Electron-cation Recombination Research Articles

Ultra-fast electron capture by electrosterically-stabilized gold nanoparticles.

Ultra-fast pre-solvated electron capture has been observed for aqueous solutions of room-temperature ionic liquid (RTIL) surface-stabilized gold nanoparticles (AuNPs; ∼9 nm). The extraordinarily large inverse temperature dependent rate constants (k(e)∼ 5 × 10(14) M(-1) s(-1)) measured for the capture of electrons in solution suggest electron capture by the AuNP surface that is on the timescale of, and therefore in competition with, electron solvation and electron-cation recombination reactions. The observed electron transfer rates challenge the conventional notion that radiation induced biological damage would be enhanced in the presence of AuNPs. On the contrary, AuNPs stabilized by non-covalently bonded ligands demonstrate the potential to quench radiation-induced electrons, indicating potential applications in fields ranging from radiation therapy to heterogeneous catalysis.

Electron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO2 in the absence and presence of solvent

We investigated photoinduced primary charge transfer processes of the sensitizer E6 on TiO2 without solvent and in contact with the organic solvent acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate [C2mim](+)[B(CN)4](-) using transient absorption spectroscopy, spectroelectrochemistry, and DFT/TDDFT calculations. E6, which belongs to a family of triarylamine dyes for solar cell applications, features two peripheral triarylamine units which are connected via diether spacer groups to the core chromophore and are designed to act as hole traps. This function was confirmed by spectroelectrochemistry, where the E6˙(+) radical cation shows a considerably blue-shifted absorption compared to dyes without these two substituents. This indicates that one of the terminal triarylamine units must carry the positive charge. After photoexcitation of E6 at 520 nm (S0 → S1 band), electrons are injected into TiO2 predominantly within the cross-correlation time (<80 fs), with some subsequent delayed electron injection (τ ca. 250 fs). Importantly, a transient Stark shift (electrochromism) is observed (time constants ca. 0.8 and 12 ps) which is related to a changing electric field generated by the E6˙(+) radical cations and injected electrons. This field induces absorption shifts of the dye species on the surface. Interestingly, these dynamics are largely unaffected by solvent molecules. However, pronounced differences are observed on longer timescales. In contact with solvent, one observes an increase in the E6˙(+) absorption band above 600 nm with a time constant of 75 ps. This is assigned to hole transfer from the core chromophore to one of the peripheral triarylamine substituents. Electron-cation recombination occurs on much longer timescales and is multiexponential, with time constants of ca. 100 μs, 1 ms and 15 ms. Because of hole trapping, it is slower than for similar dyes lacking the peripheral triarylamines. Additional experiments were performed for E6 attached to the wide band gap semiconductor ZrO2. Here, electron injection occurs into surface trap states with subsequent recombination. Another fraction of non-injecting E6 molecules in S1 quickly decays to S0 (time constants 1 and 35 ps).

Arrhenius behavior of electron attachment to CH3Br from 303 to 1100 K

Thermal electron attachment to CH3Br has been studied over the temperature range 303–1100K using two flowing-afterglow Langmuir-probe apparatuses. The reaction yielded only Br− product over this temperature range. The rate coefficient for electron attachment to CH3Br was measured to be 8±4×10−12cm3s−1 at 303K, and was observed to increase strongly with gas temperature. Rate coefficients for the reaction show Arrhenius behavior over the entire temperature range with an activation energy of 260±20meV. The results are in substantial agreement with earlier data covering a smaller temperature range. Kinetic modeling implies that this behavior and the small rate coefficient at room temperature are due to a barrier in the crossing from the neutral to the anionic potential surfaces of ∼280meV that dominates other factors in the attachment reaction. There is a hint of the Arrhenius plot reaching saturation at the highest temperatures. While examining an electron-cation recombination correction, the rate coefficient (1.8±0.4×10−9cm3s−1) of the reaction Ar++CH3Br was measured at 302K, and the ion products identified (80% CH3+ and 20% CH2Br+). A secondary reaction forming the adduct (CH3Br)CH3+ was seen to occur with a rate coefficient of 2.8±1.0×10−9cm3s−1.

Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile

The relaxation dynamics of the dye D35 has been characterized by transient absorption spectroscopy in acetonitrile and on TiO(2) and ZrO(2) thin films. In acetonitrile, upon photoexcitation of the dye via the S(0) → S(1) transition, we observed ultrafast solvation dynamics with subpicosecond time constants. Subsequent decay of the S(1) excited state absorption (ESA) band with a 7.1 ps time constant is tentatively assigned to structural relaxation in the excited state, and a spectral decay with 203 ps time constant results from internal conversion (IC) back to S(0). On TiO(2), we observed fast (<90 fs) electron injection from the S(1) state of D35 into the TiO(2) conduction band, followed by a biphasic dynamics arising from changes in a transient Stark field at the interface, with time constants of 0.8 and 12 ps, resulting in a characteristic blue-shift of the S(0) → S(1) absorption band. Several processes can contribute to this spectral shift: (i) photoexcitation induces immediate formation of D35˙(+) radical cations, which initially form electron-cation complexes; (ii) dissociation of these complexes generates mobile electrons, and when they start diffusing in the mesoporous TiO(2), the local electrostatic field may change; (iii) this may trigger the reorientation of D35 molecules in the changing electric field. A slower spectral decay on a nanosecond timescale is interpreted as a reduction of the local Stark field, as mobile electrons move deeper into TiO(2) and are progressively screened. Multiexponential electron-cation recombination occurs on much longer timescales, with time constants of 30 μs, 170 μs and 1.4 ms. For D35 adsorbed on ZrO(2), there is no clear evidence for a transient Stark shift, which suggests that initially formed cation-electron (trap state) complexes do not dissociate to form mobile conduction band electrons. Multiexponential decay with time constants of 4, 35, and 550 ps is assigned to recombination between cations and trapped electrons, and also to a fraction of D35 molecules in S(1) which decay by IC to S(0). Differential steady-state absorption spectra of D35˙(+) in acetonitrile and dichloromethane provide access to the complete D(0) → D(1) band. The absorption spectra of D35 and D35˙(+) are well described by TDDFT calculations employing the MPW1K functional.

RESPONSE OF ALANINE AND BARIUM DITHIONATE EPR DOSIMETERS

The response of alanine and barium dithionate EPR dosimeters to proton irradiation with energies ranging from 6.6-25 MeV has been investigated. EPR dosimeters were calibrated using calibrated gamma sources. Alanine dosimeters show a value 29% higher than those obtained by a Markus chamber at the same energy, and barium dithionate shows a value 22% smaller. The response of the EPR dosimeters to irradiation at a mean dose of about 40 Gy depends on the proton energy. Using experimental data, the yield of the radicals in the tracks for the alanine pellets was calculated. The yield of the radicals was determined to be proportional to the linear energy transfer (LET) on the straight-line length of the proton track, and the proportional coefficient for alanine is equal to 0.109 eV-1. In the area of the Bragg peak, the probability of recombination of the ionized electrons with cations is increased. As a result, approximately 4.6 MeV of proton energy is used for ionization that results in electron-cation recombination instead of formation of radicals, and maximum LET does not coincide with the maximum concentration of the radicals.

Probing Single-Molecule Interfacial Geminate Electron−Cation Recombination Dynamics

Interfacial electron-cation recombination in zinc-tetra (4-carboxyphenyl) porphyrin (ZnTCPP)/TiO(2) nanoparticle system has been probed at the single-molecule level by recording and analyzing photon-to-photon pair times of the ZnTCPP fluorescence. We have developed a novel approach to reveal the hidden single-molecule interfacial electron-cation recombination dynamics by analyzing the autocorrelation function and a proposed convoluted single-molecule interfacial electron-cation recombination model. Our results suggest that the fluctuations of the interfacial electron transfer (ET) reactivity modulate the ET cycles as well as the interfacial electron-cation recombination dynamics. On the basis of this model, the single-molecule electron-cation recombination time of ZnTCPP/TiO(2) system is deduced to be at time scale of 10(-5) s. The autocorrelation of photon-to-photon pair times as well as the convoluted ET model has been further demonstrated by simulation and interpreted in terms of the interfacial ET reactivity fluctuation and blinking. Our approach not only can effectively probe the single-molecule interfacial electron-cation dynamics but also can be applied to other single-molecule ground-state regeneration dynamics occurring at interfaces and within condensed phases.

Three-photon near-threshold photoionization dynamics of isooctane

The electron survival probability following three-photon (9.3 eV total) near-threshold photoionization of neat isooctane is measured with sub-50 fs time resolution. The measured dynamics are nonexponential in time and are well described by a diffusion-controlled electron-cation recombination model. Excitation-power-dependent studies indicate that the unperturbed three-photon threshold ionization is only observed for pump irradiance below 0.5 TW cm2. At excitation fields above this level, the signal is no longer cubic in the excitation irradiance, and the observed electron survival probability dramatically changes, decaying as a single exponential in time.

Radiolysis of silver ion solutions in ethylene glycol: solvated electron and radical scavenging yields

The solvated electron yield in neat ethylene glycol (1,2-ethanediol), measured using pulse radiolysis, is G ( e solv - ) 10 ns =(1.7±0.2)×10 −7 mol J −1. The rate constant of the reaction of solvated electrons with silver cations is k ( e solv - + Ag + ) =(2.8±0.1)×10 9 dm 3 mol −1 s −1 and the absorption band maximum of Ag 0 is at 350 nm. The surface plasmon band of the silver clusters appears slowly at around 400 nm with a coalescence cascade rate constant of 2×10 6 dm 3 mol −1 s −1. The free silver ions do not scavenge ethylene glycol radicals. In contrast, the γ-radiolysis reduction yield of Ag + into clusters is dose-dependent, changing from G i(Ag n )=(2.5±0.5)×10 −7 mol J −1 at low dose to G max(Ag n )=(7.5±0.5)×10 −7 mol J −1 at higher doses, when clusters accumulate. Silver cations adsorbed on clusters are able to scavenge the ethylene glycol radicals, which also contribute to their reduction for a part of G rad=(5.8±0.5)×10 −7 mol J −1. Considering the different ways the reducing radicals can be produced, it is concluded that they originate from the electron–cation recombination and from the cation–ethylene glycol reaction, but that the dissociation of excited states is a negligible path. The formation mechanisms of the radicals and the rate constants in ethylene glycol are compared with those in water and methanol.

Radiolysis of liquid water: An attempt to reconcile Monte-Carlo calculations with new experimental hydrated electron yield data at early times

A re-examination of our Monte-Carlo modeling of the radiolysis of liquid water by low linear-energy-transfer (LET ~ 0.3 keV µm–1) radiation is undertaken herein in an attempt to reconcile the results of our simulation code with recently revised experimental hydrated electron (e–aq) yield data at early times. The thermalization distance of subexcitation electrons, the recombination cross section of the electrons with their water parent cations prior to thermalization, and the branching ratios of the different competing mechanisms in the dissociative decay of vibrationally excited states of water molecules were taken as adjustable parameters in our simulations. Using a global-fit procedure, we have been unable to find a set of values for those parameters to simultaneously reproduce (i) the revised e–aq yield of 4.0 ± 0.2 molecules per 100 eV at "time zero" (that is, a reduction of ~20% over the hitherto accepted value of 4.8 molecules per 100 eV), (ii) the newly measured e–aq decay kinetic profile from 100 ps to 10 ns, and (iii) the time-dependent yields of the other radiolytic species H•, •OH, H2, and H2O2 (up to ~1 µs). The lowest possible limiting "time-zero" yield of e–aq that we could in fact obtain, while ensuring an acceptable agreement between all computed and experimental yields, was ~4.4 to 4.5 molecules per 100 eV. Under these conditions, the mean values of the electron thermalization distance and of the geminate electron–cation recombination probability, averaged over the subexcitation electron "entry spectrum," are found to be equal to ~139 Å and ~18%, respectively. These values are to be compared with those obtained in our previous simulations of liquid water radiolysis, namely ~88 Å and ~5.5%, respectively. Our average electron thermalization distance is also to be compared with the typical size (~64–80 Å) of the initial hydrated electron distributions estimated in current deterministic models of "spur" chemistry. Finally, our average probability for geminate electron–cation recombination agrees well with an estimated value of ~15% recently reported in the literature. In conclusion, this work shows that an adaptation of our calculations to a lower hydrated electron yield at early times is possible, but also suggests that the topic is not closed. Further measurements of the e–aq yields at very short times are needed. Key words: liquid water, radiolysis, electron–cation geminate recombination, electron thermalization distance, hydrated electron (e–aq), e–aq decay kinetics, time-dependent molecular and radical yields, Monte-Carlo simulations.

On the temperature dependence of the primary yield and the product Gεmax of hydrated electrons in the low-LET radiolysis of liquid water

Monte-Carlo simulations are performed to calculate the temperature dependence of the primary hydrated electron yield (Geaq-) for liquid water irradiated by low linear-energy-transfer radiation (LET ~ 0.3 keV µm–1) in the range 25–325°C. Calculations are carried out by taking properly into account the effect of the time and temperature dependencies of the water dielectric constant on the electron–cation geminate recombination. Our computed Geaq- values slightly increase with increasing temperature, in good agreement with experiment. The product Geaq- εmax(eaq-), estimated by using existing experimental data of the maximum molar extinction coefficient εmax(eaq-), remains nearly constant or slightly increases, depending on the temperature dependence chosen for εmax. Our Geaq-εmax(eaq-) values compare generally well with most experimental data, as well as with the predictions of deterministic diffusion-kinetic model calculations. Moreover, our results indicate that the static dielectric constant of water (εs) does not play any significant role on the electron–cation recombination at early times. Such a finding is inconsistent with the interpretation, proposed by certain authors in the literature, that Geaq- should in fact decrease as temperature is increased because of an increased electron–cation geminate recombination due to a lowering of εs. Finally, the temperature dependence of the hydrated electron yields, calculated at various times between 10 ps and 1 µs, shows that at low LET, the time required to establish homogeneous chemistry in the bulk of the solution is ~10–6 s in the range ~25–100°C, and that this time diminishes to ~10–7 s at higher temperatures. Key words: liquid water, radiolysis, temperature, hydrated electron (eaq-), radiolytic yields, electron–cation geminate recombination, dielectric constant, molar extinction coefficient of eaq-, homogenization time.

Magnetic field effects on TMPD cation–electron recombination fluorescence in saturated hydrocarbons

The hyperfine induced magnetic field effect on N, N, N ′, N ′-tetramethylparaphenylenediamine (TMPD) cation-geminate electron recombination fluorescence is reported as a function of temperature in squalane. Magnetic field effects were additionally observed over some limited temperature range in the liquid, supercooled liquid and solid states of some smaller saturated hydrocarbons. The temperature dependence of the magnetic field effect intensity in squalane is quantitatively analyzed in terms of the fraction of recombinations occurring after spin mixing but before spin relaxation. Numerical analysis of the recombination time distribution exposes the importance of both diffusion and tunneling in the Coulomb field.

Kinetic study of the forward and backward electron transfer between N, N, N′, N′-tetramethylbenzidine and CH 2Cl 2 in solid argon leading to a photoequilibrium

The kinetics of the two-photon ionization of N, N, N ′, N ′-tetramethylbenzidine (TMB) in dichloromethane (CH 2Cl 2)-doped solid argon using 254 nm high-pressure Hg-lamp radiation is investigated by means of UV/VIS absorption spectroscopy. The experimental findings, especially the incomplete running of the ionization reaction, can be quantitatively understood on the basis of a kinetic model which includes dissociative electron attachment to CH 2Cl 2 and associative photodetachment of the corresponding radical anion. The model implies that the underlying electron transfer reactions reach the photoequilibrium TMB+ CH 2 Cl 2 ⇌ hν 2hν TMB •++ ClHCH •⋯ Cl − . The ionization cross-section of TMB in the lowest triplet state, the cross-section of associative photodetachment and the rate constant of cation electron recombination are obtained by adapting calculated to measured irradiation time dependent concentrations of the radical cation.

Kinetics of the two-photon ionization of N, N, N′, N′-tetramethylbenzidine in CH 2Cl 2-doped solid argon

The kinetics of the two-photon ionization (TPI) of N, N, N ′, N ′-tetramethylbenzidine (TMB) in CH 2Cl 2-doped solid argon using 313 nm Hg lamp radiation is investigated by means of ultraviolet/visible (UV/VIS) absorption spectroscopy. The experimental findings can be quantitatively understood on the basis of a rate equation model. The ionization cross-section of TMB in the lowest triplet state and the rate constant of cation–electron recombination are obtained by adapting calculated to measured irradiation time-dependent concentrations of the cation.

Simulation of muonium formation in liquid hydrocarbons

Muonium formation in liquid hexane is examined by computer simulation. In track-end competition between muonium formation and cation–electron recombination, the muon is found to react with electrons from a significant part of the track end, corresponding to an energy attenuation of several tens of keV and a length of several microns. This muonium formation extends to microseconds following muon implantation. Delayed muonium formation leads to a much smaller amplitude of the muonium asymmetry than for prompt muonium formation during slowing down of the muon, and in this way may account for the missing polarization in transverse magnetic field experiments. If reaction of muons with electrons from their radiolysis tracks contributes to the experimentally observed muonium yield, the muon must thermalize between 60 and 150 nm from the last ionization of the track to reproduce the amplitudes of the muon and muonium asymmetries. For the smallest distance, 60 nm, the experimentally observed muonium asymmetry results from delayed muonium only. As the muon thermalization distance increases, prompt muonium formation also contributes, so that at 150 nm the observed asymmetry is almost entirely due to prompt muonium formation.

Near-infrared excitation of alkane ultra-violet fluorescence

We report the generation of ultra-violet fluorescence from linear and cyclic saturated hydrocarbons in neat solution when excited with femtosecond near-infrared laser pulses at 800 nm. Fluorescence decay, spectra and power dependence studies are reported. Steady-state and time-resolved experiments are in agreement with previously reported data obtained with one-photon vacuum ultra-violet, X-ray, electron impact and two-photon excitation. Our results indicate that the mechanisms leading to the fluorescing excited state are multi-photon absorption and ionisation with the latter followed by diffusion-controlled geminate cation–electron recombination.

Computer simulation studies of recombination of ions in multi-ion-pair ensembles

The problem of the diffusion-controlled recombination of ions for the case where initially two or more nonseparable pairs of oppositely charged ions are present in the system is treated by means of computer simulation. In the first stage, the calculations for the media with the short mean free paths of the free movement of ions between scattering events were performed (diffusion model). The results were compared with the results of the calculations of the electron-cation recombination in the systems characterized by long mean free path of electron between the scattering events. The scale of the deviations of the kinetics of the recombination process in the multi-pair clusters from the kinetics for the isolated pairs was estimated for both the diffusion case and the long mean free path case. The deviations of the multi-pair kinetics and escape probability from the corresponding single-pair results are significant.

Computer simulation studies of recombination of ions in multi ion-pair ensembles—II. Processes characterized by long mean free paths of charged species

The problem of the recombination of charged species for the case in which initially one or more non-separable pairs of oppositely charged ions are present in the system is treated by means of computer simulation method. The calculations of the electron–cation recombination in the systems characterized by long mean free path (MFP) of an electron between the scattering events were performed. The calculations of the recombination kinetics and escape probability for single pairs were compared with the Debye–Smoluchowski–Onsager theory. It was found that for large MFPs, of above 10–20% of the Onsager distance in the medium, the theory based on the diffusion model is no longer valid, even for isolated ion pairs. The multi-pair effects were investigated by simulation of the electron–cation recombination in clusters composed of up to ten pairs. The deviations of the multi-pair kinetics and escape probability from the corresponding single-pair results are significant. In a series of preliminary calculations it was found that, in contrast to the processes characterized by short MFPs, the mechanism of the energy and momentum transfer during the electron scattering process influences very significantly both the recombination kinetics and the escape probability.

A subpicosecond pump-probe laser study of ionization and geminate charge recombination kinetics in alkane liquids

We report on time resolved optical pump-probe investigations of the electron-cation recombination in alkane liquids. The alkanes were excited via two-photon excitation, using intense sub-picosecond pulses at 266 nm. Adding an electron scavenger to the liquid and probing the transient absorption at three different wavelengths (800 nm, 1500 nm, and 2250 nm) made it possible to distinguish the transient absorption due to excess electrons from that due to other absorbing species. The experimentally observed charge-recombination kinetics in n-hexane and n-octane could be fairly well reproduced by computer simulations in which the initial electron thermalization distance distribution was taken to be f(r)r2dr=(1/b)exp(−r/b)dr. Other distributions, such as a Gaussian, gave unsatisfactory results. The average electron thermalization distance in n-hexane was found to be 35±5 Å and in n-octane it was found to be 70±10 Å. The results for isooctane could be described either by the distribution (1/b)exp(−r/b)dr with an average thermalization distance of 25±5 Å or by a Gaussian distribution with an average thermalization distance 50±10 Å. The transient absorption in n-hexane and isooctane was observed to exhibit a red shift during the first 2 ps after the onset of the absorption. This spectral relaxation could be due to the slowing down of the ejected electron to thermal energies.

Dual sites of solvation for the electrons and cation-electron recombination observed in the radiolysis of Triton X-100 and water mixtures: homogeneous and liquid crystalline regions

Pulse radiolysis studies were carried out on mixtures of a nonionic surfactant Triton X-100 (Tx) and water. In the range 40–70% Tx, two transient absorption peaks at 630 and 720 nm were observed due to the solvation of radiolytically generated excess electrons in the palisade layer of the surfactant pseudophase and in water. Transient absorption due to the cation radical and triplet state of Tx was also observed. On shorter timescales (∼ 300 ns) a strong emission with λ max=305 nm due to the singlet state of Tx was also seen, generated by recombination of the cation radical and electrons. Scavenging studies by nitrate ions and pyrene demonstrated a preferential picking up of electrons from aqueous and surfactant pseudophases, respectively.

Picosecond transient absorption spectral studies on geminate electron—cation recombination in non-polar solvents: direct observation of the S 1 state formation

The geminate electron—cation recombination process has been measured directly for N,N,N′,N′-tetramethyl- p-phenylenediamine in n-hexane at several temperatures by using picosecond transient absorption spectroscopic techniques. The yield of the fluorescence state via the geminate recombination has been estimated to be about 0.2.