Transient spectrum from nanosecond pulse radiolysis of liquid ammonia

In addition to the absorption of the ammoniated electron, the transient absorption spectrum of irradiated liquid ammonia contains two ultraviolet bands at 250 nm and 320 nm. The latter is shown to be due neither to the NH₂^? ion nor to NH singlet. There is conflicting evidence concerning its possible assignment to NH triplet. The C value of the ammoniated electron has been found to be 3.0 at ?48°C. At ?45°C, the ammoniated electron decays with concurrent first and second order kinetics and it reacts with Cu²⁺ with the rate constant 1.5 × 10¹¹$\text{l}$ mole^?1 sec^?1.     

Some recent radiolysis works by means of a new picosecond pulse radiolysis system “Twin LINAC System”

A new picosecond pulse radiolysis Twin LINAC System has been constructed recently at Nuclear Engineering Research Laboratory at the University of Tokyo. The time resolution of the system is 20 ps and the system has a versatile capability for pulse radiolysis study. The outline of the facility is introduced. Some research carried out using the facility is presented. The examples are radiolysis of CCl₄ and n-C₂₁H₄₄.     

Microsecond pulse radiolysis of benzophenone tributyl phosphate system

It was found that an absorption peak with λmax = 560 nm in the pulse radiolysis of benzophenone-TBP system, which was assigned to the ketyl radical of benzophenone. The yields for the ketyl radical on benzophenone concentrations have been measured, which were well fitted with the Stern-Volmer equation. Analysis of the data, combining with the previous study, an energy transfer mechanism was proposed, the obtained GTBP*0 was 2.54 and the ratio of rate constants for TBP* dissociation to energy transfer was 840.     

Formation of an intermediate radical cation in the nanosecond pulse radiolysis of Malachite Green Leucocyanide in organic solvents

The malachite green leucocyanide (MGCN) was irradiated in argon or oxygen saturated solutions of n-butyl chloride, 1.2-DCE, CCl₄ and acetone with 13 ns electron pulses. Two species with absorption maxima at 620 and 480 nm were observed. The latter was attributed to the malachite green leucocyanide radical cation (MGCN^+.) and the former to the known carbonium ion of malachite green dye (MG⁺). Observation of the consecutive charge transfer via the schemes: DCE^+. → BPh^+. → MGCN^+. and DCE^+. → MGCN^+. → TMPD^+., allowed to estimate the ionization potential of MGCN molecule in the range 6.9 eV < Ip_MGCN < 8.27 eV. Presented results and literature data suggest that positive charge in MGCN^+. radical cation is located in the “aniline” part of the molecule.     

Performance of compact pulse radiolysis system using a photocathode RF gun

A compact pulse radiolysis system using a photocathode RF gun was installed at Sumitomo Heavy Industries. Some performance tests were conducted concerning the electron beam and the laser pulse. The energy and the per-pulse charge of the electron beam were measured to be 1.75 MeV max. and 1 nC. The fluctuation of the charge was restricted within 2%. The pulse widths of the electron pulse and the analyzing laser pulse were 20 ps and 15 ps, respectively. The timing jitter between electron pulse and the laser pulse was ±5.7 ps. Based on these measurements, the all-over time resolution can deduced to be about 25 ps.     

The ultra-fast process of picosecond time-resolved energy transfer in liquid cyclohexane by picosecond single-pulse radiolysis

Two formation processes of singlet states of solute molecules in irradiated liquid cyclohexane have been observed by using a single picosecond electron pulse-radiolysis technique. The slower process corresponds to the very fast formation process reported by Beck et al. The faster process is an ultra-fast process which finishes immediately after a picosecond electron pulse, and is hardly quenched by CCl₄ on triethylamine. The ratio of the faster component to the slower component in liquid cyclohexane is very different from that in liquid toluene.     

Pico-second pulse radiolysis of cyclohexane solutions

A rapid (t < 500 psec) transfer of singlet energy is observed in the picosecond pulse radiolysis of dilute (10⁻³–10⁻² M) solutions of aromatic solutes in cyclohexane. This event is not associated with any ion neutralization reaction, but is a direct transfer of energy from an excited state of cyclohexane (t_e < 500 psec) to the solute.     

Concomitant transient electrochemical and spectroscopic detection with electron pulse radiolysis

The possibility to couple a transient electrochemical detection of the redox intermediates produced by a picosecond electron accelerator is explored. The principle is demonstrated with the well-behaved methylviologen radical cation that can be reoxidised at the electrode and simultaneously detected by transient absorption.     

Photoenolization of 2-methylbenzophenone studied by picosecond and nanosecond laser spectroscopy

Picosecond and nanosecond spectroscopy methods were used to investigate the photoenolization of 2-methylbenzophenone in ethanol at room temperature. Picosecond laser photolysis revealed a T_n ← T₁ absorption spectrum with λ_max = 520 nm and a decay time of 2.8 ns. Nanosecond laser photolysis revealed two types of transient absorption: one was observed near 400 nm and decayed with two components with lifetimes of 8.1 μs and 8 ms which were assigned to cis-dienols and trans-dienols respectively; the other was observed near 530 nm and had a lifetime of 26 ns. Since this value is almost equal to the rise time of the absorptions of the cis- and/or trans-dienols, it is reasonable to assume that the transient absorption around 530 nm is due to the triplet states of either the dienols or the 1,4-biradicals.     

Reactivity between biphenyl and precursor of solvated electrons in tetrahydrofuran measured by picosecond pulse radiolysis in near-ultraviolet, visible, and infrared

The initial decrease of solvated electrons in tetrahydrofuran (THF) upon addition of biphenyl was investigated by picosecond pulse radiolysis. Transient absorption spectra derived from the biphenyl radical anion (centered at 408 and 655 nm) and solvated electrons of THF (infrared) were successfully measured in the wavelength region from 400 to 900 nm by the extension of a femtosecond continuum probe light to near-ultraviolet using a second harmonic generation of Ti:sapphire laser and a CaF2 plate. From the analysis of kinetic traces at 1300 nm considering the overlap of primary solvated electrons and partial biphenyl radical anion, C37, which is defined by the solute concentration to reduce the initial yield of solvated electrons to 1/e, was found to be 87 +/- 3 mM. The rate constant of solvated electrons with biphenyl was determined as 5.8 +/- 0.3 x 10(10) M(-1) s(-1). We demonstrate that the kinetic traces at both 408 nm mainly due to biphenyl radical anion and 1300 nm mainly due to solvated electrons are reproduced with high accuracy and consistency by a simple kinetic analysis. Much higher concentrations of biphenyl (up to 2 M) were examined, showing further increase of the initial yield of biphenyl radical anion accompanying a fast decay component. This observation is discussed in terms of geminate ion recombination, scavenging, delayed geminate ion recombination, and direct ionization of biphenyl at high concentration.     

Transient species of fullerenes studied with pulse radiolysis and laser photolysis

The excited triplet of C60 or C70 is generated via either direct excitation by laser light or energy transfer from excited states of solvent to C60 and C70. The cation radical of C60 is produced either via hole transfer from cation radical of CCl4 to C60 or via electron transfer from excited triplet of C60 to CCl4. C60 and C70 could be added to trichloromethyl radical to produce adduct radicals with different mechanisms.     

Calibration Measurements in laser-induced breakdown spectroscopy using nanosecond and picosecond lasers

The influence of laser pulse duration on laser-induced breakdown spectroscopy (LIBS) calibration curves is investigated in the present work. Two Nd:YAG lasers providing pulses of 35 ps and 5 ns, respectively, both operating at 1064 nm, have been used to create plasmas on aluminium, manganese, iron, and silicon targets and on prepared stoichiometric samples of these metals in a matrix. The time-resolved, space-averaged plasma temperatures have been deduced using Boltzmann plots, while the electron number density has been determined from the broadening of spectral lines. The effect of laser pulse duration on the plasma characteristics is discussed, and comparisons are made with previously reported data measured under similar experimental conditions. The optimum experimental conditions (i.e., time delay, gate width, laser energy) have been determined for reliable use of LIBS for quantitative analysis for both pulse durations. For each of the metals of interest, calibration curves have been constructed for concentrations ranging up to 2%.     

Distance dependence of the reaction rate for the reduction of metal cations by solvated electrons: a picosecond pulse radiolysis study

The decay of the solvated electron generated by picosecond electron pulse radiolysis is studied by broad-band transient absorption measurements in ethylene glycol solutions containing decimolar concentrations of Cu(2+), Ni(2+), and Pb(2+) metal cations. Analysis of the nonexponential kinetics of the decays reveals molecular parameters of the electron transfer reaction. It is found that the reaction occurs at long distance for Cu(2+) solutions and is only limited to contact distance in the case of Ni(2+) solutions. The distribution of reaction distance strongly depends on the free enthalpy change of the reactions.     

Gamma and pulse radiolysis of methanolic solutions of triphenylphosphine

In irradiated methanolic solutions of triphenylphosphine (Ph₃P) deaerated with argon or saturated with N₂O a transient spectrum has been detected. This spectrum is attributed to Ph₃PCH₂OH and a value of k_(CH2OH+Ph3P) = 8.8 x 10⁹mol⁻¹dm³S⁻ has been detemined. No evidence has been obtained for a reaction between e_sol⁻¹ and Ph₃P in methanol.The gamma-irradiation of oxygen-saturated methanolic solutions of Ph₃P have shown that, under these conditions, Ph₃P is oxidized to Ph₃PO according to a chain reaction mechanism. The experimental results are in good agreement with the equation for G(-Ph₃P) = G(Ph₃PO) obtained by steady-state treatment of the proposed mechanism.     

γ-radiolysis and pulse radiolysis of aqueous 4-chloroanisole

Using pulse radiolysis and steady state γ-radiolysis in combination with product analysis by HPLC the radiolytic degradation mechanism of 4-chloroanisole (4-ClAn) has been elucidated. Pulse radiolysis experiments show that OH radicals react in neutral aqueous N₂O saturated solutions with 4-ClAn by addition to all aromatic ring positions to yield hydroxycyclohexadienyl radicals (OH-adducts), k(OH + 4-ClAn) = 6.5 × 10⁹dm³mol⁻¹s⁻¹. Those OH-adducts formed on ipso positions of the molecule subsequently undergo HCl or CH₃OH elimination forming methoxyphenoxyl- and chlorophenoxyl radicals. Their yield corresponds to 20% of the OH-radicals, whereby the distribution is roughly 3:1 in favor of the methoxyphenoxyl radicals, which reflects the stronger ortho-, para-directing activity of the methoxy group. The OH-adducts decay second order, 2k = 1 × 10⁹dm³mol⁻¹s⁻¹. The presence of oxygen leads to its addition on the hydroxycyclohexadienyl radicals, k(OH-adduct + O₂) = 3.2 × 10⁸dm³mol⁻¹s⁻¹. In airfree solution the reaction of H-atom with the substrate, k(H + 4-ClAn) = 1.2 × 10⁹dm³mol⁻¹s⁻¹, results in H-adducts which decay in bimolecular reactions, 2k = 8.2 × 10⁸dm³mol⁻¹s⁻¹. The rate constant for the reaction of the solvated electrons has been determined to k(e_aq⁻ + 4-ClAn) = 2 × 10⁹dm³mol⁻¹s⁻¹. The absorption spectra of H- and OH-adducts were measured in the range of 280–450 nm. The products analysed by HPLC after γ-radiolysis in dependence of dose (100–600 Gy) are given for N₂O-, air-, oxygen- and argon saturated neutral aqueous solutions. In conditions favoring the OH radical oxidation 4-chlorophenol, 4-methoxyphenol, 5-chloro-2-methoxyphenol and 2-chloro-5-methoxyphenol were determined as final products. In the presence of Ar, where about equal amounts of OH and e_aq⁻ are present, additionally anisole could be detected. Under both reaction conditions the amount of identified products is about 20% of decomposed 4-ClAn. The reaction of e_aq⁻ leads to reductive dechlorination which corresponds quantitatively to the degradation of the substrate. In the presence of air or solutions saturated with pure oxygen predominantly hydroquinone, 4-chlorophenol and muconic acids are formed and the material balance is 50%. The efficient dechlorination (60% of the decomposed 4-ClAn) as well as ring fragmentation products as intermediates en route to complete mineralization in oxygenated solution indicate that high energy radiation is a promising method for degradation of halogenated aromatic compounds in water. Variation of dose rates from 79 Gy min⁻¹ to 266 Gy min⁻¹ did not show any influence on the product distribution.     

Intersystem crossing in ir and rh complexes studied by nanosecond and picosecond spectroscopy

Absorption spectra of the lowest triplet state were measured for cis-[IrCl₂(phen)₂ ]Cl (A), cis-[RhCl₂(bpy)₂] Cl (B), and cis-[RhBi₂(bpy)₂] Br (C) in ethanol—methanol (vlv = 4: 1) at 19°C. An S_n « S₁ absorption spectrum was also measured for the Ir complex. The time constants for T₁ formation are 26 ± 10, ≈2.4, and ≈1.3 ps for A, B, and C, respectively.     

Gas phase pulse radiolysis studies of excited states

The results and conclusions of the work of the author and co-workers on excited states proudced by pulse radiolysis of gaseous systems are summarized. Detailed consideration is given to systems containing naphthalene, for which the following subjects are discussed: (1) ionic and non-ionic mechanisms of excited state formation and their relative importance, (2) G-values of excited state formation and (3) energy partitioning in the non-ionic formation of excited states of naphthalene. Rate constants for reactions of excited states of naphthalene are summarized. The use of emission from excited states produced in ion-recombination reactions to determine rate constants for ion-recombination is also discussed.     

Folding dynamics of cytochrome C using pulse radiolysis

Pulse radiolysis is a powerful method to realize real-time observation of various redox processes, which induces various structural and functional changes occurring in biological systems. However, its application has been mainly limited to studies of the redox reactions of rather smaller biological systems such as DNA because of an undesired reaction due to various free radicals generated by pulse radiolysis. For application of pulse radiolysis to generate plenty of redox reactions of biological systems, selective redox reactions induced by electron pulses have to be developed. In this study, we report that in the presence of the high concentration of the denaturant, guanidine HCl (GdHCl), the selective reduction of the oxidized cytochrome c (Cyt c) takes place in time scales of a few microseconds by the electron transfer from the guanidine radical that is formed by the fast reaction of e(aq)(-) with GdHCl, consequently leading to folding kinetics of Cyt c. By providing insight into the folding dynamics of Cyt c, we show that the pulse radiolysis technique can be used to track the folding dynamics of various biomolecules in the presence of a denaturant including GdHCl.