Time resolved ESR spectroscopy. IV. Detailed measurement and analysis of the ESR time profile

Abstract

An ESR spectrometer for recording ESR signals with submicrosecond response time (RC=0.3 μsec) is described. Signals can be observed to within less than 1 μsec of the radiolysis pulse used to produce the radicals. The transient signals are observed directly in absorption for ease of analysis of their time dependence. Two modes of operation are possible: display of a spectrum at a fixed time after the pulse and the time profile at a fixed magnetic field. The latter mode (which was mainly used in this work) makes use of 100 data points in time, taken sequentially and averaged in a minicomputer. The magnetic field is computer controlled and is adjusted for changes in microwave frequency to maintain resonance. Analysis of the time-dependent signals was by means of the Bloch equations so modified as to take account of changes in radical concentration with time, any initial magnetization upon radical formation, and CIDEP effects produced during radical decay. The ESR signal of the hydrated electron was observed at g=2.00033±0.00003 in irradiated solutions of SO32− and, by comparison of the observed time dependence with calculated curves, shown to have zero magnetization (or spin population difference) upon formation. Its apparent relaxation time was found to vary with radical concentration (dose), suggesting relaxation by Heisenberg spin exchange. Curves taken in the presence of CH3CN were consistent with a reaction rate constant of 3.8×107M−1 sec−1, in agreement with optical data. Secondary radicals produced by reaction of e−aq such as [−O2CCH=CHCO2−]−, −O2CCH=?CO2−, ?H2CO2−, CH3? (O−)CO2− were found to have zero initial magnetization while those formed from OH, such as ?O3−, ?H2CO2−, and CH3? (O−)CO2−, showed curves indicating that the initial magnetization corresponded to the Boltzmann equilibrium as a result of relaxation of OH before reaction. This behavior persisted for CH3? (O−)CO2− from 1M lactate, showing that the relaxation time of OH is <1 nsec. The markedly different ESR response observed for ?H2CO2− [or CH3? (O−)CO2−] produced from e−aq or from OH shows very clearly that spin orientation is preserved upon reaction. Thus, the spin population of a radical such as OH (which cannot be observed directly) can be studied indirectly through the information passed to a reaction product. This experiment illustrates trapping as applied to spin populations. The reaction of both e−aq and H with −O2CC≡CCO2− produces −O2CCH=?CO2−, and those radicals formed from H show initial magnetizations for the low- and high-field lines corresponding to emission and enhanced absorption. This CIDEP is larger at lower concentration of −O2CC≡CCO2−, but some persists to very high concentrations. The increase at lower concentration must come from homogeneous radical–radical reaction of H atoms before reaction while that persisting at high concentration is probably produced by radical–radical reactions of H in the spur. Experiments with ?H2CO2− showed that the initial enhancement factors were inversely proportional to the first chemical half-life at least up to a concentration where the enhancement was 3.6 and the half-life was 15 μsec.