Deviations from exponential law and Van Hoveʹs “λ2t” limit

The deviations from a purely exponential behavior in a decay process are analyzed in relation to Van Hoveʹs “λ2t” limiting procedure. Our attention is focused on the effects that arise when the coupling constant is small but nonvanishing. We first consider a simple model (two-level atom in interaction with the electromagnetic field), then gradually extend our analysis to a more general framework. We estimate all deviations from exponential behavior at leading orders in the coupling constant.