Sudden change of shape of time-dependent molecular fluorescence intensity as a nonequilibrium phase transition

Abstract

It is the aim of this note to show that, even under fixed excitation wavelength and bandwidth, the fluorescence time profile of a given molecule may exhibit a sudden and drastic qualitative change between smooth, purely exponential, and periodically structured quantum beat decay if appropriate external conditions are continuously varied. This abrupt change is analogous to a nonequilibrium phase transition which occurs at a bifurcation point of a suitable order parameter, identified to be the beat frequency, whose branching is governed by a tunable level width which turns out to be the appropriate control parameter for the transition.