We study the survival amplitude associated to a resonance close to the crossing value of two (1-dimensional) electronic levels, one of them being bonding, and the other one anti bonding. We find the first two terms of its asymptotic, as the nuclear mass tends to infinity. In particular, the second term is described in terms of the interaction (off-diagonal) term of the 2x2 Schroedinger matrix-operator that describes the system. This is a joint work with Philippe Briet.