Effects deriving from the finite spectral line width of an irradiation source are investigated for the resonant excitation process and found to be of particular interest when the irradiation spectral width is comparable with atomic (or molecular) line widths. An application of high current interest is laser isotope separation using relatively broad band but tunable dye lasers for selective excitation. Expressions are derived in systematic fashion for the absorption coefficient and for the yield, taking into account three independent line shapes—the first describing the irradiation source, the other two describing Lorentz and Doppler broadening of the atomic (or molecular) medium. Saturation effects are included, but propagation effects are neglected. It is shown that the customary distinction between homogeneous and inhomogeneous atomic-line broadening must be modified if the irradiation is not monochromatic. A further result of practical importance is that there exists an optimum irradiation line width that maximizes the yield for resonant transfer. In this respect, the relatively broad spectral widths characteristic of dye lasers are to be regarded as an advantageous feature, contrary to what is generally assumed.
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