Influence of the movement of “over full-length cavity” cylindrical samples along the x-axis of a double TE 104 and a single TE 102 rectangular cavity on the electron paramagnetic resonance : An unusual effect analysis

Abstract

The response of the cavity to the movement of the “over full-length cavity” cylindrical samples with lengths from 50 to 100 mm along the x-axis of the Bruker double TE 104 and single TE 102 rectangular cavity has been analysed. The experimentally observed dependencies of the electron paramagnetic resonance (EPR) signal intensity versus sample position in the cavity showed the following: (i) a “sloping plateau” region, which could be approximated by the linear function (correlation coefficient, r=0.85–0.96); (ii) an additional oscillating signal superimposed on the “sloping plateau” region; (iii) an identical amplitude of the maximal signal intensity on the “sloping plateau” region for all sample lengths, within an experimental error of 1.27%. The trends of the dependencies with the “sloping plateau” were independent of the length and internal diameter of the “over full-length cavity” samples, but the width of the “sloping plateau” regions progressively increased with the increase of the sample length from ca. 30 mm for the 50-mm sample to ca. 80 mm for the 100-mm sample. Theoretical predictions of the experimental dependencies of the signal intensity versus sample position in the cavity were calculated using the “modified” and “revised” sine-squared functions, and the correlation between the observed and theoretically computed dependencies is very good. The experimental dependence of the “over full-length cavity” sample position ( x-coordinate), at which the signal intensity was a maximum, as a function of sample length, L, is nonlinear. The experimentally determined x-coordinate of these sample positions in the cavity oscillated between the upper, L− a/2, and lower, L− a, theoretically predicted limiting values, where a (23.5 mm) is the length of the active zone of the Bruker rectangular microwave cavity. Each of the above phenomena may be a source of serious errors in quantitative EPR spectroscopy. “Over full-length cavity” cylindrical samples to be compared should be of identical length and must be identically positioned in the microwave cavity.