Electron paramagnetic resonance signal intensity of a line-like sample with a variable length situated at an arbitrary position along the common sample-cavity axis. Theoretical prediction versus experimental measurement

Abstract

The integral equations used for the theoretical prediction of the relative EPR-signal intensity of variable length line-like samples situated at the central position of a microwave cavity have been computed numerically for the cases where: (i) the microwave field is non-uniform but the modulation field is uniform, and (ii) the microwave and modulation fields are non-uniform. In both cases, stress is placed on the variable length of the sample and the length of the active part of the microwave cavity. The theoretically predicted and experimentally observed electron paramagnetic resonance (EPR) signal intensity are in a very good agreement for sample lengths from 1.3 to 50 mm. However, for the cases where the sample length is greater than that of the cavity, the theoretical equations and experimental results can only be reconciled if the cavity is assumed to have an active length, * a=40 mm, considerably greater than the actual length of the cavity, a=23.5 mm. Comparison of the theoretically predicted and experimentally obtained EPR-signal intensities indicate that those parts of the sample situated in the upper and lower sample access holes of the rectangular cavity can contribute significantly to the measured signal intensity. The possible influence of this phenomenon as a significant source of error in quantitative EPR measurements is discussed. The integral equations have been generalised to describe the signal intensity for line-like sample positioned at any point along the common sample-cavity axis.