Transient probe spectra in a driven two-level atom

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Abstract
We analyze the transient probe spectra that arise when two-level atoms in an atomic beam are excited by a ‘‘pump’’ laser field and are probed by a step-function weak field on the same atomic transition. The temporal relaxation of the probe spectra to their steady-state values is found to depend critically on the initial atom–pump-field dressed-state populations and coherences. The transient spectra may be composed of absorption-emission structure that is totally absent in the steady-state limit. A simple physical interpretation of the spectra is given in terms of a conventional dressed-atom picture. In addition, we study the effects of collisions on transient probe spectra and investigate the transient buildup of several collision-induced features in the spectra.