Theoretical analysis of the phase shift measurement of lifetimes using monochromatic light

It is shown that, if one uses an essentially monochromatic but weakly modulated light source to excite atoms or molecules in a resonant way, the phase shift of the scattered light is strongly dependent on the strength of the excitation. However, for low values of the excitation strength, this dependence disappears but a result which differs by a factor of two from the result obtained for broad-band light excitation is found. In addition, the strength of the modulated component of the scattered light displays a resonant behaviour in the modulation frequency, this response being directly related to the well-known Autler-Townes effect.