Mingzhang Lin

Ultra-fast pulse radiolysis system combined with a laser photocathode RF gun and a femtosecond laser

Abstract In order to study the early events in radiation physics and chemistry, two kinds of new pulse radiolysis systems with higher time resolution based on pump-and-probe method have been developed at the Nuclear Engineering Research Laboratory, the University of Tokyo. The first one, a few picosecond (2xa0ps at FWHM) electron beam (pump) from an 18xa0MeV S-band Linac using a laser photocathode RF gun (BNL/KEK/SHI type: GUN IV) was operated with a femtosecond laser pulse (100xa0fs at FWHM), which also acted as the analyzing light (probe). The synchronization precision between the pump and the probe was 1.7xa0ps (rms). In a 1.0xa0cm sample cell, a time resolution of 12xa0ps was achieved. The second one, a picosecond (4xa0ps at FWHM) electron pulse from a 35xa0MeV S-band Linac employing a conventional thermionic gun with a sub-harmonic buncher, was synchronized with the femtosecond laser pulse, with a synchronization jitter of 2.8xa0ps (rms). A time resolution of 22xa0ps was obtained with 2xa0cm cell. This makes it possible to do the pulse radiolysis experiments in the time range from picosecond to sub-microsecond.

The formation and properties of the melatonin radical: a photolysis study of melatonin with 248 nm laser light

The photolysis of melatonin in aqueous solution has been studied spectrometrically with a 248 nm laser. The formation of hydrated electrons in a monophotonic process has been confirmed in neutral solution with a quantum yield of 0.22. Two main absorption bands at 340 and 460 nm plus an absorption shoulder resulted from the counterpart of the ejected electron, a melatonin radical, in solution. The big difference for the relative intensity of the absorption peaks under various pH conditions reveals that the melatonin radical exists in the solution through an acid-base equilibrium. In support from the pH dependence of the spectrum of the intermediate, the pKa1 for the doubly-protonated melatonin radical against the mono-protonated melatonin cation radical was estimated to be -0.95 and the pKa2 for the mono-protonated melatonin cation and melatonin neutral radical was 4.5 +/- 0.5. This work will benefit the basic understanding about melatonin as a UV-light protector, as a light receptor and the antioxidation functions of melatonin.

Temperature dependence of ketyl radical in aqueous benzophenone solutions up to 400 °C: A pulse radiolysis study

Pulse radiolysis studies with aqueous benzophenone solutions were performed over the range 25–400u2006°C, including conditions beyond the critical point of water. The addition of eaq− to benzophenone forms either ketyl anion in alkaline solutions or neutral ketyl radical in acidic/neutral solutions. Although both have an intense and sharp absorption band in the UV region and a weak but broad band in the visible region, their spectra are clearly different. The peak of the visible band is at 545 nm for the neutral ketyl radical and 610 nm for the anion at room temperature. With increasing temperature, the absorption peak of the ketyl radical shifts to a shorter wavelength while that of the anion shifts to a substantially longer wavelength. The decay of the ketyl radical is not monotonically temperature-dependent: its rate constant (2k/e) increases with temperature up to 100u2006°C and then ndecreases at higher temperatures. The rate constant for the addition of eaq− to benzophenone can be considered to be diffusion controlled and the Arrhenius plot can be fitted with a curved line. The temperature dependence of the rate constant for the reaction of eaq− with proton was determined up to 400u2006°C by the competition method using benzophenone as the reference solute. It was also found that the addition of OH˙ radical to benzophenone forms a transient species with a strong absorption band centered at 380 nm and the peak seems to be slightly blue-shifted with temperature. The decay of this species increases rapidly with temperature, indicating the enhanced conversion to stable products at high temperatures.

Laser photolysis study on the reaction of nitrate radical with tributylphosphate and its analogues—comparison with sulfate radical

Time resolved laser photolysis was carried out to study the reaction of NO3˙ radical toward a series of organic phosphates (tributylphosphate (TBP) and its analogues) with different length of side alkyl groups such as CH3, C2H5, C3H7, CH(CH3)2 and C4H9 in water, acetonitrile, and the mixtures of water and acetonitrile, respectively. For comparison, a similar investigation on SO4˙− radical was also conducted. The oxidation reaction of NO3˙ and SO4˙− radical with the phosphates was attributed to mainly an H-abstract reaction. The kinetics study shows that the rate constants of the reaction between the two radicals and phosphates increase remarkably with increasing of the length of the alkyl groups, e.g. in the case of NO3˙ radical in acetonitrile, the rate constant increases by almost 4 orders of magnitude from TMP (–CH3) to TBP (–C4H9). The composition of the solvent also greatly affects the reactivity but it exhibits an opposite effect of the two radicals, the rate constant of NO3˙ increases with the mole fraction of acetonitrile while that of SO4˙− decreases. The present study would be helpful for understanding the mechanism of deterioration of TBP in the Purex process.

Current status of the ultra-fast pulse radiolysis system at NERL, the University of Tokyo

In order to investigate the early events in radiation physics and chemistry, a new pulse radiolysis system with higher time resolution based on the pump-and-probe method has been developed at Nuclear Engineering Research Laboratory (NERL), The University of Tokyo. Electron pulses with duration of a few picoseconds are generated from an 18 MeV S-band linear accelerator (linac) by using a combination of a laser photocathode rf-gun and a chicane-type magnetic compressor. The pulses are synchronized with a femtosecond laser pulse as the analyzing light. The precision of synchronization between the pump and the probe has attained within 1.6 ps (rms). By converting the fundamental laser into white light continuum, it allows to measure in the wavelength range from 400 to 1100 nm. With a sample cell having an optical path of 1 mm, a time resolution of 5 ps has been achieved.

Observation of hydrated electron, (SCN)2.- and CO3.- radical in high temperature and supercritical water

Transient radicals (hydrated electron, (SCN)2.- and CO3.-) formed in supercritical water have been observed by the pulse radiolysis technique. The change of spectra of these radicals with temperature has been measured. It was found that the spectra and absorption coefficients of the radicals, e-aq and (SCN)2.-, are strongly dependent on the temperature of the water. Since it was found that the absorption spectrum and molar absorption coefficient of CO3.- radical seem to be almost independent of temperature, G-values of OH and e-aq could be derived. Then, the absolute values of the absorption coefficients for the radicals could be calculated. The G-values of the radical products in water radiolysis tend to increase with increasing temperature up to 400°C. Based on the above observation, radiolysis of supercritical water is discussed.

Time-dependent radiolytic yields at room temperature and temperature-dependent absorption spectra of the solvated electrons in polyols

Abstract The molar extinction coefficients at the absorption maximum of the solvated electron spectrum have been evaluated to be 900, 970, and 1000 mol−1·m2 for 1,2-ethanediol (12ED), 1,2-propanediol (12PD), and 1,3-propanediol (13PD), respectively. These values are two-third or three-fourth of the value usually reported in the published report. Picosecond pulse radiolysis studies have aided in depicting the radiolytic yield of the solvated electron in these solvents as a function of time from picosecond to microsecond. The radiolytic yield in these viscous solvents is found to be strongly different from that of the water solution. The temperature dependent absorption spectra of the solvated electron in 12ED, 12PD, and 13PD have been also investigated. In all the three solvents, the optical spectra shift to the red with increasing temperature. While the shape of the spectra does not change in 13PD, a widening on the blue side of the absorption band is observed in 12ED and 12PD at elevated temperatures.