Abstract. The purpose of the work is to investigate the decay of long-lived states created by an alternating radiation and especially when the incident beam is terminated. For the case of recoilless interaction the population law of the scatterer differs from the Lorentzian one and hence the mode of decay is not an exponent. It is shown that for scattering and transmission the detected intensity consists of two terms - one for excited slates in the source formed before the opening of the shutter and a second one accounting for states formed after the opening of the shutter. The contribution of each term is calculated. In addition the line broadenings in transmission spectra are shown.

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