Influence of the movement of "over full-length cavity" cylindrical samplesalong the x-axis of a double TE104 and a single TE102 rectangular cavity on the electron paramagnetic resonance - An unusual effect analysis

The response of the cavity to the movement of the "over full-length cavity" cylindrical samples with lengths from 50 to 100mm along the x-axis of the Bruker double TE104 and single TE102 rectangular cavity has been analysed. The experimentally observed dependencies of the electron paramagnetic reson ance (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 addit ional oscillating signal superimposed on the "sloping plateau" region; (iii ) an identical amplitude of the maximal si anal intensity on the "sloping p lateau" region for all sample lengths, within an experimental error of 1.27 %. The trends of the dependencies with the "sloping plateau" were independe nt of the length and internal diameter of the "over full-length cavity" sam ples, but the width of the "sloping plateau" regions progressively increase d with the increase of the sample length from ca. 30 mm for the 50-mm sampl e to ca. 80 mm for the 100-mm sample. Theoretical predictions of the experi mental dependencies of the signal intensity versus sample position in the c avity were calculated using the "modified" and "revised" sine-squared funct ions, and the correlation between the observed and theoretically computed d ependencies is very good. The experimental dependence of the "over full-len gth cavity" sample position (x-coordinate), at which the signal intensity w as a maximum, as a function of sample length, L, is nonlinear. The experime ntally determined x-coordinate of these sample positions in the cavity osci llated between the upper, L - a/2, and lower, L - a, theoretically predicte d limiting values, where a (23.5 mm) is the length of the active zone of th e Bruker rectangular microwave cavity. Each of the above phenomena may be a source of serious errors in quantitative EPR spectroscopy. "Over full-leng th cavity" cylindrical samples to be compared should be of identical length and must be identically positioned in the microwave cavity. (C) 2001 Elsev ier Science B.V. All rights reserved.