Sample pulse radiolysis

Figure 8 The time-dependent behavior of the hydrated electron obtained in the subpicosecond pulse radiolysis of neat water using 2-mm optical path sample cell, monitored at the wavelength of 780 nm.

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. 

The experiments were carried out using the subpicosecond pulse radiolysis system [77] described in Sec. 10.2.2 ( Subpicosecond Pulse Radiolysis ). Considering the signal intensity and the degradation of the time resolution, a sample cell with the optical length of 2 mm was mostly used. The sample was saturated by Ar gas to eliminate the scavenging effect by the remaining O2 gas. 

Pulse radiolysis was performed using e from a linear accelerator at Osaka University [42 8]. The e has an energy of 28 MeV, single-pulse width of 8 nsec, dose of 0.7 kGy, and a diameter of 0.4 cm. The probe beam for the transient absorption measurement was obtained from a 450-W Xe lamp, sent into the sample solution with a perpendicular intersection of the electron beam, and focused to a monochromator. The output of the monochromator was monitored by a photomultiplier tube (PMT). The signal from the PMT was recorded on a transient digitizer. The temperature of the sample solution was controlled by circulating thermostated aqueous ethanol around the quartz sample cell. Sample solution of M (5 x 10 -10 M) was prepared in a 1 x 1 cm rectangular Suprasil cell. 

A method for preparing a-methylstyrene to investigate its radiation-induced polymerization yields samples which exhibit reproducible kinetics. The kinetic results are interpreted as indicating that free radicals, carbonium ions, and carbanions can all propagate simultaneously, the relative importance of each species depending upon the dryness of the monomer and all associated glassware. This viewpoint is further supported by data from a preliminary investigation of the transients formed in a-methylstyrene, as studied by the pulse radiolysis technique. 

Let us consider the data on the dependence of the kinetics of et decay at 77 K on the radiation dose. As seen from Fig. 11, over the dose range 3 x 1019 - 3.6 x 102° eV cm 3, the kinetics of et decay is virtually independent of the dose. At the same time, at lower doses, the decay of et is significantly slowed down. For example, for a dose of 1019 eV cm-3, the change in optical density of y-irradiated samples at the maximum (585 nm) of the et absorption spectrum with time is also described by eqn. (5), but the slope of the kinetic curve the coefficient M in eqn. (5)] is smaller by almost a factor of two [28] than for the curve of Fig. 11. Further investigations by pulse radiolysis technique with spectrophotometric recording of et showed that, at a still lower dose (6 x 1017 eV cm"3) no decay of et in water-alkaline matrices is observed at all [43] while at high doses (5 x 1021 eV cm"3) for the same samples, the decay of efr does occur [43]. A decrease in the rate of etr decay via the reaction with O at small doses was also reported in ref. 44. This behaviour of the kinetic curves seems to reflect special features of the spatial distribution of etr and 0 particles in samples irradiated with different doses. 

Wilkinson F, Willsher CJ, Warwick P, Land EJ, Rushton FAP (1984) Diffuse reflectance pulse radiolysis of opaque samples. Nature 311 40-42... 

Fig. 3.9. Method used to study the diffuse reflectance of opaque samples in pulse radiolysis (Adams et al., 1991). The insert shows the method used to mount the sample. The electrons pass through the thin aluminium plate and the probe light through the quartz window. The scattered beam is analysed in the normal way with a monochromator, care being taken to exclude normal reflected light.

Radiolysis of low temperature alkane liquid also gives rise to trapped radical ions (23). Thermal annealing of the irradiated samples gives rise to luminescence characteristic of excited states of the solvent and, or, solutes present (23,24). These data conform exactly to those obtained in pulse radiolysis studies. 

Samples were irradiated by a 10 ps single or 2 ns electron pulse from a 35 MeV linear accelerator for pulse radiolysis studies (17). The fast response optical detection systems of the pulse radiolysis system for absorption spectroscopy (18) is composed of a very fast response photodiode (R1328U, HTV.), a transient digitizer (R7912, Tektronix), a computer (PDP-11/34) and a display unit. The time resolution is about 70 ps which is determined by the rise time of the transient digitizer. 

Pulse radiolysis was modeled after flash photolysis. The time resolution of laser flash photolysis has always been better than for pulse radiolysis. There are multiple reasons for this effect. (1) Flash photolysis equipment is cheaper than electron accelerators so there have been many more practitioners of the art. (2) Photons do not repel each other so it is possible to focus a larger number of them in a small volume over a short time period than it is possible to do for electrons. (3) The velocity of relativistic electrons in a dense material is much higher than photons in the same material so sample thicknesses must be much thinner for pulse radiolytic experiments than for flash photolytic experiments, thus meaning that signals would be smaller. 

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