Femtosecond pulse radiolysis

With the development of the picosecond pulse radiolysis, the kinetics data of the geminate ion recombination have been directly obtained. The history of picosecond and subpicosecond pulse radiolysis is shown in Fig. 7. Very recently, the first construction of the femtosecond pulse radiolysis and the improvement of the subpicosecond pulse radiolysis started in Osaka University. 

Both Construction of the First Femtosecond Pulse Radiolysis and Improvement of the Subpicosecond Pulse Radiolysis started in Osaka University (2002) ... 

Figure 7 History of picosecond and subpicosecond pulse radiolysis and the start of construction of the first femtosecond pulse radiolysis.

Further detailed kinetics of the geminate recombination of electrons and positive ions and their application to the advanced technology will be studied by higher time resolution of the femtosecond pulse radiolysis and both by the higher S/N ratio and the wider wavelength monitoring light of the improved subpicosecond pulse radiolysis shown in Fig. 7. 

Several decades ago, picosecond pulse radiolysis was as eagerly anticipated as the femtosecond pulse radiolysis today. Knowing what followed thereafter... 

The subpicosecond pulse radiolysis [74,77] detects the optical absorption of short-lived intermediates in the time region of subpicoseconds by using a so-called stroboscopic technique as described in Sec. 10.2.2 ( History of Picosecond and Subpicosecosecond Pulse Radiolysis ). The short-lived intermediates produced in a sample by an electron pulse are detected by measuring the optical absorption using a very short probe light (a femtosecond laser in our system). The time profile of the optical absorption can be obtained by changing the delay between the electron pulse and the probe light. 

Yang J, Kondoh T, Kozawa T, Yoshida H, Tagawa S. (2006) Pulse radiolysis based on a femtosecond electron beam and a femtosecond laser light with double-pulse injection technique. Rad Phys Chem 75 1034-1040. 

Ogata A, Nakajima K, Kozawa T, Yoshida Y. (1996) Femtosecond single-bunched linac for pulse radiolysis based on laser wakefield acceleration. IEEE Trans Plasma Science 24 453-459. 

Saeki A, Kozawa T, Tagawa S. (2006) Picosecond pulse radiolysis using femtosecond white light with a high S/N spectrum acquisition system in one beam shot. Nucl Instrum Meth A 556 391-396. 

Muroya Y, Lin M, Watanabe T, Wu G, Kobayashi T, Yoshii K, Ueda T, Uesaka M, Katsumura Y. (2002) Ultra-fast pulse radiolysis system combined with a laser photocathode RF gun and a femtosecond laser. Nucl Instrum Meth A 489 554-562. 

Yang J, Kondoh T, Yoshida A, Yoshida Y. (2006) Double-decker femtosecond electron beam accelerator for pulse radiolysis. Rev Sei Instrum 77 043302. 

Photocathode-based picosecond electron accelerators are conceptually simpler than pre-bunched thermionic systems, although they require reasonably powerful, multicomponent femtosecond or picosecond laser systems to drive the photocathode. In addition, the availability of synchronized laser pulses allows the development of advanced detection capabilities with unprecedented time resolution. The combination of ease of use and powerful detection methods has stimulated strong interest in photocathode gun systems. Since the installation ofthe first photocathode electron gun pulse radiolysis system at BNL [5,13], four additional photocathode-based facilities have become operational and two more are in progress. The operational centers include the ELYSE facility at the Universite de Paris-Sud XI in Orsay, France [7,8], NERL in Tokai-Mura, Japan [9,10], Osaka University [11,12], and Waseda University in Tokyo [13]. Facilities under development are located at the Technical University of Delft, the Netherlands, and the BARC in Mumbai, India. 

Saeki A., Kozawa T., Kashiwagi S., Okamoto K., Isoyama G., Yoshida Y.,Tagawa S., Synchronization of femtosecond UV-IR laser with electron beam for pulse radiolysis studies, Nucl. Instr. and Meth. A, 2005, 546, 627-633. 

A simplified view of the early processes in electron solvation is given in Figure 7. Initially, electron pulse radiolysis was the main tool for the experimental study of the formation and dynamics of electrons in liquids (Chapter 2), first in the nanosecond time range in viscous alcohols [23], later in the picosecond time range [24,25]. Subsequently, laser techniques have achieved better time resolution than pulse radiolysis and femtosecond pump-probe laser experiments have led to observations of the electron solvation on the sub-picosecond to picosecond time scales. The pioneering studies of Migus et al. [26] in water showed that the solvation process is complete in a few hundreds of femtoseconds and hinted at the existence of short-lived precursors of the solvated electron, absorbing in the infrared spectral domain (Fig. 8). The electron solvation process could thus be depicted by sequential stepwise relaxation cascades, each of the successive considered species or... 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

Figure 2. Transient absorption spectrum of visible aqueous electrons generated by different exdtation methods of pure liquid water pulse radiolysis (60j, pico-second pulse photolysis (58), and femtosecond UV photolysis (6). The long-lived spectra obtained by three different pulsed methods correspond to a broad absorption band of relaxed hydrated electrons (in an s-like ground state) centered around...

In this chapter we mainly limit ourselves to the time region from several femtoseconds to milliseconds and to processes that are induced by illumination with light. One could roughly divide this time region into two parts, from femtoseconds to nanoseconds and from nanoseconds to milliseconds. The latter time scale can be investigated with electronic detection, and the described methods can also be used for processes that are initiated in other ways, for example, in stopped-flow experiments where reactants are rapidly mixed or pulse-radiolysis experiments where a short electron pulse induces a chemical reaction. After an introduction to the basic principles of transient absorption, we first treat slow transient absorption measurements (nanoseconds-milliseconds). 

One final example of ultrafast kinetics performed at radiolysis facilities is the study of excited states of radical ions. An accelerator pulse can be used to generate radical species, which can then be excited by a pump laser beam and probed with femtosecond resolution by another laser pulse with variable optical delay. This application does not depend on precise correlation of the electron and laser pulses and can be done at almost all radiolysis facilities. The availability of femtosecond lasers in photocathode facilities places all the necessary components to hand. Effective pump-probe measurements will require significant concentrations of radical ions. This can be accomplished by frequency-quadrupling a 5-9 nanosecond Nd YAG pulse to irradiate the photocathode, thereby creating a macropulse containing several tens of nanocoulombs which will produce a high concentration of radicals for the pump-probe experiment. 

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