SPECTROSCOPIC APPLICATIONS OF THE PLASMA DISPERSION FUNCTION .2. AN ASYMMETRIC LINESHAPE MODEL AND THE COMPLEX SUSCEPTIBILITY

摘要

It is first shown that, when magnetization processes and resonant excitation of states in a spin system occur in the presence of internal and/or external random perturbing line-broadening mechanisms, the complex magnetic susceptibility of the system can be written in terms of the plasma dispersion function. This function arises in the description of linear wave propagation in plasmas and inherently contains information related to random phenomena, such as thermal motions, that affect wave propagation, This property is transferred to the complex magnetic susceptibility, which is the solution of the Bloch equations, through a parameter that can be associated with actual random perturbing processes affecting the resonant phenomenon, The resulting absorption and dispersion profiles that make up the so generalized complex susceptibility have a Voigtian shape; this can be used to fit spectral lines. Next, as a variety of mechanisms of diverse origin may affect the profiles of the absorptive and bi dispersive parts of either the plasma dispersion function or the complex susceptibility in such a way to make them asymmetric, a further generalization is made by introducing an asymmetry factor. A general relationship linking the Voigtian and the asymmetry parameters is obtained, This relationship indicates that the degree of asymmetry of a spectral line should not exceed its degree of Gaussian deformation measured in units of Lorentzian width. The obtained generalized relationship for the complex susceptibility could be useful in fitting experimental EPR or NMR absorption or dispersion spectra to understand the resonance phenomenon better when it occurs in the presence of both random perturbing and line-asymmetrizing processes. Finally, it is pointed out that the profiles associated with the real and imaginary parts of the asymmetrized plasma dispersion function and generalized asymmetric complex susceptibility thus obtained constitute a new spectroscopic lineshape model, This model can be applied in other spectroscopies (optical, Mossbauer, etc.) to fit experimental absorption and dispersion spectra which in general show some degree of asymmetry. (C) 1997 Elsevier Science B.V. [References: 21]