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

It is first shown that, when magnetization processes and resonant exci tation of states in a spin system occur in the presence of internal an d/or external random perturbing line-broadening mechanisms, the comple x magnetic susceptibility of the system can be written in terms of the plasma dispersion function. This function arises in the description o f linear wave propagation in plasmas and inherently contains informati on related to random phenomena, such as thermal motions, that affect w ave 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 proce sses affecting the resonant phenomenon, The resulting absorption and d ispersion profiles that make up the so generalized complex susceptibil ity have a Voigtian shape; this can be used to fit spectral lines. Nex t, as a variety of mechanisms of diverse origin may affect the profile s of the absorptive and bi dispersive parts of either the plasma dispe rsion function or the complex susceptibility in such a way to make the m asymmetric, a further generalization is made by introducing an asymm etry factor. A general relationship linking the Voigtian and the asymm etry parameters is obtained, This relationship indicates that the degr ee of asymmetry of a spectral line should not exceed its degree of Gau ssian deformation measured in units of Lorentzian width. The obtained generalized relationship for the complex susceptibility could be usefu l in fitting experimental EPR or NMR absorption or dispersion spectra to understand the resonance phenomenon better when it occurs in the pr esence of both random perturbing and line-asymmetrizing processes. Fin ally, it is pointed out that the profiles associated with the real and imaginary parts of the asymmetrized plasma dispersion function and ge neralized 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 absorpt ion and dispersion spectra which in general show some degree of asymme try. (C) 1997 Elsevier Science B.V.