The influence of laser fluctuations on the excitation of auto-ionizing Rydb
erg states close to an ionization threshold is investigated. Based on quant
um defect theory a theoretical framework is developed within which all thre
shold phenomena originating from the infinitely many Rydberg states converg
ing towards the ionization threshold as well as the stochastic influence of
the exciting laser field can be described conveniently. It is demonstrated
that the competition between the coherent process of auto-ionization and t
he incoherent processes induced by the fluctuating laser field leads to a v
ariety of novel phenomena.