Time resolved ESR spectroscopy. II. The behavior of H atom signals

Abstract

The time dependence of the ESR signals of H atoms formed in acid water by pulse radiolysis has been examined at times $\text{>2\hspace{0.17em}μ}\sec$ after the pulse. As in steady‐state experiments, the two lines are of very similar amplitudes but with the low‐field line appearing in emission. A growth of signal is observed for the first 4–10 μsec depending upon the microwave power and dose per pulse. At the higher microwave powers $\text{(>1\hspace{0.17em}}\mathrm{mW})$ an oscillatory decay is observed which must be caused by the transient response of the spin system. An attempt has been made to explain the over‐all behavior of the ESR signals by introducing the assumption that the signal enhancement arises upon nuclear spin dependent recombination. This effect is included in the Bloch equations by a production term for M_z which is proportional to radical concentration (taken to decay by a second‐order kinetic curve). The results of this calculation were very successful in fitting all details of the observed curves including the initial growth and the oscillatory behavior. An alternative assumption, that the singal enhancement occurred upon radical formation, did not lead to agreement with the observations. Because the initial growth is a direct consequence of the Bloch equations it is not necessary to invoke any chemical complications, such as a precursor of lifetime $\text{∼ 5\hspace{0.17em}μ}\sec$ , to explain this phenomenon. The ESR signals of H atoms in acid were found to double upon addition of tert‐butyl alcohol as an OH scavenger, and a much larger fractional increase was found in neutral, N₂O saturated water. These results are taken to show that the reaction $\mathrm{H+OH}$ is not selective as to the spin state of the H atom and so as a result produces no signal enhancement, while the reactions $\mathrm{H+H}$ and $\mathrm{H+R}$ do lead to signal enhancement.