Ultrafast dynamics for electron photodetachment from aqueous hydroxide

Charge-transfer-to-solvent reactions of hydroxide induced by 200 nm monophotonic or 337 and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump-probe ultrafast laser spectroscopy. Transient absorption kinetics of the hydrated electron, e(aq) (-), have been observed, from a few hundred femtoseconds out to 600 ps, and studied as function of hydroxide concentration and temperature. The geminate decay kinetics are bimodal, with a fast exponential component ( approximately 13 ps) and a slower power "tail" due to the diffusional escape of the electrons. For the biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8 times higher than for the monophotonic 200 nm excitation (31% versus 17.5% at 25 degrees C, respectively), due to the broadening of the electron distribution. The biphotonic electron detachment is very inefficient; the corresponding absorption coefficient at 400 nm is <4 cm TW(-1) M(-1) (assuming unity quantum efficiency for the photodetachment). For [OH(-)] between 10 mM and 10 M, almost no concentration dependence of the time profiles of solvated electron kinetics was observed. At higher temperature, the escape fraction of the electrons increases with a slope of 3x10(-3) K(-1) and the recombination and diffusion-controlled dissociation of the close pairs become faster. Activation energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The semianalytical theory of Shushin for diffusion controlled reactions in the central force field was used to model the geminate dynamics. The implications of these results for photoionization of water are discussed.     

Novel geminate recombination channel after indirect photoionization of water

We studied the photolysis of neat protonated and heavy water using pump-probe and pump-repump-probe spectroscopy. A novel recombination channel is reported leading to ultrafast quenching (0.7 ± 0.1 ps) of almost one third of the initial number of photo-generated electrons. The efficiency and the recombination rate of this channel are lower in heavy water, 27 ± 5% and (0.9 ± 0.1 ps)(-1), respectively. Comparison with similar data measured after photodetachment of aqueous hydroxide provides evidence for the formation of short-lived OH:e(-) (OD:e(-)) pairs after indirect photoionization of water at 9.2 eV.     

Electron detachment and relaxation of OH-(aq)

Femtosecond transient absorption spectroscopy is used to study the primary reaction dynamics of photoinduced electron detachment of the hydroxide ion in water, OH- (aq). The electron is detached by excitation of OH- (aq) to the charge-transfer-to-solvent (CTTS) state at 200 nm. The subsequent relaxation processes are probed in the spectral range from 193 to 800 nm with femtosecond time resolution. We determine both the time-dependent quantum yields of OH- (aq), OH(aq), and e-(aq), and we observe a transient spectral signature which is assigned to relaxation of hot (OH-)* ions formed via solvent-assisted conversion of the excited CTTS state to OH-. The primary quantum yield of OH(aq) is 65 +/- 5%, while recombination with e-(aq) reduces the yield to 34% after 5 ps and 12% after 200 ps. The yield of hot (OH-)* ions is 35 +/- 5%. Rotational anisotropy measurements of OH- (aq) and OH(aq) indicate a reorientation time for OH- (aq) of 1.9 ps, while no rotational anisotropy is resolved for the OH(aq) radical within our time resolution of 0.3 ps. This is consistent with the notion that OH(aq) radicals formed after electron detachment are only weakly bound to the hydrogen bond network of water. The assignment of the experimental data is supported by a series of electronic structure calculations of simple complexes of OH- (H(2)O)(n).     

Absence of a signature of aqueous I(2P(1/2)) after 200-nm photodetachment of I-(aq)

Ultrafast pump-broadband probe spectroscopy was used to study the transient photoproducts following 200-nm photodetachment of I(-)(aq). Resonant detachment at 200 nm in the second charge-transfer-to-solvent (CTTS) band of I(-)(aq) is expected to produce an electron and iodine in its spin-orbit excited state, I*((2)P(1/2)). The transients in solution following photodetachment were probed from 200 to 620 nm. Along with strong absorption in the visible region due to solvated electrons and a strong bleach of the I(-)(aq) ground-state absorption, a weaker transient absorption near 260 nm was observed that is consistent with a previously assigned ground-state I((2)P(3/2)) charge-transfer band. However, no evidence was found for an equivalent I*(aq) charge-transfer absorption, and I((2)P(3/2)) was produced within the instrument response. This suggests either that I* is electronically relaxed in less than 300 fs or that excitation in the second CTTS band does not in fact lead to I*. The consequences for previous experimental work where I*(aq) production has been postulated, as well as for halogen electron ejection mechanisms, are discussed. In addition, the broad spectral coverage of this study reveals in the bleach recovery the rapid cooling of the solvent surrounding the re-formed iodide after geminate recombination of the iodine with the solvated electron.     

Photexcitation of aqueous ruthenium(II)-tris-(2,2'-bipyridine) with high-intensity femtosecond laser pulses

We report a femtosecond pump-probe study on the photochemistry of concentrated aqueous solutions of [RuII(bpy)3]2+, as a function of pump power (up to 2 TW/cm2) at 400 nm excitation. The transient absorption spectra in the 345-660 nm range up to 1 ns time delay enable the observation of the following photoproducts: the triplet 3MLCT (metal-to-ligand-charge-transfer) excited state, the reduced form [RuII(bpy)3]+, the oxidized species [RuIII(bpy)3]3+, and the solvated electron e(aq). The 3MLCT state is formed within the excitation pulse and undergoes vibrational relaxation in 3-5 ps, as evidenced by the shift of the ligand-centered (LC) absorption band below 400 nm. Even at the highest pump powers, the majority of e(aq) originates from multiphoton ionization of [RuII(bpy)3]2+ and not from the solvent, generating [RuIII(bpy)3]3+ as a byproduct. At 10 ps time delay, the total concentration of the three product species is balanced by the depleted concentration of [RuII(bpy)3]2+, even at the highest fluences used, indicating that no further reaction products significantly contribute to the overall photochemistry. On the 100 ps time scale, most probably diffusion-controlled reduction of ground-state [RuII(bpy)3]2+ by solvated electrons occurs, next to recombination between e(aq) and [RuIII(bpy)3]3+.     

Femtosecond photolysis of aqueous formamide

In this work, we investigate the primary photodynamics of aqueous formamide. The formamide was photolyzed using 200 nm femtosecond pulses, and formation of products and their relaxation was followed with approximately 300 fs time resolution using probe pulses covering the range from 193 to 700 nm. Following excitation, the majority of formamide molecules (approximately 80%) converts the electronic excitation energy to vibrational excitation, which effectively is dissipated to the solvent through vibrational relaxation in just a few picoseconds. The vibrational relaxation is observed as a distinct modulation of the electronic absorption spectrum of formamide. The relaxation process is modeled by a simple one-dimensional wavepacket calculation. A smaller fraction of the excited formamide molecules dissociates to the CHO and NH2 radical pairs, of which 50% escape recombination. In addition to the electronic excitation of formamide, we also observe a small contribution from one-photon ionization of formamide and two-photon ionization and dissociation of the water solvent.     

Time-resolved photoelectron spectroscopy of solvated electrons in aqueous NaI solution

Time-resolved photoelectron spectroscopy was used to study the energetics and dynamics of solvated electrons in aqueous solution. Solvated electrons are generated by ultrafast photodetachment in a 100 mM aqueous NaI solution. Initially, an ensemble of strongly bound ("cold") solvated electrons and an ensemble of weakly bound ("hot") electrons in an unequilibrated solvent environment are observed. We report an ultrafast recombination channel for the "hot" electrons with a rate of (800 fs)(-1) which is in competition with thermalization occurring with a rate of (1.1 ps)(-1). The thermalized electrons recombine with the iodide radical with a rate of (22 ps)(-1). About 35% of the thermalized electrons escape geminate recombination and form free, solvated electrons. The vertical detachment energy for the solvated electron is determined to be 3.40 eV. No indication for a surface-bound electron at lower binding energies was observed.     

Time-resolved photoelectron imaging of large anionic methanol clusters: (methanol)n-(n approximately 145-535)

The dynamics of an excess electron in size-selected methanol clusters is studied via pump-probe spectroscopy with resolution of approximately 120 fs. Following excitation, the excess electron undergoes internal conversion back to the ground state with lifetimes of 260-175 fs in (CH3OH)n- (n=145-535) and 280-230 fs in (CD3OD)n- (n=210-390), decreasing with increasing cluster size. The clusters then undergo vibrational relaxation on the ground state on a time scale of 760+/-250 fs. The excited state lifetimes for (CH3OH)n- clusters extrapolate to a value of 157+/-25 fs in the limit of infinite cluster size.     

Control of bond-cleaving reactions of free protonated tryptophan ion by femtosecond laser pulses

The excited-state dynamics of protonated tryptophan ions is investigated by photoinduced fragmentation in the gas phase. In contrast to the neutral molecule that decays on the nanosecond time scale, the protonated species exhibits an ultrafast decay with two time constants of about 400 fs and 15 ps. In addition, after UV excitation by a pump photon at 266 nm, specific photofragments, and in particular the NH3-loss channel, can be enhanced by the absorption of a probe photon at 800 nm. The bond-cleaving reactions can thus be controlled by a variation of the pump/probe delay.     

Ultrafast plasmon-induced electron transfer from gold nanodots into TiO2 nanoparticles

By using femtosecond transient absorption spectroscopy with visible pump and IR probe to observe generation of injected electrons, we could directly observe plasmon-induced electron transfer from 10 nm gold nanodots to TiO2 nanocrystalline film. It was revealed that the reaction time was within 240 fs and the yield was about 40%.     

Flash photolysis and pulse radiolysis studies on collagen Type I in acetic acid solution

An investigation of the photochemical properties of collagen Type I in acetic acid solution was carried out using nanosecond laser irradiation. The transient spectra of collagen solution excited at 266 nm show two bands. One of them with maximum at 295 nm and the second one with maximum at 400 nm. The peak at 400 nm is assigned to tyrosyl radicals. The first peak of the transient absorption spectra at 295 nm is probably due to photoionisation producing collagen radical cation. The transient for collagen solution in acetic acid at 640 nm was not observed. It is evidence that there is no hydrated electron in the irradiated collagen solution. The reactions of hydrated electrons and (*)OH radicals with collagen have been studied by pulse radiolysis. In the absorption spectra of products resulting from the reaction of collagen with e(aq)(-) no characteristic maximum absorption in UV and visible light region has been observed. In the absorption spectra of products resulting from the reaction of the hydroxyl radicals with collagen two bands have been observed. The first one at 320 nm and the second one at 405 nm. Reaction of (*)OH radicals with tyrosine residues in collagen chains gives rise to Tyr phenoxyl radicals (absorption at 400 nm).     

Interfacial electron transfer between the photoexcited porphyrin molecule and TiO2 nanoparticles: effect of catecholate binding

Interfacial electron transfer (ET) dynamics of 5,10,15-trisphenyl-20-(3,4-dihydroxybenzene) porphyrin (TPP-cat) adsorbed on TiO2 nanoparticles has been studied by femtosecond transient absorption spectroscopy in the visible and near-IR region exciting at 400 and 800 nm. TPP-cat molecule forms a charge transfer (CT) complex with TiO2 nanoparticles through the catechol moiety with the formation of a five-membered ring. Optical absorption measurements have shown that the Q-band of TPP-cat interacts strongly with TiO2 due to chelation; however, the Soret band is affected very little. Optical absorption measurements indicate that the catechol moiety also interacts with TiO2 nanoparticles showing the characteristic band of pure catechol-TiO2 charge transfer (CT) in the visible region. Electron injection has been confirmed by monitoring the cation radical, instant bleach, and injected electron in the conduction band of TiO2 nanoparticles. Electron injection time has been measured to be < 100 fs and recombination kinetics has been best fitted with a multiexponential function, where the majority of the injected electrons come back to the parent cation radical with a time constant of approximately 800 fs for both excitation wavelengths. However, the reaction channel for the electron injection process has been found to be different for both wavelengths. Excitation at 800 nm, found to populate the CT state of the Q-band, and from the photoexcited CT state electron injection into the conduction band, takes place through diffusion. On the other hand, with excitation at 400 nm, a complicated reaction channel takes place. Excitation with 400 nm light excites both the CT band of Cat-TiO2 and also the Soret band of TPP-cat. We have discussed the reaction path in the TPP-cat/TiO2 system after exciting with both 400 and 800 nm laser light. We have also compared ET dynamics by exciting at both wavelengths.     

Electron photodetachment from aqueous anions. 3. Dynamics of Geminate pairs derived from photoexcitation of mono- vs polyatomic anions

Photostimulated electron detachment from aqueous inorganic anions is the simplest example of solvent-mediated electron transfer reaction. As such, this photoreaction became the subject of many ultrafast studies. Most of these studied focused on the behavior of halide anions, in particular, iodide, that is readily accessible in the UV. In this study, we contrast the behavior of these halide anions with that of small polyatomic anions, such as pseudohalide anions (e.g., HS(-)) and common polyvalent anions (e.g., SO(3)(2-)). Geminate recombination dynamics of hydrated electrons generated by 200 nm photoexcitation of aqueous anions (I(-), Br(-), OH(-), HS(-), CNS(-), CO(3)(2-), SO(3)(2-), and Fe(CN)(6)(4-)) have been studied. Prompt quantum yields for the formation of solvated, thermalized electrons and quantum yields for free electrons were determined. Pump-probe kinetics for 200 nm photoexcitation were compared with kinetics obtained at lower photoexcitation energy (225 or 242 nm) for the same anions, where possible. Free diffusion and mean force potential models of geminate recombination dynamics were used to analyze these kinetics. These analyses suggest that for polyatomic anions (including all polyvalent anions studied) the initial electron distribution has a broad component, even at relatively low photoexcitation energy. There seems to be no well-defined threshold energy below which the broadening of this electron distribution does not occur, as is the case for halide anions. The constancy of (near-unity) prompt quantum yields vs the excitation energy as the latter is scanned across the lowest charge-transfer-to-solvent band of the anion is observed for halide anions but not for other anions: the prompt quantum yields are considerably less than unity and depend strongly on the excitation energy. Our study suggests that halide anions are in the class of their own; electron photodetachment from polyatomic, especially polyvalent, anions exhibits qualitatively different behavior.     

Simplicity Beneath Complexity: Counting Molecular Electrons Reveals Transients and Kinetics of Photodissociation Reactions

Time‐resolved pump–probe gas‐phase X‐ray scattering signals, extrapolated to zero momentum transfer, provide a measure of the number of electrons in a system, an effect that arises from the coherent addition of elastic scattering from the electrons. This allows to identify reactive transients and determine the chemical reaction kinetics without the need for extensive scattering simulations or complicated inversion of scattering data. We examine the photodissociation reaction of trimethylamine and identify two reaction paths upon excitation to the 3p state at 200 nm: a fast dissociation path out of the 3p state to the dimethyl amine radical (16.6±1.2 %) and a slower dissociation via internal conversion to the 3s state (83.4±1.2 %). The time constants for the two reactions are 640±130 fs and 74±6 ps, respectively. Additionally, it is found that the transient dimethyl amine radical has a N?C bond length of 1.45±0.02 Å and a C?N?C bond angle of 118°±4°.     

Absorption spectra of e-beam-excited Ne, Ar, and Kr, pure and in binary mixtures

A technique using the broadband emission of a laser plume as probe radiation is applied to record UV-visible (190-510 nm) absorption spectra of Ne, Ar, and Kr, pure and in binary mixtures under moderate e-beam excitation up to 1?MW/cm(3). In all the rare gases and mixtures, the absorption spectra show continuum related to Rg(2) (+) homonuclear ions [peaking at λ～285, 295, and 320 nm in Ne, Ar, and Kr(Ar/Kr), respectively] and a number of atomic lines related mainly to Rg(?)(ms) levels, where m is the lowest principal quantum number of the valence electron. In argon, a continuum related to Ar(2) (?) (λ～325?nm) is also recorded. There are also trains of narrow bands corresponding to Rg(2) (?)(npπ?(3)Π(g))←Rg(2) (?)(msσ?(3)Σ(u) (+)) transitions. All the spectral features mentioned above were reported in literature but have never been observed simultaneously. Although charge transfer to a homonuclear ion of the heavier additive is commonly believed to dominate in binary rare-gas mixtures, it is found in this study that in Ne/Kr mixture, the charge is finally transferred from the buffer gas Ne(2) (+) ion not to Kr(2) (+) but to heteronuclear NeKr(+) ion.     

Consecutive Photoinduced Electron Transfer (conPET): The Mechanism of the Photocatalyst Rhodamine 6G

The dye rhodamine 6G can act as a photocatalyst through photoinduced electron transfer. After electronic excitation with green light, rhodamine 6G takes an electron from an electron donor, such as N,N-diisopropylethylamine, and forms the rhodamine 6G radical. This radical has a reduction potential of around −0.90 V and can split phenyl iodide into iodine anions and phenyl radicals. Recently, it has been reported that photoexcitation of the radical at 420 nm splits aryl bromides into bromide anions and aryl radicals. This requires an increase in reduction potential, hence the electronically excited rhodamine 6G radical was proposed as the reducing agent. Here, we present a study of the mechanism of the formation and photoreactions of the rhodamine 6G radical by transient absorption spectroscopy in the time range from femtoseconds to minutes in combination with quantum chemical calculations. We conclude that one photon of 540 nm light produces two rhodamine 6G radicals. The lifetime of the photoexcited radicals of around 350 fs is too short to allow diffusion-controlled interaction with a substrate. A fraction of the excited radicals ionize spontaneously, presumably producing solvated electrons. This decay produces hot rhodamine 6G and hot rhodamine 6G radicals, which cool with a time constant of around 10 ps. In the absence of a substrate, the ejected electrons recombine with rhodamine 6G and recover the radical on a timescale of nanoseconds. Photocatalytic reactions occur only upon excitation of the rhodamine 6G radical, and due to its short excited-state lifetime, the electron transfer to the substrate probably takes place through the generation of solvated electrons as an additional step in the proposed photochemical mechanism.     

Turn‐On Luminescent Probes for the Real‐Time Monitoring of Endogenous Hydroxyl Radicals in Living Cells

The utilization of semiconductor quantum dots (QDs) as optical labels for biosensing and biorecognition has made substantial progress. However, the development of a suitable QD‐based luminescent probe that is capable of detecting individual reactive oxygen species (ROS) represents a great challenge, mainly because the fluorescence of QDs is quenched by a wide variety of ROS. To overcome this limitation, a novel QD‐based turn‐on luminescent probe for the specific detection of ^.OH has been designed, and its application in monitoring the endogenous release of ^.OH species in living cells is demonstrated. Metal citrate complexes on the surfaces of the QDs can act as electron donors, injecting electrons into the LUMO of the QDs, while ^.OH can inject holes into the HOMO of the QDs. Accordingly, electron–hole pairs are produced, which could emit strong luminescence by electron–hole recombination. Importantly, this luminescent probe does not respond to other ROS.     

Direct observation of electronic relaxation dynamics in adenine via time-resolved photoelectron spectroscopy

We present femtosecond time-resolved photoelectron spectra of adenine in a molecular beam, recorded at pump wavelengths of 250, 267, and 277 nm. This leads to initial excitation of the bright S2(pipi*). Close to the band origin (277 nm), the lifetime is several picoseconds. Higher vibronic levels (267 and 250 nm excitation) show much shorter lifetimes of t < 50 fs, and we observe strong coupling between S2(pipi*) and S1(npi*). Rapid internal conversion (t < 50 fs) populates the lower lying S1(npi*) state which has a lifetime of 750 fs. At 267 nm, we found evidence for an additional channel which is consistent with the dissociative S3(pisigma*) state, previously proposed as an ultrafast relaxation pathway from S2(pipi*).     

Photoproduction and determination of hydroxyl radicals in aqueous solutions of Fe(III)–tartrate complexes: a quantitative assessment

Benzene has been used as a probe to determine the photogeneration of ^·OH radicals in aqueous solutions containing Fe(III) and tartrate (Tar) when they are irradiated (λ?≥?313?nm, 250?W). Products were determined by HPLC. Results show that tartrate has a positive effect on the photogeneration of ^·OH. The influence of pH, temperature, and concentrations of Fe(III) and Tar were systematically investigated. Production of ^·OH radicals was greatest at pH 3.0. Concentrations of ^·OH increased with increasing Fe(III) and tartrate concentrations, but at a certain ratio they decreased. The quantum yield of ^·OH radicals at 328?K was nearly twice that at 298?K.     

Strand breakage in poly(C), poly(A), single- and double-stranded DNA induced by nanosecond laser excitation at 193 nm.

Single- and double-stranded calf thymus DNA and two polynucleotides (0.4 mM) were studied in aqueous solution at pH approximately 7 using pulsed, 20 ns laser excitation at 193 nm. Monophotonic ionization of the nucleic acids is suggested from the linear dependences of the concentration of ejected electrons and the number of single- and double-strand breaks (ssb, dsb, respectively) on laser intensity (IL) in the range (0.2-3) x 10(6) W cm-2. The quantum yields of formation of hydrated electrons (phi e-) and ssb and dsb (phi ssb and phi dsb) are therefore independent of IL. In contrast, under 248 nm excitation these quantum yields increase linearly with IL under otherwise comparable conditions. Nevertheless, several effects and mechanistic implications are analogous using lambda exc = 193 and 248 nm. For polycytidylic acid, poly(C), in Ar-saturated solution for example, the efficiency of ssb per radical cation (eta RC = phi ssb/phi e-) is similar to the efficiency of ssb per OH radical (eta OH). For polyadenylic acid, poly(A), and single- and double-stranded DNA eta RC (lambda exc = 193 nm) is significantly smaller than eta OH. The ratio phi ssb (N2O)/phi ssb (Ar) is approximately 2 for poly(C), approximately 4 for poly(A) approximately 10 for DNA; the conversion of hydrated electrons into OH radicals in N2O-saturated solution and smaller eta RC than eta OH values in the case of DNA account for these results. For double-stranded DNA phi dsb does not depend on IL but increases linearly with the dose, indicating an accumulative effect of two ssb to generate one dsb. The critical distance for this event is 60-85 phosphoric acid diester bonds.