Time Dependence of Saturation in Microwave Spectroscopy

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Abstract
Previously derived expressions for the absorption coefficient for microwave spectroscopy under conditions of partial saturation are critically examined and extended to times shortly after the onset of the radiation. It is found that the absorption coefficient varies with time as an exponentially damped sine wave. It is shown that the absorption coefficient and the population difference do not have the same time dependence and the discrepancy is explained as being the result of population changes caused by collisions. A semiclassical argument is given for the validity of the differential equations obtained from the concept of strong collisions. The relation between the time constant of the exponential damping and the conventional linewidth is discussed.