The quantum and classical Fano parameter $q$

The Fano resonance has been a familiar and important feature in atomic and molecular physics for more than half a century. Typically, the combination of a discrete state with one or more continua results in an asymmetric peak in the ionization spectrum. The peak-shape, called the Fano profile, can be expressed by a simple formula derived by Fano in 1935. However, the interpretation of its characteristic parameter $q$, which represents the asymmetry of the peak in the formula, is not necessarily intuitively transparent. The Fano resonance is not necessarily a quantum effect, but it is a manifestation of a certain physical mechanism in various systems, both quantum and classical. Through the derivation of $q$ from the known classical pictures with the classical coupled oscillator, we interpret the $q$ value with a geometrical view. And further, we introduce a complex valued $q$ parameter for the description of the resonance with a damped oscillator.

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