Two-dimensional spectroscopy: Theory of nonstationary, time-dependent absorption and its application to femtosecond processes

In this paper, we present a formalism for a generalized linear susceptibility applicable to femtosecond (fs) processes. As an application, we analyze the recent experimental results of the real-time fs transition spectra of the photodissociation of NaI and NaBr reported by Zewail et al. In this connection we also investigate in general the behaviors of decaying and trapping of the molecule in an excited electronic state coupled to a continuum.