The purpose of this review is to give a simple introduction to the basic concepts of Multichannel Quantum Defect Theory (MQDT). The same simple principles used to study atomic Rydberg levels can be extended to treat more complicated systems like high excited states in molecules and their interactions with Rydberg series and continua. MQDT exploits the subtle interplay between the boundary conditions of ion-electron collision at infinity and at the core, and this information permits a detailed description of the origins of features observed in Rydberg spectra. Problems approaching chemical complexity are possible to study by using MQDT as an interface between frequency and time domain experiments, and ab initio theory.
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