Primary Processes In Radiation Chemistry Research Articles

Improvements in time resolution and signal-to-noise ratio in a compact pico-second pulse radiolysis system

A compact pico-second pulse radiolysis system has been developing at Waseda University for studying primary processes in radiation chemistry. The system is composed of a photo-injector system and a pico-second all-solid-state laser system. An infrared (IR) and an ultraviolet (UV) laser pulses are obtained from mode-locked Nd:YLF laser system and used for generation of the white light continuum as a probe light and the irradiation to the Cu cathode of a photo-cathode RF-gun, respectively. To improve signal-to-noise (S/N) ratio and time resolution of this pulse radiolysis system, we optimized both probe light and pump electron beam. As a result, our pico-second pulse radiolysis system has been enough to study the primary processes of radiation chemistry. The experimental results and the improvements of our system are described in this paper.

Pulse radiolysis based on a femtosecond electron beam and a femtosecond laser light with double-pulse injection technique

A new pulse radiolysis system based on a femtosecond electron beam and a femtosecond laser light with oblique double-pulse injection was developed for studying ultrafast chemical kinetics and primary processes of radiation chemistry. The time resolution of 5.2 ps was obtained by measuring transient absorption kinetics of hydrated electrons in water. The optical density of hydrated electrons was measured as a function of the electron charge. The data indicate that the double-laser-pulse injection technique was a powerful tool for observing the transient absorptions with a good signal to noise ratio in pulse radiolysis.

Motivation and development of ultrafast laser-based accelerator techniques for chemical physics research

The products of radiation induced chemical reactions are determined by rapid primary processes such as energy transfer, thermalization and solvation. Ultrafast photoionization experiments on liquid water demonstrate that these initial events occur on time scales <5ps and involve a complicated interplay between electronic relaxation and vibrational energy redistribution. These experiments also show that the chemical processes originating from ionizing radiation are unique and cannot be reproduced by laser photons alone. Due to the lack of a suitable femtosecond source of ionizing radiation, knowledge of the primary processes in radiation chemistry remains poor. To address this issue a 20TW laser system has been constructed to obtain subpicosecond electron pulses with energies in the 1–10MeV range. In addition to the production of femtosecond electron pulses, future efforts will be directed towards using this laser for accelerating heavier particles such as protons and generating hard X-rays.

Development of compact picosecond pulse radiolysis system

A pulse radiolysis technique is known as a powerful method to study the primary processes in radiation chemistry. However, a large-sized accelerator and a huge cost should be required for the construction and operation of pulse radiolysis system. Very recently, the new compact picosecond pulse radiolysis system composed of the small size accelerator and laser system has been developed at RISE, Waseda University. To evaluate the performance of the system, the absorption measurement of hydrated electrons was carried out as a preliminary experiment. As a result, the time resolution was estimated to be approximately 26ps. It is analytically calculated to be approximately 30ps (FWHM) with the convolution among the beam size, the bunch length of electron beam, the laser spot size, and the laser pulse length. The experimental value is in good agreement with the calculated one. The system has been developed to get better S/N ratio and will be applied for the study of primary processes in radiation chemistry of polymer system.

Formation and Dissociation Dynamics of Molecular Superexcited States

Abstract This is a survey of recent progress in experimental studies of spectroscopy and dynamics of molecules in the superexcited states. The Platzman’s concept of the superexcited states deduced from the theoretical analysis of the interaction of ionizing radiation with matter has been recently substantiated experimentally. Most of the observed superexcited states are assigned to high Rydberg states which are vibrationally (and/or rotationally), doubly, or inner-core excited, and converge to each ion state. Non-Rydberg superexcited states are also observed. The dissociation dynamics of these states as well as the products are very different from those of the lower states excited below ionization thresholds. The neutral dissociation is unexpectedly more important than ionization. Molecular superexcited states are described as reaction intermediates in some fundamentally important collision processes, e.g., electron-ion or ion-ion recombination, Penning ionization, and electron attachment, and are described also as important species in the primary processes of radiation chemistry, VUV-photochemistry, and plasmas. Comments are presented on molecular superexcited states in the condensed phase. Finally, the future perspectives of the present studies are summarized.

Spectroscopy and dynamics of molecular superexcited states. Aspects of primary processes of radiation chemistry

A survey is given of recent progress in experimental studies of spectroscopy and dynamics of molecules in the superexcited states. The Platzman’s idea of the superexcited states deduced from the theoretical analysis of the interaction of ionizing radiation with matter has been recently substantiated experimentally. Most of the observed superexcited states are assigned to high Rydberg states which are vibrationally (or/and rotationally), doubly, or inner-core excited, and converge to each of ion states. Non-Rydberg superexcited states are also observed. The dissociation dynamics of these states as well as the products are highly different from those of the states excited below ionization thresholds. The neutral dissociation is unexpectedly important in comparison with ionization. Some remarks are presented of superexcited states in the condensed phase from the viewpoint to understand the primary processes in radiation chemistry.

Development of laser-synchronized picosecond pulse radiolysis system

A new picosecond pulse radiolysis system for absorption spectroscopy by using a 20 ps single electron pulse from the 28 MeV L-band electron linac and a single femtosecond laser pulse was developed for the research of the ultra-fast phenomena in primary processes of radiation chemistry. The electron pulse was used for the irradiation source and the laser pulse was used for the analyzing light. Both pulses are synchronized within several picoseconds by controlling a common radio frequency. The available analyzing light was from 360 to 1000 nm by using white-light continuum.

Studies of geminate ion recombination and formation of excited states in liquid n-dodecane by means of a new picosecond pulse radiolysis system

A new ps pulse radiolysis system for optical absorption spectroscopy from visible to infrared regions was developed to investigate the primary processes of radiation chemistry. 10 ps electron pulses from a 28 MeV electron linac and ps diode lasers were used as irradiation source and analyzing light, respectively. The ps diode lasers were synchronized with the electron pulses. The time profile of the absorption was obtained by changing the time difference between the electron and the laser pulses. The time resolutions of the system were a few tens ps. The time-dependent behavior of the cation radicals, electrons and excited states in primary processes in liquid n-dodecane was studied by using the new system. The decays of the cation radicals and electrons showed the geminate ion recombination based on the diffusion theory in the time region within 2 ns.

Stochastic simulation of primary processes in radiation chemistry

A review of recent work on the computer simulation method as applied to investigation of primary processes in radiation chemistry is presented. The method was applied to the ion recombination in nonpolar liquids for small spurs of ionization containing 2 – 4 ion pairs and for recombination in larger (25 – 50) structures corresponding to spherical blobs and cyllindrical tracks of ionization. The trapped electron decay by tunnelling to acceptor molecules in low temperature glasses was also simulated. The kinetics of the electron decay was calculated and was compared with the kinetics calculated on a base of the approximate analytical models of the electron tunnelling.

Dependence of the G value on scavenger concentration

In the study of the primary processes of radiation chemistry, many experiments have been done about the dependence of the G value on scavenger concentration. In this paper, we analyze this dependence using the theory of the random walk, that is, by the discrete approach, comparing it with the results by the continuous one. We explain from this the “√ c-law” at low concentration, which is the typical experimental result. Furthermore, we show the difference between the two approaches at high concentration. The dimensional study gives the singular result, for two-dimensional case, that the “√ c-law” does not hold.