FITTING OF THE DERIVATIVE VOIGT ESR LINE UNDER CONDITIONS OF MODULATION BROADENING

An analysis is made of the effect of the magnetic-field-modulation amp litude on the Voigt lineshape in electron spin resonance spectroscopy. The problem is solved numerically using the Fourier expansion method. Only the coefficient of the first harmonic in the Fourier expansion, corresponding to the output of the phase-sensitive detector, is analyz ed. The modulation-broadened first-derivative Voigt ESR line, here aft er referred to as the derivative Voigt, is fitted with the sum of a fi rst-derivative Gaussian function and a first-derivative Lorentzian fun ction of equal linewidths to extract the Lorentzian contribution of th e line. Fitting to the sum is applicable as long as the amplitude of m odulation is less than the true peak-to-peak linewidth of ESR line. Mo dulation broadening substantially contributes only to the derivative G aussian component of the derivative Voigt line. The same effect is obs erved for the modulation-broadened derivative Lorentzian lineshape, wh ich also can be fitted to the sum function. The linewidth of the Loren tzian component of the modulation-broadened derivative Lorentzian is c onstant if the modulation amplitude is less than the peak-to-peak line width. For the modulation-broadened derivative Voigt line, the goodnes s of the fit depends on the ratio of the Gaussian component to the Lor entzian component of the derivative Voigt line. When the Lorentzian co mponent of the derivative Voigt is dominant, the linewidth of the Lore ntzian component can be extracted for a broader range of the modulatio n amplitudes than when the Gaussian component is dominant. (C) 1994 Ac ademic Press, Inc.