ANALYSIS OF THE MOVEMENT OF LINE-LIKE SAMPLES OF VARIABLE-LENGTH ALONG THE X-AXIS OF A DOUBLE TE104 AND A SINGLE TE102 RECTANGULAR CAVITY

Movement of line-like samples with lengths from 5 to 50 mm along the x -axis of the double TE104 rectangular cavity has been analyzed. The ob served dependencies of the EPR signal intensity versus sample position showed: (i) a sharp maximum for sample lengths from 5 to 20 mm; (ii) a plateau, over which the EPR signal intensity remained constant withi n experimental errors of 0.26-1.07%, for lengths from 30 to 40 mm; and (iii) a ''sloping plateau,'' which could be approximated by the linea r function (correlation, r = 0.98) for sample length 50 mm, Theoretica l values of the experimentally observed dependencies of the intensity versus sample position were calculated using the modified sine-squared function and the correlation between observed and theoretically predi cted dependencies is very good. The experimental dependence of the EPR signal intensity versus the sample length for samples situated at the same point in the cavity was nonlinear with a maximum for the 40-mm s ample. The dependence of the EPR signal intensity upon the movement of a large cylindrical sample (o.d. 4 mm and length 100 mm) along the x- axis of the cavity was similar to that found for the 50-mm sample, How ever, an additional oscillating signal superimposed on the sloping pla teau was observed. The presence of a large sample fixed in the complem entary cavity of the double TE104 cavity caused an additional deformat ion of the signal intensity for a 30-mm sample which was moving in the first cavity. The primary effect was that the plateau was replaced by a region in which the intensity increased linearly with sample positi on, r = 0.99. Each of the above phenomena may be a source of significa nt errors in quantitative EPR spectroscopy, Cylindrical samples to be compared should be of identical length and internal diameter. Accurate and precise positioning of each sample in the microwave cavity is ess ential. (C) 1997 Academic Press.