Resonant interaction of smooth laser pulses with two level atoms

The authors have studied the fluorescence spectrum of a two-level atom interacting with a resonant smooth laser pulse. Assuming a filter width larger than the Fourier width of the pulse, an approximate analytic closed form for the power spectrum valid for any pulse shape is obtained. In these conditions the power spectrum exhibits a time-developing three-peak structure, while the energy spectrum presents a single line, unless the pulse is near rectangular. Theoretical energy spectra are in qualitative agreement with measurements of the resonance fluorescence emitted by strontium atoms irradiated by short laser pulses.