Pulse-pump-probe radiolysis

Proton inventory technique. 21.9-220 Pseudo-first-order kinetics, 16 Pulse-accelerated-flow method. 255 Pulse radiolysis, 266-268 Pump-probe technique. 266... 

Ethylene glycol is a very viscous liquid and the molecule presents two close OH groups. It has to be noticed that, among all the different solvents studied by pulse radiolysis, the transition energy of the solvated electron absorption band is maximum in liquid ethylene glycol. For these reasons, the electron in EG seems to have a special behaviour and it is of great interest to study the dynamics of the formation of equilibrated solvated electron. Within this context, the present communication deals with the dynamics of solvation in EG of electrons produced by photoionisation of the solvent at 263 nm. The formation of solvated electrons is followed by pump-probe transient absorption spectroscopy in the visible spectral range from 425 to 725 nm and also in near IR. For the first time, the absorption spectrum of the precursor of the equilibrated electron is observed in EG. Our results are shortly compared by those obtained in water and methanol. 

Access to subpicosecond electron pulses has already been achieved at Osaka University by a new double-decker accelerator concept. In order to reduce the time jitter for the detection of the optical absorption signals in pulse radiolysis studies, the light pulse used for the pump-probe system is Cerenkov emission which is produced in the same cell by a synchronized second electron beam and is concomitant with the electron path. The distance between the axes of the two beams is 1.6 mm. The pulse durations of these electron pulses, which are both produced by delayed beams issued from the same laser, are 430 + 25 fs and 510 20 fs, respectively, and the charge per pulse is 0.65 nC. An electron bunch of 100 fs and 0.17 nC has already been generated. 

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. 

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. 

Pump-probe techniques use similar pulses to excite the sample to create the chemistry and to observe the reaction (see figure 5). The observation pulse is delayed so that time can be varied (see figure). The technique can be used both for absorption and emission (up conversion). The time resolution is then only limited by the width of the pulse that does the excitation. Multiple experiments must be done because each experiment will only measure one time. The approach can be used both for flash photolysis and for pulse radiolysis. 

This discourse tries to give an overview of the current state-of-the-art instrumentation in real-time pulse radiolysis experiments utilizing optical, conductometric and other methods. Pump-and-probe techniques for the sub-nanosecond time domain are believed to be beyond the scope of this discussion. 

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