Deviations from exponential law and Van Hove's "lambda(2)t" limit

The deviations from a purely exponential behavior in a decay process are an alyzed in relation to Van Hove's "lambda(2)t" limiting procedure. Our atten tion is focused on the effects that arise when the coupling constant is sma ll but nonvanishing. We first consider a simple model (two-level atom in in teraction with the electromagnetic field), then gradually extend our analys is to a more general framework. We estimate all deviations from exponential behavior at leading orders in the coupling constant. (C) 1999 Elsevier Sci ence B.V. All rights reserved.