYDRATED ELECTRON FORMATION ON LASER EXCITATION OF P‐PYRIDOXYL AMINO ACIDS and PROTEINS

Evidence from a characteristic transient spectrum, its lifetime and quenching by N2O demonstrates that XeCl excimer laser excitation of the reduced form of pyridoxal 5'-phosphate bound to lysine residues will produce the hydrated electron. The phenoxyl-type radical coproduct was also observed. The results identify a useful label of protein structures for site-specific electron generation which can occur outside the region of light absorption by aromatic residues of protein. The results also represent the first observation of hydrated electron formation from flash excitation of a vitamin B6 cofactor.

Picosecond study of electron ejection in aqueous phenol and phenolate solutions

The photoionization of aqueous phenol and phenolate solutions was investigated using picosecond absorption spectroscopy. Phenol and phenolate were excited in the first singlet excited state using a single picosecond pulse of 27 ps duration, at 265 nm (4th harmonic of a Nd–YAG laser). Hydrated electron formation was ascertained by following the kinetics of formation and disappearance at 630 nm with a picosecond pulse obtained by stimulated Raman scattering (SRS). Similarly to the ferrocyanide ion, the excited phenolate ion undergoes a very fast electron ejection process. The lifetime of this process is shorter than the time resolution of our experimental device. In the excited phenol molecule, hydrated electron formation is retarded, the delay between the half-rise times of the exciting pulse and the electron absorption signal being (12±1) ps. This delay is not reduced at low pH. These observations and the high yield of ̄eaq formation are best explained by a consecutive two photon process, the second photon being absorbed by the excited singlet state S1 and the deprotonation occuring after ejection of the electron. The kinetics of hydrated electron disappearance with NO3− as scavenger, are in excellent agreement with literature values. Using H3O+ and Cd++ to scavenge the hydrated electron, its initial formation yield remains constant, showing that Cd++ does not react with any hydrated electron precursor. This difference between our results and those obtained in picosecond pulse radiolysis shows that the precursor of the hydrated electron is not the same in the two techniques. The precursor obtained in pulse radiolysis appears to be more reactive with scavengers. A possible explanation is that after phenolate or phenol excitation, the resultant internal state of the aromatic molecule is capable of undergoing electron transfer to the adjacent solvent molecules, thus producing at least partially hydrated electrons directly.

FLASH PHOTOLYSIS OF TRYPTOPHAN AND N‐ACETYL‐L‐TRYPTOPHANAMIDE; THE EFFECT OF BROMIDE ON TRANSIENT YIELDS

Abstract— The initial yields of the cation and neutral radicals and the triplet are greatly enhanced when high concentrations of Br‐ are present during the flash photolysis of aqueous solutions containing either tryptophan or N‐acetyl‐L‐trytophanamide. The present study is an attempt to elucidate the mechanism by which Br‐ induces these effects. The results obtained indicate that the initial event involves an interaction between the fluorescent state and Br‐ to promote the formation of a long‐lived radical precursor that may be the triplet. It is shown that all of the Br‐‐induced neutral and cation radical formation originate from this long‐lived state. Furthermore, it was found that the mechanism of radical production from the Br‐‐induced long‐lived precursor does not involve hydrated electron formation.

Hydrated electron formation from photoexcited [Ru(CN) 6] 4− and [W(CN) 8] 4− ions

Hydrated electron formation from photoexcited [Ru(CN) 6] 4− and [W(CN) 8] 4− ions

Reactions in spurs: mechanisms of hydrated electron formation in radiolysis of water

Hydrated electron and solute anion formation in H2O and D2O has been investigated with the picosecond pulse radiolysis technique. The spectrum of e–aq in D2O is slightly shifted towards the blue as compared with that in H2O. The yields of e–aq in liquid H2O and D2O are larger than those of e– in the corresponding vapours. Use of salicylate anion (Salaq) as an electron scavenger shows that the yields of its electron adduct (Sal–aq) are considerably larger than those of the e–aq. The importance of electronically excited water molecules in the formation of these extra yields in indicated. Three possible mechanisms to explain the extra yields are considered: (i) solvent assisted e–aq formation from excited water; (ii) energy transfer from excited water to OH–aq leading to e–aq formation; the OH–aq is postulated to result from ionic dissociation of vibrationally excited water molecules in spurs; and (iii) interaction of Salaq and excited water to produce Sal–aq.

Flash photolysis of N-acetyl-L-tryptophanamide; evidence for radical production without hydrated electron formation

Electron counting methods show that approximately 50% of the neutral radicals produced in the flash photolysis of N-acety-L-tryptophanamide (NATA) are not accompanied by hydrated electron formation. Thus, the neutral radical cannot originate solely from the photoionization reaction.

Intensity dependence in laser flash photolysis experiments: Hydrated electron formation from ferrocyanide, tyrosine, and tryptophan

The light intensity dependence in monophotonic and consecutive biphotonic processes under laser flash photolysis conditions is derived. In a monophotonic process the relation between photoproduct yield and exciting light intensity is linear but there is saturation at high intensity. In a biphotonic process there are three different regions. The relation is quadratic at low intensity, linear at medium intensity, and there is saturation at high intensity. The difference between the two linear relations is discussed. These results are applied for analyzing the process of hydrated electron formation from excited states of various substances. The process is found to be monophotonic for ferrocyanide. For tyrosine and tryptophan in aqueous neutral solutions, under the conditions of the laser experiments, a biphotonic process is observed. For the tyrosinate ion (alkaline solution pH 12) the findings are not conclusive. Other work on this topic is discussed with reference to the analysis.

Wavelength and Temperature Dependent Effects on Hydrated Electron Formation and Fluorescence of β-Naphtholate

The absorption spectrum of Na β-naphtholate shows an increase in optical density at its long wavelength edge as a function of temperature. However, fluorescence yield shows a red-edge effect: it decreases with temperature when excited anywhere in the last absorption band except at its long wavelength edge, where no dependence upon temperature is observed. When β-naphtholate is confined in a transparent ethylene-glycol water glass at low temperatures, the fluorescence peak is shifted towards the blue by ∼2000 cm−1. The formation of solvated electrons from the 1S excited state of β-naphtholate was also found to be dependent upon the excitation wavelength, no electrons being formed at λexc>365 nm. The various observations are interpreted in terms of a mechanism by which electrons are dissociated from the excited singlet state prior to solvent relaxation in competition with internal conversion to the fluorescent, solvent relaxed level.

On the Hydrated Electron Formation from Photoexcited Mo(CN)84− Ion

On the Hydrated Electron Formation from Photoexcited Mo(CN)84− Ion

Excited State Chemistry of the Ferrocyanide Ion in Aqueous Solution. I. Formation of the Hydrated Electron

Excitation of the Fe(CN)64− ion in aqueous solution into the 1T1u or 1T2g excited singlet state, below 313 nm, leads to hydrated electron formation in competition with internal conversion to the lowest excited singlet 1T1g state. The limiting quantum yield of hydrated electron formation is a linear function of quantum energy between 313 and 228 nm, reaching φe≃ 0.9 at 228 and 214 nm. Above 313 nm hydrated electron formation is not observed, but photoaquation is. Dependence of Φe on scavenger (N2O) concentration is observed. The involvement of CTTS character in the process, and the role of rapid solvent rearrangement in the dissociation of the excited state, are discussed.

Photochemistry of the Ferrocyanide Ion in Aqueous Solution: Hydrated Electron Formation and Aquation

AbstractLight absorption by the Fe(CN−)4−6 ion in aqueous solution at selected wavelengths between 214 and 313 nm results in the formation of excited states which yield the reaction sequence: with φe, the quantum yield, decreasing from 0.9 at the lowest to 0.1 at the highest wavelength. At and above 337.1 nm φe = 0. The role of charge transfer to solvent (CTTS) intermediates in this process is demonstrated. Photoaquation according to magnified image is found to have very low, possibly zero quantum yield in this region. The role of acid‐base equilibria, e. g. magnified image and of the protonated species, in the photochemistry of cyanoferrate (II) species is discussed.

Formation and Reactions of Hydrated Electrons in UVIrradiated Hexacyanoferrate(II) Solutions

ヘキサシアノ鉄(II)酸廃水溶液の光照射による水和電子の生成ならびに反応を穫々の電子捕提剤を用いて調べた。254mmの波畏の光励起により生じるヘキサシアノ鉄(II)酸イオンの1T1u状態が周囲の水分子と熱的に相互作用することにより,電子脱離が起こることが示される。 ヘキサシアノ鉄(II)酸塩の吸収スペクトルおよび水和電子生成の澱子収量に対する環境効果を研究し,さらに脱離電子のイオン雰囲気をBronsted-Bjerrum理論を適用して調べる。光化学的に生成された水和電子と放射線化学における水和電子を比較しつつ,水和電子を研究する簡便な方法としての本系の特畏を考察する。 さらに水和電子の関与する反応における水素同位体効果,凍結水溶液における電子の挙動,本系を適用したOHラジカルの反応に関する研究結果を報告するo

Primary Radical Yields in Pulse-Irradiated Alkaline Aqueous Solution

Primary radical yields of hydrated electrons, H atoms, and OH radicals are determined by measuring hydrated electron formation following a 4 microsecond pulse of X rays. The pH dependence of free radical yields beyond pH 12 is determined by observation of the hydrated electrons.

The photolysis of aqueous systems at 1849 Å II. Solutions containing Cl - , Br - , SO 2- 4 or OH - ions

Quantum yields of nitrogen and hydrogen are recorded for the photolysis at 1849 Å of aqueous solutions of NaCl, KBr, Na 2 SO 4 , NaOH or HCl containing N 2 O or CH 3 OH or both. It is concluded: (i) That the extinction coefficients and quantum yields of hydrated electron formation are—Cl - 3800± 300m -1 cm -1 and 0.43±0.02; Br - , 13000 ± 1000 M -1 cm -1 and 0.34± 0.02; SO 4- 2 , 260 ± 30m -1 cm -1 and 0.71 ±0.04 and OH - , 3600±600 M -1 cm -1 and 0.11. (ii) That k ( e - aq +H + ) = (1.49 ± 0.16) x k ( e - sq. +N 2 O). (iii) That in alkaline solutions a chain oxidation of methanol by N 2 O can occur, (iv) That the application of the Noyes (1955) equation leads to unacceptably large values of the root mean square diffusive displacement.