Time resolved ESR spectroscopy. I. A kinetic treatment of signal enhancements

Abstract

ESR experiments with time resolution on the order of 1 μsec have been carried out upon aqueous solutions during repetitive pulse radiolysis with a 2.8 MeV electron beam. The ESR spectrometer was operated with no field modulation, and spectra were presented as absorption curves. The radicals $\mathrm{ĊH}({\mathrm{CO}}_2^{-}{)}_2\mathrm{,\hspace{0.17em}}{\mathrm{ĊH}}_2{\mathrm{CO}}_2^{-}$ , and hydroxycyclohexadienyl, formed in each case by reactions of OH, were studied and each spectrum showed evidence of a nonequilibrium population of spin levels, with the low field lines of the latter two radicals observed in emission. For ${\mathrm{ĊH}}_2{\mathrm{CO}}_2^{-}$ this emission persists for $\text{∼ 50\hspace{0.17em}μ}\sec$ . The observed signals are described by enhancement factors V (fractional excess signal) of 0.23, 3.1, and 12.3, respectively. From the short spin relaxation times (1.4 μsec was measured for ${\mathrm{ĊH}}_2{\mathrm{CO}}_2^{-}$ ) it is evident that emission existing at times much greater than this relaxation period must be the result of a continuous regeneration of the population differences by the radical‐radical reactions. The form of the kinetic equations for the population of the spin levels is considered and an intrinsic enhancement factor defined (Vt_1/2/T₁) which takes into account the rate of radical reaction (through the initial second‐order half‐life t_1/2) and the spin relaxation times. This quantity is found to be fundamental to the radical system and is, within experimental error, the same for observations in both steady‐state and pulse experiments where the radical lifetimes differ by a factor of $\text{∼ 10}$ . The equations developed successfully describe the time dependence of the observed ESR signals. The intrinsic enhancement factors are large (30–100) and represent $\text{∼ 5\%–10\%}$ adiabatic character of the recombination when so interpreted. The kinetic theory developed does not require that such large enhancements appear in the NMR spectra of the reaction products.