Time-Dependence of ESR Intensities

Abstract

The quantitative information we have about many aspects of CIDEP is obtained by the time-resolved ESR technique^1-3. The radicals under investigation are generated by a pulsed light¹ or electron source², or alternatively by a modulated source, e.g. a rotating sector or an electronically modulated light source. The signal intensity is recorded as a function of time from the instant of time when radical formation starts. Rapid response ESR spectrometers^{1, 2, 3} with a time resolution in the microsecond domain have been developed and this makes it possible, at least in principle, to determine T.₁, ω₁, chemical decay constants, and various polarization terms from the time dependence of the intensity. Such a determination requires a very careful curve fitting and the purpose of this chapter is to provide the necessary theory⁵. The approach is based on the Bloch equations, which have been modified to include chemical reactions and polarization production. The time resolution or response time of the spectrometer, is a lower limit to the time constants that can be determined^{3, 6}. Whenever the response time is comparable to or larger than the smallest time constant (e.g T₁) then one must include the effect of the spectrometer response on the intensity curve and this is discussed at the end of the chapter.