EXPERIMENTAL TECHNIQUES Steady-state techniques In radiolytic investigations the conversion (i.e., the ratio of transformed and starting molecules) is low (0.0001-0.01), therefore sensitive analytical techniques are needed. The final products are analyzed by the usual analytical techniques, like spectrophotometry, GC, HPLC and others. In radiolytic reactions excited molecules, cations, free electrons, anions and radicals are the intermediates. Their lifetimes of are short, their direct observation is difficult. Exceptional cases when the reactions of intermediates are hindered, for instance if they are “frozen” in a solid matrix, e.g. in polyethylene irradiation. The alkyl radicals may live for several hours and can be observed by EPR spectroscopy.

SCAVENGER METHOD, CHARGED SPECIES An indirect technique for studying intermediates is the scavenger method. Yields decrease, increase or remain unchanged in the presence of scavengers, new components appear among products. For scavenging excited molecules, compounds with low excitation energies are used, e.g. aromatic hydrocarbons. Electron scavenging is carried out with N2O additive, e.g.: N2O+eaq−+H2O  N2+OH−+OH k = 7109 mol−1 dm3 s−1 In hydrocarbons alkyl halides,like CH3Br, may be used:   CH3Br + e−CH3 + Br− CH3 + RH  CH4 + R NH3 or cyclopropane are used as positive-ion scavenger

SCAVENGER METHOD, RADICALS The detection of free radicals can be achieved by using unsaturated hydrocarbons (e.g. ethylene) or other substances with unsaturated bonds or with unpaired electron orbital (I2, O2, benzoquinone, stable free radicals, etc). I2 reactions are often studied by radioactive iodine. I2 reacts with many radicals with diffusion-controlled rate: R + I2  RI + I 2I  I2   Scavengers are applied in 10-3 mol dm−3 concentration. This concentration is sufficient to scavenge all the radicals that escape the spur or track, at this concentration the scavenger cannot compete with reactions taking in the spur.

PULSE RADIOLYSIS In pulse radiolysis short electron pulses, accelerated to several MeV, are directed to the sample to initiate chemical reactions. The time dependence of build-up and decay of intermediates are followed by various detection systems. Most often used technique is the optical detection. In optical detection the intermediates are observed by their light absorptions. The setup is composed of two parts, first part, including light source for sample illumination, lens system, shutter preventing heating and photolysis of sample, light filters, and the measuring cell itself, is housed in the irradiation room. The other parts are outside the radiation shielding and Faraday cage protecting against electromagnetic interference.

PULSE RADIOLYSIS, SETUP Schematic representation of a pulse radiolysis setup. 1 – xenon lamp, 2 – shutter, 3 – lenses, 4 – accelerator window, 5 – sample, 6 – focusing lens, 7 – light guide cable, 8 – monochromator, 9 – photomultiplier, 10 – digital oscilloscope, 11 – computer.

PULSE RADIOLYSIS, KINETIC CURVES

Light intensity Time Effect of ‘backing-off’ on light intensity signal. The upper figure shows the oscilloscope screen with pulsed light. The transient light intensity disturbance caused by light absorption of intermediate (shown in circle) is enlarged by the ‘backing-off’ method as shown in the lower figure.

Derivation of an absorption spectrum.